Hexakis‐Adducts of [60]Fullerene with Different Addition Patterns: Templated Synthesis,Physical Properties,and Chemical Reactivity

Representatives of two classes of hexakis‐adducts of C₆₀ were prepared by templated synthesis strategies. Compound 8 with a dipyridylmethano addend in a pseudo‐octahedral addition pattern was obtained by DMA‐templated addition (DMA=9,10‐dimethylanthracene; Scheme 1) and served as the starting material for the first supramolecular fullerene dimer 2 . Hexakis‐adduct 12 also possesses a pseudo‐octahedral addition pattern and was obtained by a sequence of tether‐directed remote functionalization, tether removal, and regioselective bis‐functionalization (Scheme 2). With its two diethynylmethano addends in trans‐1 position, it is a precursor for fascinating new oligomers and polymers that feature C₆₀ moieties as part of the polymeric backbone (Fig. 1). With the residual fullerene π‐electron chromophore reduced to a `cubic cyclophane'‐type sub‐structure (Fig. 4), and for steric reasons, 8 and 12 no longer display electrophilic reactivity. As a representative of the second class of hexakis‐adducts, (±)‐ 1 , which features six addends in a distinct helical array along an equatorial belt, was prepared by a route that involved two sequential tether‐directed remote functionalization steps (Schemes 3 and 5). In compound (±)‐ 1 , π‐electron conjugation between the two unsubstituted poles of the carbon sphere is maintained via two (E)‐stilbene‐like bridges (Fig. 4). As a result, (±)‐ 1 features very different chemical reactivity and physical properties when compared to hexakis‐adducts with a pseudo‐octahedral addition pattern. Its reduction under cyclic voltammetric conditions is greatly facilitated (by 570 mV), and it readily undergoes additional, electronically favored Bingel additions at the two sterically well‐accessible central polar 6‐6 bonds under formation of heptakis‐ and octakis‐adducts, (±)‐ 30 and (±)‐ 31 , respectively (Scheme 6). The different extent of the residual π‐electron delocalization in the fullerene sphere is also reflected in the optical properties of the two types of hexakis‐adducts. Whereas 8 and 12 are bright‐yellow (end‐absorption around 450 nm), compound (±)‐ 1 is shiny‐red, with an end‐absorption around 600 nm. This study once more demonstrates the power of templated functionalization strategies in fullerene chemistry, providing addition patterns that are not accessible by stepwise synthetic approaches.     

Supramolecular Fullerene Chemistry: A Comprehensive Study of Cyclophane‐Type Mono‐ and Bis‐Crown Ether Conjugates of C70

The covalently templated bis‐functionalization of C₇₀, employing bis‐malonate 5 tethered by an anti‐disubstituted dibenzo[18]crown‐6 (DB18C6) ether, proceeds with complete regiospecificity and provides two diastereoisomeric pairs of enantiomeric C₇₀ crown ether conjugates, (±)‐ 7a and (±)‐ 7b , featuring a five o'clock bis‐addition pattern that is disfavored in sequential transformations (Scheme 1). The identity of (±)‐ 7a was revealed by X‐ray crystal‐structure analysis (Fig. 6). With bis‐malonate 6 containing a syn‐disubstituted DB18C6 tether, the regioselectivity of the macrocylization via double Bingel cyclopropanation changed completely, affording two constitutionally isomeric C₇₀ crown ether conjugates in a ca. 1 : 1 ratio featuring the twelve ( 16 ) and two o'clock ((±)‐ 15 ) addition patterns, respectively (Scheme 3). The X‐ray crystal‐structure analysis of the twelve o'clock bis‐adduct 16 revealed that a H₂O molecule was included in the crown ether cavity (Figs. 7 and 8). Two sequential Bingel macrocyclizations, first with anti‐DB18C6‐tethered ( 5 ) and subsequently with syn‐DB18C6‐tethered ( 6 ) bis‐malonates, provided access to the first fullerene bis‐crown ether conjugates. The two diastereoisomeric pairs of enantiomers (±)‐ 28a and (±)‐ 28b were formed in high yield and with complete regioselectivity (Scheme 9). The cation‐binding properties of all C₇₀ crown‐ether conjugates were determined with the help of ion‐selective electrodes (ISEs). Mono‐crown ether conjugates form stable 1 : 1 complexes with alkali‐metal ions, whereas the tetrakis‐adducts of C₇₀, featuring two covalently attached crown ethers, form stable 1 : 1 and 1 : 2 host‐guest complexes (Table 2). Comparative studies showed that the conformation of the DB18C6 ionophore imposed by the macrocyclic bridging to the fullerene is not particularly favorable for strong association. Reference compound (±)‐ 22 (Scheme 4), in which the DB18C6 moiety is attached to the C₇₀ sphere by a single bridge only and, therefore, possesses higher conformational flexibility, binds K⁺ and Na⁺ ions better by factors of 2 and 20, respectively. Electrochemical studies demonstrate that cation complexation at the crown ether site causes significant anodic shifts of the first reduction potential of the appended fullerene (Table 3). In case of the C₇₀ mono‐crown ether conjugates featuring a five o'clock functionalization pattern, addition of 1 equiv. of KPF₆ caused an anodic shift of the first reduction wave in the cyclic voltammogram (CV) by 70 to 80 mV, which is the result of the electrostatic effect of the K⁺ ion bound closely to the fullerene core (Fig. 14). Addition of 2 equiv. of K⁺ ions to C₇₀ bis‐crown ether conjugates resulted in the observation of only one redox couple, whose potential is anodically shifted by 170 mV with respect to the corresponding wave in the absence of the salt (Fig. 16). The synthesis and characterization of novel tris‐ and tetrakis‐adducts of C₇₀ are reported (Schemes 5 and 6). Attempts to prepare even more highly functionalized derivatives resulted in the formation of novel pentakis‐ and hexakis‐adducts and a single heptakis‐adduct (Scheme 7), which were characterized by ¹H‐ and ¹³C‐NMR spectroscopy (Fig. 10), as well as matrix‐assisted laser‐desorption‐ionization mass spectrometry (MALDI‐TOF‐MS). Based on predictions from density‐functional‐theory (DFT) calculations (Figs. 12 and 13), structures are proposed for the tris‐, tetrakis‐, and pentakis‐adducts.     

Optically Active Macrocyclic cis‐3 Bis‐Adducts of C60: Regio‐ and Stereoselective Synthesis,Exciton Chirality Coupling,and Determination of the Absolute Configuration,and First Observation of Exciton Coupling between Fullerene Chromophores in a Chiral Environment

A series of optically active cis‐3 bis‐adducts, such as (R,R,^fC)‐ 16 (Scheme 6), was obtained regio‐ and diastereoselectively by Bingel macrocyclization of C₆₀ with bis‐malonates, which contain optically active tethers derived from 1,2‐diols. The absolute configuration of the inherently chiral addition pattern in cis‐3 bis‐adducts had previously been determined by comparison of calculated and experimental circular dichroism (CD) spectra. Full confirmation of these earlier assignments was now obtained by an independent method based on semiempirical AM1 (`Austin Model 1') and OM2 (`Orthogonalization Method 2') calculations combined with ¹H‐NMR spectroscopy. It was found computationally that bis‐malonates [CHR(OCOCH₂COOEt)]₂, which contain (R,R)‐ or (S,S)‐butane‐2,3‐diol derivatives as optically active tethers, preferentially form out‐out cis‐3 bis‐adducts of C₆₀ as a single diastereoisomer in which the alkyl groups R adopt a gauche conformation, while the two glycolic H‐atoms are in an antiperiplanar (ap) and the ester linkages to the fullerene in a gauche relationship (Figs. 2 and 5). In contrast, in the less favorable diastereoisomer, which should not form, the alkyl groups R adopt an ap and the H‐atoms a gauche conformation, while the ester bridges to the fullerene remain, for geometric reasons, locked in a gauche conformation. According to the OM2 calculations, the geometry of the fully staggered tether in the free bis‐malonates closely resembles the conformation of the tether fragment in the bis‐adducts formed. These computational predictions were confirmed experimentally by the measurement of the coupling constant between the vicinal glycolic H‐atoms in the ¹H‐NMR spectrum. For (R,R,^fC)‐ 16 , ³J(H,H) was determined as 7.9 Hz, in agreement with the ap conformation, and, in combination with the calculations, this allowed assignment of the ^fC‐configuration to the inherently chiral addition pattern. This conformational analysis was further supported by the regio‐ and diastereoselective synthesis of cis‐3 bis‐adducts from bis‐malonates, including tethers derived from cyclic glycol units with a fixed gauche conformation of the alkyl residues R at the glycolic C‐atoms. Thus, a bis‐malonate of (R,R)‐cyclohexane‐1,2‐diol provided exclusively cis‐3 bis‐adduct (R,R,^fC)‐ 20 in 32% yield (Scheme 7). Incorporation of a tether derived from methyl 4,6‐O,O‐benzylidene‐α‐D ‐glucopyranoside into the bis‐malonate and Bingel macrocyclization diastereoselectively produced the cis‐3 stereoisomer (α,D ,^fA)‐ 22 (Scheme 8) as the only macrocyclic bis‐adduct. If the geometry of the alkyl groups R at the glycolic C‐atoms of the tether component deviates from a gauche relationship, as in the case of tethers derived from exo cis‐ and trans‐norbornane‐2,3‐diol or from trans‐cyclopentane‐1,2‐diol, hardly any macrocyclic product is formed (Schemes 5 and 9). The absolute configurations of the various optically active cis‐3 bis‐adducts were also assigned by comparison of their CD spectra, which are dominated by the chiroptical contributions of the inherently chiral fullerene chromophore (Figs. 1, 3, and 4). A strong chiral exciton coupling was observed for optically active macrocyclic cis‐3 bis‐adducts of C₆₀ with two appended 4‐(dimethylamino)benzoate ((S,S,^fC)‐ 26 ; Fig. 6) or meso‐tetraphenylporphyrin ((R,R,^fC)‐ 28 ; Fig. 7) chromophores. Chiral exciton coupling between two fullerene chromophores was observed for the first time in the CD spectrum of the threitol‐bridged bis‐fullerene (R,R)‐ 35 (Fig. 9).     

The Orthogonal (e,e,e)‐Tris‐Adduct of 9,10‐Dimethylanthracene with C60‐Fullerene: A Hidden Cornerstone of Fullerene Chemistry. Preliminary Communication

Tris(9′,10′‐dimethyl[9,10]ethanoanthracene[11′,12′: 1,9;11″,12″: 16,17;11′′′,12′′′: 30,31])[5,6]fullerene C₆₀, the orthogonal (e,e,e)‐tris‐adduct of C₆₀ and 9,10‐dimethylanthracene, was obtained from [4+2]‐cycloaddition (Diels–Alder reaction) at room temperature. The thermally unstable orange red (e,e,e)‐tris‐adduct was purified by chromatography and was isolated in the form of red monoclinic crystals. Its C₃‐symmetric addition pattern was established spectroscopically. Its structure could be further investigated by single crystal X‐ray diffraction. The (e,e,e)‐tris‐adduct of C₆₀ and 9,10‐dimethylanthracene has earlier been suggested as intermediate and reversibly formed critical component in ‘template directed’ addition reactions of C₆₀. This previously elusive compound has now been isolated and structurally characterized.     

Cyclophane-Type Fullerene-dibenzo[18]crown-6 Conjugates with trans-1, trans-2, and trans-3 Addition Patterns: Regioselective Templated Synthesis,X-Ray Crystal Structure,Ionophoric Properties,and Cation-Complexation-Dependent Redox Behavior

The fullerene-crown ether conjugates (±)- 1 to (±)- 3 with trans-1 ((±)- 1 ), trans-2 ((±)- 2 ), and trans-3 ((±)- 3 ) addition patterns on the C-sphere were prepared by Bingel macrocyclization. The trans-1 derivative (±)- 1 was obtained in 30% yield, together with a small amount of (±)- 2 by cyclization of the dibenzo[18]crown-6(DB18C6)-tethered bis-malonate 4 with C₆₀ (Scheme 1). When the crown-ether tether was further rigidified by K⁺-ion complexation, the yield and selectivity were greatly enhanced, and (±)- 1 was obtained as the only regioisomer in 50% yield. The macrocyclization, starting from a mixture of tethered bis-malonates with anti ( 4 ) and syn ( 10 ) bisfunctionalized DB18C6 moieties, afforded the trans-1 ((±)- 1 , 15%), trans-2 ((±)- 2 , 1.5%), and trans-3 ((±)- 3 , 20%) isomers (Scheme 2). Variable-temperature ¹H-NMR (VT-NMR) studies showed that the DB18C6 moiety in C₂-symmetrical (±)- 1 cannot rotate around the two arms fixing it to the C-sphere, even at 393 K. The planar chirality of (±)- 1 was confirmed in ¹H-NMR experiments using the potassium salts of (S)-1,1′-binaphthalene-2,2′-diyl phosphate ((+)-(S)- 19 ) or (+)-(1S)-camphor-10-sulfonic acid ((+)- 20 ) as chiral shift reagents (Fig. 1). The DB18C6 tether in (±)- 1 is a true covalent template: it is readily removed by hydrolysis or transesterification, which opens up new perspectives for molecular scaffolding using trans-1 fullerene derivatives. Characterization of the products 11 (Scheme 3) and 18 (Scheme 4) obtained by tether removal unambiguously confirmed the trans-1 addition pattern and the out-out geometry of (±)- 1 . VT-NMR Studies established that (±)- 2 is a C₂-symmetrical out-out trans-2 and (±)- 3 a C₁-symmetrical in-out trans-3 isomer. Upon changing from (±)- 1 to (±)- 3 , the distance between the DB18C6 moiety and the fullerene surface increases and, correspondingly, rotation of the ionophore becomes increasingly facile. The ionophoric properties of (±)- 1 were investigated with an ion-selective electrode membrane (Fig. 2 and Table 2), and K⁺ was found to form the most stable complex among the alkali-metal ions. The complex between (±)- 1 and KPF₆ was characterized by X-ray crystal-structure analysis (Figs. 3 and 4), which confirmed the close tangential orientation of the ionophore atop the fullerene surface. Addition of KPF₆ to a solution of (±)- 1 resulted in a large anodic shift (90 mV) of the first fullerene-centered reduction process, which is attributed to the electrostatic effect of the K⁺ ion bound in close proximity to the C-sphere (Fig. 5). Smaller anodic shifts were measured for the KPF₆ complexes of (±)- 2 (50 mV) and (±)- 3 (40 mV), in which the distance between ionophore and fullerene surface is increased (Table 3). The effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of (±)- 1 were investigated (Table 4). These are the first-ever observed effects of cation complexation on the redox properties of the C-sphere in fullerene-crown ether conjugates.     

Enzyme‐Mediated Preparation of (+)‐ and (−)‐β‐Irone and (+)‐ and (−)‐cis‐γ‐Irone from Irone alpha®

The (−)‐ and (+)‐β‐irones ((−)‐ and (+)‐ 2 , resp.), contaminated with ca. 7 – 9% of the (+)‐ and (−)‐trans‐α‐isomer, respectively, were obtained from racemic α‐irone via the 2,6‐trans‐epoxide (±)‐ 4 (Scheme 2). Relevant steps in the sequence were the LiAlH₄ reduction of the latter, to provide the diastereoisomeric‐4,5‐dihydro‐5‐hydroxy‐trans‐α‐irols (±)‐ 6 and (±)‐ 7 , resolved into the enantiomers by lipase‐PS‐mediated acetylation with vinyl acetate. The enantiomerically pure allylic acetate esters (+)‐ and (−)‐ 8 and (+)‐ and (−)‐ 9 , upon treatment with POCl₃/pyridine, were converted to the β‐irol acetate derivatives (+)‐ and (−)‐ 10 , and (+)‐ and (−)‐ 11 , respectively, eventually providing the desired ketones (+)‐ and (−)‐ 2 by base hydrolysis and MnO₂ oxidation. The 2,6‐cis‐epoxide (±)‐ 5 provided the 4,5‐dihydro‐4‐hydroxy‐cis‐α‐irols (±)‐ 13 and (±)‐ 14 in a 3 : 1 mixture with the isomeric 5‐hydroxy derivatives (±)‐ 15 and (±)‐ 16 on hydride treatment (Scheme 1). The POCl₃/pyridine treatment of the enantiomerically pure allylic acetate esters, obtained by enzymic resolution of (±)‐ 13 and (±)‐ 14 , provided enantiomerically pure cis‐α‐irol acetate esters, from which ketones (+)‐ and (−)‐ 22 were prepared (Scheme 4). The same materials were obtained from the (9S) alcohols (+)‐ 13 and (−)‐ 14 , treated first with MnO₂, then with POCl₃/pyridine (Scheme 4). Conversely, the dehydration with POCl₃/pyridine of the enantiomerically pure 2,6‐cis‐5‐hydroxy derivatives obtained from (±)‐ 15 and (±)‐ 16 gave rise to a mixture in which the γ‐irol acetates 25a and 25b and 26a and 26b prevailed over the α‐ and β‐isomers (Scheme 5). The (+)‐ and (−)‐cis‐γ‐irones ((+)‐ and (−)‐ 3 , resp.) were obtained from the latter mixture by a sequence involving as the key step the photochemical isomerization of the α‐double bond to the γ‐double bond. External panel olfactory evaluation assigned to (+)‐β‐irone ((+)‐ 2 ) and to (−)‐cis‐γ‐irone ((−)‐ 3 ) the strongest character and the possibility to be used as dry‐down note.     

A New Approach to Sesquiterpene Arenes of the 9,11‐Drimenyl Type (= [(1E,2RS,4aRS,8aRS)‐Octahydro‐2,5,5,8a‐tetramethylnaphthalen‐1(2H)‐ylidene] methyl Type)

A new reaction sequence for the synthesis of the sesquiterpene arenes (±)‐wiedendiol B ((±)‐ 1 ) and the siphonodictyal B derivative (±)‐ 21 consists in the coupling of (±)‐drimanoyl chloride ((±)‐ 3 ) with lithiated and appropriately substituted aromatic synthons to furnish the ketones (±)‐ 7 and (±)‐ 17 which were reduced to the benzyl alcohols (±)‐ 8a,b and (±)‐ 18a,b , respectively (Schemes 5, 4, and 12). The 9,11‐double bond of the drimenes (±)‐ 9 and (±)‐ 19 was formed by elimination of H₂O from the benzyl alcohols (±)‐ 8a,b and (±)‐ 18a,b (Schemes 6 and 12). New alternatives were applied to this elimination reaction involving either the pyridine ? SO₃ complex or chloral as reagents.     

Macrocyclization on the fullerene core: Direct regio- and diastereoselective multi-functionalization of [60]fullerene,and synthesis of fullerene-dendrimer derivatives

The macrocyclization between buckminsterfullerene, C₆₀, and bis-malonate derivatives in double Bingel reaction provides a versatile and simple method for the preparation of covalent bis-adducts of C₆₀ with high regio- and diastereoselectivity. A combination of spectral analysis, stereochemical considerations, and X-ray crystallography (Fig. 2) revealed that out of the possible in-in, in-out, and out-out stereoisomers, the reaction of bis-malonates linked by o-, m-, or p-xylylene tethers afforded only the out-out ones (Scheme 1). In contrast, the use of larger tethers derived from 1,10-phenanthroline also provided a first example, (±)- 19 (Scheme 2), of an in-out product. Starting from optically pure bis-malonate derivatives, the new bis-functionalization method permitted the diastereoselective preparation of optically active fullerene derivatives (Schemes 4 and 5) and, ultimately, the enantioselective preparation (enantiomeric excess ee > 97%) of optically active cis-3 bis-adducts whose chirality results exclusively from the addition pattern (Fig. 6). The macrocyclic fixation of a bis-malonate with an optically active, 9,9′-spirobi[9H-fluorene]-derived tether to C₆₀ under generation of 24 and ent- 24 with an achiral addition pattern (Scheme 4) was found to induce dramatic changes in the chiroptical properties of the tether chromophore such as strong enhancement and reversal of sign of the Cotton effects in the circular dichroism (CD) spectra (Figs. 4 and 5). By the same method, the functionafized bis-adducts 50 and 51 (Schemes 10 and 11) were prepared as initiator cores for the synthesis of the fullerene dendrimers 62 , 63 , and 66 (Schemes 12 and 13) by convergent growth. Finally, the new methodology was extended, to the regio- and diastereoselective construction of higher cyclopropanated adducts. Starting from mono-adduct 71 , a clipping reaction provided exclusively the all-cis-2 tris-adduct (±)- 72 (Scheme 14), whereas the similar reaction of bis-adduct 76 afforded the all-cis-2 tetrakis-adduct 77 (Scheme 15). Electrochemical investigations by steady-state voltammetry (Table 2) in CH₂Cl₂ (+0.1M Bu₄NPF₆) showed that all macroeyciic bis(methano)fullerenes underwent multiple reduction steps, and that regioisomerism was not much influencing the redox potentials, All cis-2 bis-adducts gave an instable dianion which decomposed during the electrochemical reduction. In CH₂Cl₂, the redox potential of the fullerene core in dendrimers 62, 63 , and 66 is not affected by differences in size and density of the surrounding poly(ether-amide) dendrons. The all-cis-2 tris- and tetrakis(meihano)fullercnes (±) -72 and 77 , respectively, are reduced at more negative potential than previously reported all-e tris- and tetrakis-adducts with methane bridges that are also located along an equatorial belt. This indicates a larger perturbation of the original fullerene π-chromophore and a larger raise in LUMO energy in the former derivatives.     

A New Family of C3‐Symmetrical Carbohydrate Receptors

The formation of the new optically active C₃‐symmetrical receptors (S,S,S)‐ 2 – 4 (Fig. 1), incorporating 1,3,5‐triphenylbenzene and 1,3,5‐tris(phenylethynyl)benzene platforms as ‘floors' and ‘ceilings', is described. The tris(phenylethynyl)benzene derivatives 9 and (S,S,S)‐ 10 (Scheme 1) for the three‐fold peptide coupling to yield the macrocyclic skeletons (Scheme 2) were prepared starting from 1,3,5‐triethynylbenzene by the Sonogashira cross‐coupling reaction. The optical rotations of the three macrocycles (S,S,S)‐ 2 – 4 , two of which ((S,S,S)‐ 2 and (S,S,S)‐ 3 ) are constitutional isomers, differ significantly, which is explained by differential twists induced into the macrocyclic skeletons by the leucine spacer in these bridges. 1 : 1 Host–guest complexes of (S,S,S)‐ 2 – 4 with octyl glucosides (Fig. 3) in CDCl₃ are of modest stability (K_a≤270 M ^?1 at 300 K). In these complexes, the monosaccharides are most probably nesting on one of the H‐bonding faces of the receptor rather than being accommodated in the cavity.     

A Joint Theoretical and Experimental Insight into the Electronic Structure of Chromophores Derived from 6H,12H‐5,11‐Methanodibenzo[b,f][1,5]diazocine

We report on the synthesis and electronic spectra of the chiral, donor‐acceptor (push‐pull) chromophores (±)‐ 4 and (±)‐ 5 with a 6H,12H‐5,11‐methanodibenzo[b,f][1,5]diazocine scaffold (Scheme 1 and Fig. 2). The electronic structures of these compounds were investigated at a quantum‐chemical level (Figs. 2 and 3). The chemical reactivity of 6H,12H‐5,11‐methanodibenzo[b,f][1,5]diazocine ((±)‐ 11 ) towards aromatic electrophilic substitution (Scheme 2 and Table) provided additional information about its electronic structure and confirmed nonnegligible delocalization of the lone pair of the bridge‐head N‐atoms in this heterocyclic system.     

Metabolism of Deuterated Isomeric 6,7‐Dihydroxydodecanoic Acids in Saccharomyces cerevisiae – Diastereo‐ and Enantioselective Formation and Characterization of 5‐Hydroxydecano‐4‐lactone (=4,5‐Dihydro‐5‐(1‐hydroxyhexyl)furan‐2(3H)‐one) Isomers

The chemical synthesis of deuterated isomeric 6,7‐dihydroxydodecanoic acid methyl esters 1 and the subsequent metabolism of esters 1 and the corresponding acids 1a in liquid cultures of the yeast Saccharomyces cerevisiae was investigated. Incubation experiments with (6R,7R)‐ or (6S,7S)‐6,7‐dihydroxy(6,7‐²H₂)dodecanoic acid methyl ester ((6R,7R)‐ or (6S,7S)‐(6,7‐²H₂)‐ 1 , resp.) and (±)‐threo‐ or (±)‐erythro‐6,7‐dihydroxy(6,7‐²H₂)dodecanoic acid ((±)‐threo‐ or (±)‐erythro‐(6,7‐²H₂)‐ 1a , resp.) elucidated their metabolic pathway in yeast (Tables 1–3). The main products were isomeric ²H‐labeled 5‐hydroxydecano‐4‐lactones 2 . The absolute configuration of the four isomeric lactones 2 was assigned by chemical synthesis via Sharpless asymmetric dihydroxylation and chiral gas chromatography (Lipodex ^® E). The enantiomers of threo‐ 2 were separated without derivatization on Lipodex ^® E; in contrast, the enantiomers of erythro‐ 2 could be separated only after transformation to their 5‐O‐(trifluoroacetyl) derivatives. Biotransformation of the methyl ester (6R,7R)‐(6,7‐²H₂)‐ 1 led to (4R,5R)‐ and (4S,5R)‐(2,5‐²H₂)‐ 2 (ratio ca. 4 : 1; Table 2). Estimation of the label content and position of (4S,5R)‐(2,5‐²H₂)‐ 2 showed 95% label at C(5), 68% label at C(2), and no ²H at C(4) (Table 2). Therefore, oxidation and subsequent reduction with inversion at C(4) of 4,5‐dihydroxydecanoic acid and transfer of ²H from C(4) to C(2) is postulated. The 5‐hydroxydecano‐4‐lactones 2 are of biochemical importance: during the fermentation of Streptomyces griseus, (4S,5R)‐ 2 , known as L‐factor, occurs temporarily before the antibiotic production, and (?)‐muricatacin (=(4R,5R)‐5‐hydroxy‐heptadecano‐4‐lactone), a homologue of (4R,5R)‐ 2 , is an anticancer agent.     

Binuclear trans‐Bis(β‐iminoaryloxy)palladium(II) Complexes Doubly Linked with Pentamethylene Spacers: Structure‐Dependent Flapping Motion and Heterochiral Association Behavior of the Clothespin‐Shaped Molecules

The synthesis, structure, and solution‐state behavior of clothespin‐shaped binuclear trans‐bis(β‐iminoaryloxy)palladium(II) complexes doubly linked with pentamethylene spacers are described. Achiral syn and racemic anti isomers of complexes 1 – 3 were prepared by treating Pd(OAc)₂ with the corresponding N,N′‐bis(β‐hydroxyarylmethylene)‐1,5‐pentanediamine and then subjecting the mixture to chromatographic separation. Optically pure (100 % ee) complexes, (+)‐anti‐ 1 , (+)‐anti‐ 2 , and (+)‐anti‐ 3 , were obtained from the racemic mixture by employing a preparative HPLC system with a chiral column. The trans coordination and clothespin‐shaped structures with syn and anti conformations of these complexes have been unequivocally established by X‐ray diffraction studies. ¹H NMR analysis showed that (±)‐anti‐ 1 , (±)‐anti‐ 2 , syn‐ 2 , and (±)‐anti‐ 3 display a flapping motion by consecutive stacking association/dissociation between cofacial coordination planes in [D₈]toluene, whereas syn‐ 1 and syn‐ 3 are static under the same conditions. The activation parameters for the flapping motion (ΔH^≠ and ΔS^≠) were determined from variable‐temperature NMR analyses as 50.4 kJ mol^?1 and 60.1 J mol^?1 K^?1 for (±)‐anti‐ 1 , 31.0 kJ mol^?1 and ?22.7 J mol^?1 K^?1 for (±)‐anti‐ 2 , 29.6 kJ mol^?1 and ?57.7 J mol^?1 K^?1 for syn‐ 2 , and 35.0 kJ mol^?1 and 0.5 J mol^?1 K^?1 for (±)‐anti‐ 3 , respectively. The molecular structure and kinetic parameters demonstrate that all of the anti complexes flap with a twisting motion in [D₈]toluene, although (±)‐anti‐ 1 bearing dilated Z‐shaped blades moves more dynamically than I‐shaped (±)‐anti‐ 2 or the smaller (±)‐anti‐ 3 . Highly symmetrical syn‐ 2 displays a much more static flapping motion, that is, in a see‐saw‐like manner. In CDCl₃, (±)‐anti‐ 1 exhibits an extraordinary upfield shift of the ¹H NMR signals with increasing concentration, whereas solutions of (+)‐anti‐ 1 and the other syn/anti analogues 2 and 3 exhibit negligible or slight changes in the chemical shifts under the same conditions, which indicates that anti‐ 1 undergoes a specific heterochiral association in the solution state. Equilibrium constants for the dimerizations of (±)‐ and (+)‐anti‐ 1 in CDCl₃ at 293 K were estimated by curve‐fitting analysis of the ¹H NMR chemical shift dependences on concentration as 26 M ^?1 [K_D(racemic)] and 3.2 M ^?1 [K_D(homo)], respectively. The heterochiral association constant [K_D(hetero)] was estimated as 98 M ^?1, based on the relationship K_D(racemic)=1/2 K_D(homo)+1/4 K_D(hetero). An inward stacking motif of interpenetrative dimer association is postulated as the mechanistic rationale for this rare case of heterochiral association.     

Molecular Switching: A Fully Reversible,Optically Active Photochemical Switch Based on a Tetraethynylethene‐1,1′‐Binaphthalene Hybrid System

The synthesis, characterization, and physical properties of a novel, fully reversible, light‐driven molecular switch, (R,R)‐ 1 /(R,R)‐ 2 , based on a tetraethynylethene‐1,1′‐binaphthalene hybrid system are presented. trans‐Configured (R,R)‐ 1 was synthesized in 57% yield by Stille cross‐coupling between stannylated tetraethynylethene 3 and 3‐iodo‐1,1′‐binaphthalene derivative (R)‐ 4 (cf. Scheme 2). The cis‐isomer (R,R)‐ 2 was prepared from (R,R)‐ 1 by photoisomerization. X‐Ray crystal‐structure analyses were obtained for both cis‐ and trans‐forms of the photoswitch (Figs. 1 and 2). In the crystalline state, molecules of the cis‐isomer (R,R)‐ 2 exhibit intramolecular edge‐to‐face (C−H⋅⋅⋅π) interactions between naphthalene rings of the two 1,1‐binaphthalene moieties (Fig. 3). The switching properties were investigated by electronic absorption spectroscopy (Table and Fig. 4): irradiation at λ=398 nm converts trans‐isomer (R,R)‐ 1 into cis‐isomer (R,R)‐ 2 , whereas switching occurs in the opposite direction upon irradiation at λ=323 nm. No thermal interconversion between the two isomers was observed in CH₂Cl₂ at room temperature over a period of 2 – 3 months, and the system possesses good resistance against photofatigue (Fig. 5). Investigations of the circular dichroism of (R,R)‐ 1 and (R,R)‐ 2 in CH₂Cl₂ solution showed that the chiral binaphthalene moieties induce a weak Cotton effect in the achiral tetraethynylethene core (Fig. 6). System (R,R)‐ 1 /(R,R)‐ 2 represents one of the rare switches allowing two‐way photochemical interconversions, not perturbed by thermal‐isomerization pathways.     

Achiral and Chiral Higher Adducts of C70 by Bingel Cyclopropanation

Five optically active isomeric C₇₀ bis-adducts with (R)-configured chiral malonate addends were prepared by Bingel cyclopropanation (Scheme 1) and their circular dichroism (CD) spectra investigated in comparison to those of the corresponding five bis-adducts with (S)-configured addends (Fig. 2). Pairs of diastereoisomers, in which the inherently chiral addition patterns on the fullerene surface have an enantiomeric relationship, display mirror-image shaped CD spectra that are nearly identical to those of the corresponding pairs of enantiomers (Fig. 3, b and c). This result demonstrates that the Cotton effects arising from the chiral malonate addends are negligible as compared to the chiroptical contribution of the chirally functionalized fullerene chromophore. A series of four stereoisomeric tetrakis-adducts (Fig. 4) was prepared by Bingel cyclopropanation starting from four stereoisomeric bis-adducts. A comparison of the CD spectra of both series of compounds showed that the magnitude of the Cotton effects does not decrease with increasing degree of functionalization (Fig. 5). Bingel cyclopropanations of C₇₀ in Me₂SO are dramatically faster than in apolar solvents such as CCl₄, and the reaction of bis-adducts (±)- 13 and 15 with large excesses of diethyl 2-bromomalonate and DBU generated, via the intermediacy of defined tetrakis-adducts (±)- 16 and 17 , respectively, a series of higher adducts including hexakis-, heptakis-, and octakis-adducts (Table 1). A high regioselectivity was observed up to the stage of the hexakis-adducts, whereas this selectivity became much reduced at higher stages of addition. The regioselectivity of the nucleophilic cyclopropanations of C₇₀ correlates with the coefficients of the LUMO (lowest unoccupied molecular orbital) and LUMO+1 at the positions of preferential attack calculated by restricted Hartree-Fock – self-consistent field (RHF-SCF) methods (Figs. 9 – 11). Based on predictions from molecular-orbital calculations (Fig. 11) and the analysis of experimental ¹³C-NMR data (Fig. 7, a), the structure of a unique hexakis-adduct ((±)- 22 , Fig. 12), prepared from (±)- 13 , was assigned. The C₂-symmetrical compound contains four 6−6-closed methanofullerene sub-structures in its polar regions (at the bonds C(1)−C(2), C(31)−C(32), C(54)−C(55), and C(59)−C(60)), and two 6−5-open methanofullerene sub-structures parallel to the equator (at C(22)−C(23) and C(26)−C(27)). The 6−5-open sub-structures are formed by malonate additions to near-equatorial 6−5 bonds with enhanced LUMO coefficients, followed by valence isomerization (Fig. 12).     

Stereoselective Preparation of 3‐Amino‐2‐fluoro Carboxylic Acid Derivatives,and Their Incorporation in Tetrahydropyrimidin‐4(1H)‐ones,and in Open‐Chain and Cyclic β‐Peptides

The preparation of (2S,3S)‐ and (2R,3S)‐2‐fluoro and of (3S)‐2,2‐difluoro‐3‐amino carboxylic acid derivatives, 1 – 3 , from alanine, valine, leucine, threonine, and β³h‐alanine (Schemes 1 and 2, Table) is described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N‐dibenzyl‐2‐amino‐3‐hydroxy and 3‐amino‐2‐hydroxy carboxylic acid esters is discussed (Fig. 1). The fluoro‐β‐amino acid residues have been incorporated into pyrimidinones ( 11 – 13 ; Fig. 2) and into cyclic β‐tri‐ and β‐tetrapeptides 17 – 19 and 21 – 23 (Scheme 3) with rigid skeletons, so that reliable structural data (bond lengths, bond angles, and Karplus parameters) can be obtained. β‐Hexapeptides Boc[(2S)‐β³hXaa(αF)]₆OBn and Boc[β³hXaa(α,αF₂)]₆‐OBn, 24 – 26 , with the side chains of Ala, Val, and Leu, have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and many intermediates are fully characterized by IR‐ and ¹H‐, ¹³C‐ and ¹⁹F‐NMR spectroscopy, by MS spectrometry, and by elemental analyses, [α]_D and melting‐point values.     

Synthesis and High‐Resolution NMR Structure of a β3‐Octapeptide with and without a Tether Introduced by Olefin Metathesis

Bridging between (i)‐ and (i+3)‐positions in a β³‐peptide with a tether of appropriate length is expected to prevent the corresponding 3₁₄‐helix from unfolding (Fig. 1). The β³‐peptide H‐β³hVal‐β³hLys‐β³hSer(All)‐β³hPhe‐β³hGlu‐β³hSer(All)‐β³hTyr‐β³hIle‐OH ( 1 ; with allylated βhSer residues in 3‐ and 6‐position), and three tethered β‐peptides 2 – 4 (related to 1 through ring‐closing metathesis) have been synthesized (solid‐phase coupling, Fmoc strategy, on chlorotrityl resin; Scheme). A comparative CD analysis of the tethered β‐peptide 4 and its non‐tethered analogue 1 suggests that helical propensity is significantly enhanced (threefold CD intensity) by a (CH₂)₄ linker between the β³hSer side chains (Fig. 2). This conclusion is based on the premise that the intensity of the negative Cotton effect near 215 nm in the CD spectra of β³‐peptides represents a measure of ‘helical content’. An NMR analysis in CD₃OH of the two β³‐octapeptide derivatives without (i.e., 1 ) and with tether (i.e., 4 ; Tables 1–6, and Figs. 4 and 5) provided structures of a degree of precision (by including the complete set of side chain–side chain and side chain–backbone NOEs) which is unrivaled in β‐peptide NMR‐solution‐structure determination. Comparison of the two structures (Fig. 5) reveals small differences in side‐chain arrangements (separate bundles of the ten lowest‐energy structures of 1 and 4 , Fig. 5, A and B ) with little deviation between the two backbones (superposition of all structures of 1 and 4 , Fig. 5, C ). Thus, the incorporation of a CH₂? O? (CH₂)₄? O? CH₂ linker between the backbone of the β³‐amino acids in 3‐ and 6‐position (as in 4 ) does accurately constrain the peptide into a 3₁₄‐helix. The NMR analysis, however, does not suggest an increase in the population of a 3₁₄‐helical backbone conformation by this linkage. Possible reasons for the discrepancy between the conclusion from the CD spectra and from the NMR analysis are discussed.     

Stereoselective and Enantioselective Syntheses of the Four Stereoisomers of Muscol from (3RS)‐Muscone

Two trans stereoisomers of 3‐methylcyclopentadecanol (=muscol), (1R,3R)‐ 2 and (1S,3S)‐ 2 , were efficiently synthesized from (3RS)‐3‐methylcyclopentadecanone (=muscone; (3RS)‐ 1 ) by a highly stereoselective reduction (Scheme). L‐Selectride^® (=lithium tri(sec‐butyl)borohydride) was used, followed by the enantiomer resolution by lipase QLG (Alcaligenes sp.). The cis stereoisomers of muscol, (1S,3R)‐ 2 and (1R,3S)‐ 2 , were obtained by the Mitsunobu inversion of (1R,3R)‐ 2 and (1S,3S)‐ 2 , respectively (Scheme). The absolute configuration of (1R,3R)‐ 2 was determined by X‐ray crystal‐structure analysis of its 3‐nitrophthalic acid monoester, 2‐[(1R,3R)‐3‐methylcyclopentadecyl hydrogen benzene‐1,2‐dicarboxylate ((1R,3R)‐ 3b ), and by oxidation of (1R,3R)‐ 2 to (3R)‐muscone.     

Design,Synthesis, NMR‐Solution and X‐Ray Crystal Structure of N‐Acyl‐γ‐dipeptide Amides That Form a βII′‐Type Turn

Conformational analysis of γ‐amino acids with substituents in the 2‐position reveals that an N‐acyl‐γ‐dipeptide amide built of two enantiomeric residues of unlike configuration will form a 14‐membered H‐bonded ring, i.e., a γ‐peptidic turn (Figs. 1 – 3). The diastereoselective preparation of the required building blocks was achieved by alkylation of the doubly lithiated N‐Boc‐protected 4‐aminoalkanoates, which, in turn, are readily available from the corresponding (R)‐ or (S)‐α‐amino acids (Scheme 1). Coupling two such γ‐amino acid derivatives gave N‐acetyl and N‐[(tert‐butoxy)carbonyl] (Boc) dipeptide methyl amides ( 1 and 10 , resp.; Fig. 2, Scheme 2); both formed crystals suitable for X‐ray analysis, which confirmed the turn structures in the solid state (Fig. 4 and Table 4). NMR Analysis of the acetyl derivative 1 in CD₃OH, with full chemical‐shift and coupling assignments, and, including a 300‐ms ROESY measurement, revealed that the predicted turn structure is also present in solution (Fig. 5 and Tables 1 – 3). The results described here are yet another piece of evidence for the fact that more stable secondary structures are formed with a decreasing number of residues, and with increasing degree of predictability, as we go from α‐ to β‐ to γ‐peptides. Implications of the superimposable geometries of the actual turn segments (with amide bonds flanked by two quasi‐equatorial substituents) in α‐, β‐, and γ‐peptidic turns are discussed.     

One‐Pot,Asymmetric and Diastereoselective Four‐Component Synthesis of Polyfunctional (Z)‐Alkenyl Methyl Sulfones with Three Stereogenic Centers

The reaction of 1‐(trimethylsilyloxy)cyclopentene ( 9 ) with (±)‐1,3,5‐triisopropyl‐2‐(1‐(RS)‐{[(1E)‐2‐methylpenta‐1,3‐dienyl]oxy}ethyl)benzene ((±)‐ 4a ) in SO₂/CH₂Cl₂ containing (CF₃SO₂)₂NH, followed by treatment with Bu₄NF and MeI gave a 3.0 : 1 mixture of (±)‐(2RS)‐2{(1RS,2Z,4SR)‐2‐methyl‐4‐(methylsulfonyl)‐1‐[(RS)‐1‐(2,4,6‐triisopropylphenyl)ethoxy]pent‐2‐en‐1‐yl}cyclopentanone ((±)‐ 10 ) and (±)‐(2RS)‐2‐{(1RS,2Z)‐2‐methyl‐4‐[(SR)‐methylsulfonyl]‐1‐[(SR)‐1‐(2,4,6‐triisopropylphenyl)ethoxy]pent‐2‐en‐1‐yl}cyclopentanone ((±)‐ 11 ). Similarly, enantiomerically pure dienyl ether (−)‐(1S)‐ 4a reacted with 1‐(trimethylsilyloxy)cyclohexene ( 12 ) to give a 14.1 : 1 mixture of (−)‐(2S)‐2‐{(1S,2Z,4R)‐2‐methyl‐4‐(methylsulfonyl)‐1‐[(S)‐1‐(2,4,6‐triisopropylphenyl)ethoxy]pent‐2‐enyl}cyclohexanone ((−)‐ 13a ) and its diastereoisomer 14a with (1S,2R,4R) or (1R,2S,4S) configuration. Structures of (±)‐ 10 , (±)‐ 11 , and (−)‐ 13a were established by single‐crystal X‐ray crystallography. Poor diastereoselectivities were observed with the (E,E)‐2‐methylpenta‐1,3‐diene‐1‐ylethers (+)‐ 4b and (−)‐ 4c bearing ( 1 S )‐1‐phenylethyl and (1S)‐1‐(pentafluorophenyl)ethyl groups instead of the Greene's auxiliary ((1S)‐(2,4,6‐triisopropylphenyl)ethyl group). The results demonstrate that high α/β‐syn and asymmetric induction (due to the chiral auxiliary) can be obtained in the four‐component syntheses of the β‐alkoxy ketones. The method generates enantiomerically pure polyfunctional methyl sulfones bearing three chiral centers on C‐atoms and one (Z)‐alkene moiety.     

Synthesis of Amphiphilic Fullerene Derivatives and Their Incorporation in Langmuir and Langmuir‐Blodgett Films

Various amphiphilic fullerene derivatives were prepared by functionalization of [5,6]fullerene‐C₆₀‐I_h (C₆₀) with malonate or bis‐malonate derivatives obtained by esterification of the malonic acid mono‐esters 5 – 7 . Cyclopropafullerene 10 was obtained by protection of the carboxylic acid function of 6 as a tert‐butyl ester, followed by Bingel addition to C₆₀ and a deprotection step (Scheme 2). The preparation of 10 was also attempted directly from the malonic acid mono‐ester 6 under Bingel conditions. Surprisingly, the corresponding 3′‐iodo‐3′H‐cyclopropa[1,9][5,6]fullerene‐C₆₀‐I_h‐3′‐carboxylate 11 was formed instead of 10 (Scheme 3). The general character of this new reaction was confirmed by the preparation of 15 and 16 from the malonic acid mono‐esters 13 and 14 , respectively (Scheme 4). All the other amphiphilic fullerene derivatives were prepared by taking advantage of the versatile regioselective reaction developed by Diederich and co‐workers which led to macrocyclic bis‐adducts of C₆₀ by a cyclization reaction at the C‐sphere with bis‐malonate derivatives in a double Bingel cyclopropanation. The bis‐adducts 37 – 39 with a carboxylic acid polar head group and four pendant long alkyl chains of different length were prepared from diol 22 and acids 5 – 7 , respectively (Scheme 9). In addition, the amphiphilic fullerene derivatives 45, 46, 49, 54 , and 55 bearing different polar head groups and compound 19 with no polar head group were synthesized (Schemes 11–13, 15, and 5, resp.). The ability of all these compounds to form Langmuir monolayers at the air‐water interface was investigated in a systematic study. The films at the water surface were characterized by their surface pressure vs. molecular area isotherms, compression and expansion cycles, and Brewster‐angle microscopy. The spreading behavior of compound 10 was not good, the two long alkyl chains in 10 being insufficient to prevent aggregation resulting from the strong fullerene‐fullerene interactions. While no films could be obtained from compound 19 with no polar head group, all the corresponding amphiphilic fullerene bis‐adducts showed good spreading characteristics and reversible behavior upon successive compression/expansion cycles. The encapsulation of the fullerene in a cyclic addend surrounded by four long alkyl chains is, therefore, an efficient strategy to prevent the irreversible aggregation resulting from strong fullerene‐fullerene interactions usually observed for amphiphilic C₆₀ derivatives at the air‐water interface. The balance of hydrophobicity to hydrophilicity was modulated by changing the length of the surrounding alkyl chains or the nature of the polar head group. The best results in terms of film formation and stability were obtained with the compounds having the largest polar head group, i.e. 45 and 46 , and dodecyl chains. Finally, the Langmuir films obtained from the amphiphilic fullerene bis‐adducts were transferred onto solid substrates, yielding high‐quality Langmuir‐Blodgett films.