Synthesis of Linear Oligomers of (R)-3-Hydroxybutyrate and Solid-State Structural Investigations by electron microscopy and X-ray scattering

Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 10⁵ Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at ?70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15-, 30-, and 45mer. Discrete fragments are also isolated when P(3-HB) is heated under reflux (89°) in neat Et₃N. Linear oligo(3-HB) derivatives ( 3-7 ) containing up to 96 3-HB units are synthesized using an exponential segment-coupling strategy. These oligomers are used to calibrate size-exclusion chromatography columns for the analysis of oligo(3-HB) samples from the different sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron microscopy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair-pin’-type conformation. High-molecular-weight P(3-HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Å packing within the tight folds of P(3-HB) is discussed in view of its possible function in ion transport through cell membranes.     

Synthese monodisperser linearer und cyclischer Oligomere der (R)-3-Hydroxybuttersäure mit bis zu 128 Einheiten

Monodisperse Linear and Cyclic Oligo[(R)-3-hydroxybutanoates] Containing up to 128 Monomeric Units Using benzyl ester/(tert-butyl)diphenylsilyl ether protection, (COCl)₂/pyridine esterification conditions, and a fragment-coupling strategy (with H₂/Pd-C debenzylation and HF · pyridine desilylation), linear oligomers of (R)-3-hydroxybutanoic acid (3-HB) containing up to 128 3-HB building blocks (mol. weight > 11 000 Da) are assembled (Schemes 1,2,5, and 6). In contrast to the previously employed protecting-group combination, and due to the low-temperature esterifying conditions, this procedure leads to monodisperse oligomers: all steps occur without loss of single 3-HB units. The product oligomers with two, one, and no terminal protecting groups (mostly prepared in multi-gram amounts) are characterized by all standard spectroscopic methods, especially by mass spectroscopy (Figs. 2 and 3), by their optical activity, and by elemental analyses. Cyclization of the oligo[(R)-3-hydroxybutanoic acids] with up to 32 3-HB units, using thiopyridine activation and CuBr₂ for the ring closure, produces oligolides consisting of up to 128 ring atoms (Scheme 7). Mixed oligolides containing 3-HB and (R)-3-hydroxypentanoic units are prepared from the corresponding linear trimers, using Yamaguchi's method for the ring closure (Scheme 8 and Fig.4 (X-ray crystal structures of two folded conformers)). Comparisons of melting points (Table 1), of [α] values (Tables 2 and 3), of ¹H-NMR coupling constants (Table 3), and of molecular volume/hydroxyalkanoate unit (Table 4) of linear and cyclic oligomer derivatives and of the high-molecular-weigh polymer show that the monodisperse oligomers appear to be surprisingly good models for the polymer. Besides this insight, our synthesis is supplying the samples to further test the role of P(3-HB) (ca. 140 units) as a component of complexes forming channels through cell-wall phospholipid bilayers.     

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.     

Preparation,Structure, and Properties of All Possible Cyclic Dimers (Diolides) of 3-Hydroxybutanoic Acid

In connection with the proposed structure of a trans-membrane cellular ion channel consisting of a complex between poly[(R)-3-hydroxy butanoate] (P(3-HB)) and calcium polyphosphate, CaPP_i (ca. 150 units each), which is supposed to contain s-cis-bonds or even more highly strained ester conformations, we have prepared and studied the properties of the cyclic dimer of 3-HB, the diolide 1 . All possible forms of 1 , the rac-, the meso-, and the enantiomerically pure (R,R)- and (S,S)-compounds were prepared, purified, and characterized. The synthesis (Scheme 1) started from dimethyl succinate with the key step being the Baeyer-Villiger oxidation of the rac- and meso-2,5-dimethylcyclohexane-1,4-diones 5 . The rac-diolide 1 was resolved by preparative chromatography on a Chiralcel OD column (Fig.1). The crystal structures of rac- 1 (Fig.3) and of meso- 1 (Fig.5) were determined by X-ray diffraction: the diolides 1 contain s-cis-ester bonds and an ester group with a conformation half way to the transition state of rotation (Fig.2). Strain energies for the diolides 1 of up to 17.8 kcal/mol are suggested. Accordingly, these compounds show reactivities similar to those of carboxylic-acid anhydrides or even acid chlorides. They cannot be chromatographed on silica gel, and they react with primary, secondary, and tertiary alcohols, and with amines to form derivatives of open chain 3-HB ‘dimers’, hydroxy acids 6 , esters 7 , and amides 8 (Scheme 2). The rate of acid-catalyzed ring opening of the diolides 1 with alcohols has been measured (Fig.6 and 7). From the results described, we conclude that it is unlikely for strained and reactive ester conformations to occur as part of ion channels through phospholipid bilayers of cells.     

Molecular Recognition of Pyranosides by a Family of Trimeric, 1,1′-Binaphthalene-Derived Cyclophane Receptors

The synthesis and carbohydrate-recognition properties of a new family of optically active cyclophane receptors, 1 – 3 , in which three 1,1′-binaphthalene-2,2′-diol spacers are interconnected by three buta-1,3-diynediyl linkers, are described. The macrocycles all contain highly preorganized cavities lined with six convergent OH groups for H-bonding and complementary in size and shape to monosaccharides. Compounds 1 – 3 differ by the functionality attached to the major groove of the 1,1′-binaphthalene-2,2′-diol spacers. The major grooves of the spacers in 2 are unsubstituted, whereas those in 1 bear benzyloxy (BnO) groups in the 7,7′-positions and those in 3 2-phenylethyl groups in the 6,6′-positions. The preparation of the more planar, D₃-symmetrical receptors (R,R,R)- 1 (Schemes 1 and 2), (S,S,S)- 1 (Scheme 4), (S,S,S)- 2 (Scheme 5), and (S,S,S)- 3 (Scheme 8) involved as key step the Glaser-Hay cyclotrimerization of the corresponding OH-protected 3,3′-diethynyl-1,1′-binaphthalene-2,2′-diol precursors, which yielded tetrameric and pentameric macrocycles in addition to the desired trimeric compounds. The synthesis of the less planar, C₂-symmetrical receptors (R,R,S)- 2 (Scheme 6) and (S,S,R)- 3 (Scheme 9) proceeded via two Glaser-Hay coupling steps. First, two monomeric precursors of identical configuration were oxidatively coupled to give a dimeric intermediate which was then subjected to macrocyclization with a third monomeric 1,1′-binaphthalene precursor of opposite configuration. The 3,3′-dialkynylation of the OH-protected 1,1′-binaphthalene-2,2′-diol precursors for the macrocyclizations was either performed by Stille (Scheme 1) or by Sonogashira (Schemes 4, 5, and 8) cross-coupling reactions. The flat D₃-symmetrical receptors (R,R,R)- 1 and (S,S,S)- 1 formed 1 : 1 cavity inclusion complexes with octyl 1-O-pyranosides in CDCl₃ (300 K) with moderate stability (ΔG⁰ ca. −3 kcal mol⁻¹) as well as moderate diastereo- (Δ(ΔG⁰) up to 0.7 kcal mol⁻¹) and enantioselectivity (Δ(ΔG⁰)=0.4 kcal mol⁻¹) (Table 1). Stoichiometric 1 : 1 complexation by (S,S,S)- 2 and (S,S,S)- 3 could not be investigated by ¹H-NMR binding titrations, due to very strong signal broadening. This broadening of the ¹H-NMR resonances is presumably indicative of higher-order associations, in which the planar macrocycles sandwich the carbohydrate guests. The less planar C₂-symmetrical receptor (S,S,R)- 3 formed stable 1 : 1 complexes with binding free enthalpies of up to ΔG⁰=−5.0 kcal mol⁻¹ (Table 2). With diastereoselectivities up to Δ(ΔG⁰)=1.3 kcal mol⁻¹ and enantioselectivities of Δ(ΔG⁰)=0.9 kcal mol⁻¹, (S,S,R)- 3 is among the most selective artificial carbohydrate receptors known.     

Synthesis and Characterization of Chiral Bis(aminato)iridate(I) and Aminato‐Bridged Iridium(I) Complexes

The two dinuclear Ir^I complexes [Ir₂(μ‐Cl)₂ {(R)‐(S)‐PPF‐PPh₂}₂] ( 1 ; (R)‐(S)‐PPF‐PPh₂=(S)‐1‐(diphenylphosphino)‐2‐[(R)‐1‐(diphenylphosphino)ethyl]ferrocene and [Ir₂(μ‐Cl)₂{(R)‐binap}₂] ( 3 ; (R)‐binap=(R)‐[1,1′‐binaphthalene]‐2,2′‐diylbis[diphenylphosphine]) smoothly react with 4 equiv. of the lithium salt of aniline to afford the new bis(anilido)iridate(I) (=bis(benzenaminato)iridate(1‐)) complexes Li[Ir(NHPh)₂{(R)‐(S)‐PPF‐PPh₂}] ( 4 ) and Li[Ir(NHPh)₂{(R)‐binap}] ( 5 ), respectively. The anionic complexes 4 and 5 react upon protonolysis to give the dinuclear aminato‐bridged derivatives [Ir₂(μ‐NHPh)₂{(R)‐(S)‐PPF‐PPh₂}₂] ( 6 ) and [Ir₂(μ‐NHPh)₂{(R)‐binap}₂] ( 7 ), which were characterized by X‐ray crystallography. None of the new complexes 4 – 7 shows catalytic activity in the hydroamination of olefins.     

An Efficient Synthesis of Optically Pure (R)- and (S)-2-(aminomethyl)alanine ((R)- and (S)-ama) and (R)- and (S)-2-(aminomethyl)leucine ((R)- and (S)-aml)

An efficient synthesis of enantiomerically pure (R)- and (S)-2-(aminomethyl)alanine ((R)- and (S)-Ama) 1a and (R)- and (S)-2-(aminomethyl)leucine ((R)- and (S)-Aml) 1b is described (Schemes 1 and 2). Resolution of the racemic amino acids was achieved using L -phenylalanine cyclohexylamide ( 2 ) as chiral auxiliary. The free amino acids 1a, b were converted to the N^α-Boc,N^γ-Z-protected derivatives 11a, b (Scheme 3) ready for incorporation into peptides. Based on the three crystal structures of the diastereoisomeric peptides 8a, 8b , and 9b , the absolute configurations in both series were determined. β-Turn type-I geometries were observed for structures 8b and 9b , whereas 8a crystallized in an extended backbone conformation.     

Synthesis of Amino-Acid-, Carbohydrate-, Coumarin-, and Biotin-Labelled Oligo[(R)-3-Hydroxybutanoic Acids] (OHB)

The benzyl esters 1 – 3 of oligo[(R)-3-hydroxybutanoic acids] (OHB) containing 2, 16, or 32 HB units were coupled at the hydroxy terminus with arginine (by esterification with carbodiimide), with glucose (by acetalization with glucosyl trichloroacetimidate), and with 7-(dimethylamino)coumarin-4-acetic acid and biotin (by amide formation through a glycine linker) to give, after deprotection(s), the corresponding `labelled' OHB acids 7 – 9 , 12 , 13 , 25 , 26 , 33 , and 34 (Schemes 1, 4, and 5). The respective novel 16- and 32mer derivatives exhibit distinct water solubility (Table) or may be detected (in minute amounts) by fluorescence spectroscopy, properties required for biochemical investigations.     

Zur Konfiguration des Vitamin-D3-Metaboliten 25, 26-Dihydroxycholecalciferol: Synthese von (25S,26)- und (25R,26)-Dihydroxycholecalciferol

Configuration of the Vitamin-D₃-Metabolite 25,26-Dihydroxycholecalciferol: Synthesis of (25S,26)- and (25R,26)-Dihydroxycholecalciferol For selective synthesis of the title compounds, (25S)- 1b and (25R)- 1b (Scheme 1), the protected cholesterol precursors (25S)- 6 and (25R)- 6 were prepared from stigmasterol-derived steroid-units 4a-d and C₅-side chain building blocks 5a–d by Grignard- or Wittig-coupling (Scheme 2), the configuration at C(25) of the target compounds being already present in the C₅-units. Conversion of the cholesterol intermediates to the corresponding vitamin-D₃ derivatives was carried out via the 7,8-didehydrocholesterol compounds (25S)- 2b and (25R)- 2b (Scheme 1), using the established photochemical-thermal transformation of the 5,7-diene system to the seco-triene system of cholecalciferol. The configuration at C(25) of the cholesterol precursors as assigned on basis of the known configuration of the C₅-units used, was found to be in agreement with the result of a single crystal X-ray analysis on compound 11 . The configuration at C(25) remained untouched on conversion of the cholesterol ring system to the seco-triene system of vitamin D₃ as evident from comparison of the lanthanide-induced CD. Cotton effects observed for (25S)- 3b and (25S) 1b . 25,26-Dihydroxycholecalciferol observed as a natural vitamin-D₃ metabolite has (25S)-configuration.     

Synthesis of Monodisperse Macromolecular Bicyclic and dendritic compounds from (R)-3-hydroxybutanoic acid and benzene-1,3,5-tricarboxylic acid and analysis by fragmenting MALDI-TOF mass spectroscopy

As previously shown, oligo- and poly(β-hydroxyalkanoates) have a high tendency to form lamellar crystallites with ca. 50-Å thickness which corresponds to chain lengths of 16 units (Fig. 1). To have monodisperse model compounds, we have now prepared bicyclic derivatives with three parallel ( 27 – 29 ) or two parallel and an antiparallel chain ( 68 – 70 ) consisting of up to 16 3-hydroxybutanoate (3-HB) units. We also prepared dendritic compounds ( 71 – 75 , 82 – 85 ) containing oligo(3-HB) chains which cannot possibly be arranged as in the lamellae; the branching units were prepared from trimesic acid (= benzene-1,3,5-tricarboxylic acid). So far, none of the prepared samples formed crystals or contained crystalline domains which would have been suitable for single-crystal or powder-diffraction X-ray analysis. The terminally deprotected dendrimers ( 74 , 75 , and 85 ) are multi-anionic (up to 12 peripheral CO₂H groups) and biodegradable. The macromolecular HB derivatives (molecular weight up to 10150 Da) have been fully characterized by IR, ¹H- and ¹³C-NMR, [α]_D, and elemental analysis. Especially important is the analysis by mass spectrometry with the MALDI-TOF technique (Fig. 2), proving that the products are monodisperse; application of a new variation of this MS method (post source decay = PSD or fragment analysis by structural time of flight = FAST^TM) allows for the observation of metastable fragment ions and, thus, is a tool for structural oligomer analysis (Fig. 3).     

Triple Helicates with Golden Strands: Self‐Assembly of M2Au6 Complexes from Gold(I) Metallaligands and Iron(II), Cobalt(II) or Zinc(II) Cations

Alkynyl gold(I) metallaligands [(AuC≡Cbpyl)₂(μ‐diphosphine)] (bpyl=2,2′‐bipyridin‐5‐yl; diphosphine=Ph₂P(CH₂)_nPPh₂, [n=3 (L^Pr), 4 (L^Bu), 5 (L^Pent), 6 (L^Hex)], dppf (L^Fc), Binap (L^Binap) and Diop (L^Diop)) react with MX₂ (M=Fe, Zn, X=ClO₄; M=Co, X=BF₄) to give triple helicates [M₂(L^R)₃]X₄. These complexes, except those containing the semirigid L^Binap metallaligand, present similar hydrodynamic radii (determined by diffusion NMR spectroscopy measurements) and a similar pattern in the aromatic region of their ¹H NMR spectra, which suggests that in solution they adopt a compact structure where the long and flexible organometallic strands are folded. The diastereoselectivity of the self‐assembly process was studied by using chiral metallaligands, and the absolute configuration of the iron(II) complexes with L^Binap and L^Diop was determined by circular dichroism spectroscopy (CD). Thus, (R)‐L^Binap or (S)‐L^Binap specifically induce the formation of (Δ,Δ)‐[Fe₂((R)‐L^Binap)₃](ClO₄)₄ or (Λ,Λ)‐[Fe₂((S)‐L^Binap)₃](ClO₄)₄, respectively, whereas (R,R)‐ or (S,S)‐L^Diop give mixtures of the ΔΔ‐ and ΛΛ‐diastereomers. The ΔΔ helicate diastereomer is dominant in the reaction of Fe^II with (R,R)‐L^Diop, whereas the ΛΛ isomer predominates in the analogous reaction with (S,S)‐L^Diop. The photophysical properties of the new dinuclear alkynyl complexes and the helicates have been studied. The new metallaligands and the [Zn₂(L^R)₃]⁴⁺ helicates present luminescence from [π→π*] excited states mainly located in the C≡Cbpyl units.     

Chiral [2H]-Labelled Methylene Groups in Trienoic- and Dienoic Fatty Acids: A Facile Approach via Asymmetric Epoxidation of [2H] Allyl Alcohols

Chiral [²H] -labelled methylene groups flanked by two double bonds within (poly)unsaturated fatty acids are readily available from trans-2,3-epoxy[2,3-²H₂] alk-4-yn-l-ols, obtained in their turn by asymmetric epoxidation of the corresponding (E)-[2,3-²H₂] alk-2-en-4-yn-l-ols (see Scheme 3). The procedure is exemplified for (8S,3Z,6Z,9Z)-[7,8-²H₂] trideca-3,6,9-trienoic acid ((8S)- 11 ) and (8R)- 11 (Scheme 4) as well as for (5S,3Z,6Z)-[4,5^?2H₂]deca-3,6-dienoic acid ((5S)- 13 ) and (5R)- 13 (Scheme 5).     

Solid-State CP/MAS 13C-NMR Spectra of Oligolides derived from 3-hydroxybutanoic acid

The solid-state CP/MAS ¹³C-NMR spectra (cross-polarization/magic-angle spinning ¹³C-NMR) of eight lower cyclic and one linear oligomers and several polymers of (R)-3-hydroxybutanoic acid (3-HB) are reported. The polymeric samples of different origin and molecular weight give remarkably similar and well resolved spectra, indicating considerable similarity in the conformations of the molecules and homegeneity in the solid-state environment. The crystalline cyclic oligomers 1 – 8 containing 3–9 units of 3-HB give very well resolved spectra. The number of nonequivalent positions in the solid state can be identified and is in accord with structures from X-ray diffraction where these were determined. The spectra of the oligolides become increasingly similar to those of the polymer as the ring size increases. This spectral evidence supports the view of a homogeneous and well defined conformation for the polymeric material (as proposed previously, based on other experiments).     

Synthesis and Characterization of Ruthenium Complexes with Substituted Pyrazino[2,3‐f][1,10]‐phenanthroline (=Rppl; R=Me,COOH, COOMe)

A series of substituted pyrazino[2,3‐f][1,10]‐phenanthroline (Rppl) ligands (with R=Me, COOH, COOMe) were synthetized (see 1 – 4 in Scheme 1). The ligands can be visualized as formed by a bipyridine and a quinoxaline fragment (see A and B ). Homoleptic [Ru(R¹ppl)₃](PF₆)₂ and heteropleptic [Ru(R¹ppl){(R²)₂bpy}₂](PF₆)₂ (R¹=H, Me, COOMe and R²=H, Me) metal complexes 5 – 7 and 8 – 13 , respectively, based on these ligands were also synthesized and characterized by conventional techniques (Schemes 2 and 3, resp.). In the heteroleptic complexes, the R¹‐ppl ligand reduces at a less‐negative potential than the bpy ligand, reflecting the acceptor property conferred by the quinoxaline moiety. The potentiality of some of these complexes as solar‐cell dyes is discussed.     

Synthesis of the Oviposition-Deterring Pheromone (ODP) in Rhagoletis cerasi L. Preliminary communication

For the assignment of the configuration at C(8) and C(15) of the natural oviposition-deterring pheromone 1 in Rhagoletis cerasi L., the four possible stereoisomers of 1 are synthesized. By condensing the C₆ building blocks (5R)- 4 and (5S)- 4 with the boron enolates of the C₁₀ building blocks (4S)- 13 and (4R)- 13 , followed by decarboxylative dehydration, all stereoisomers of 16 are available (Scheme 5). Glucosylation of 16 followed by formation of the taurin amide gives, after deprotection, the four stereoisomers (8R,15S)- 1 , (8R,15R)- 1 , (8R,15S) -1 , and (8S,15S)- 1 (Scheme 6).     

Intramolecular Asymmetric Amidations of Sulfonamides and Sulfamates Catalyzed by Chiral Dirhodium(II) Complexes

Enantioselective intramolecular amidation of aliphatic sulfonamides was achieved for the first time by means of chiral carboxylatodirhodium(II) catalysts in conjunction with PhI(OAc)₂ and MgO in high yields and with enantioselectivities of up to 66% (Scheme 3, Table 1). The best results were obtained with [Rh₂{(S)‐nttl)₄] and [Rh₂{(R)‐ntv)₄] as catalysts ((S)‐nttl=(αS)‐α‐(tert‐butyl)‐1,3‐dioxo‐2H‐benz[de]isoquinoline‐2‐acetato, (R)‐nto=(αR)‐α‐isopropyl‐1,3‐dioxo‐2H‐benz[de] isoquinoline‐2‐acetato). In addition, these carboxylatodirhodium(II) catalysts were also efficient in intramolecular amidations of aliphatic sulfamates esters, although the enantioselectivity of these latter reactions was significantly lower (Scheme 4, Table 3).     

On the Solution Structure of PHB: Preparation and NMR Analysis of Isotopically Labeled Oligo[(R)‐3‐hydroxybutanoic Acids] (OHBs)

While the chain conformation of poly‐ and oligo[(R)‐3‐hydroxybutanoate] (PHB, OHB) is known to be 2₁‐ and 3₁‐helical in stretched fibers and in the crystalline state, respectively (Fig. 2), the structure in solution is unknown. To be able to determine the NMR‐solution structure, specifically labeled linear oligomers have been prepared: a 16‐mer consisting of alternating pairs of fully ¹³C‐labeled and non‐labeled residues ( 1 ) and a 20‐mer containing an O‐¹³CH(¹³CH₂D)‐¹³CHD^Si‐¹³CO residue in position 9 (from the O‐terminus) and a fully ¹³C‐labeled residue in position 12 ( 2 ), both with (t‐Bu)Ph₂Si protection at the O‐ and Bn protection at the C‐terminus. The labeled (R)‐3‐hydroxybutanoic acid building blocks were prepared by Noyori hydrogenation of the ethyl ester of fully ¹³C‐labeled acetoacetic acid, and the D‐atoms were incorporated by D₂/Pd‐C reduction of a previously reported dibromo‐1,3‐dioxinone 8 (Scheme 1). The oligomers were obtained by a series of fragment couplings (Schemes 2 and 3). 600‐MHz NMR COSY, HSQC, ROESY, and cross‐correlated relaxation measurements (Figs. 4 – 6, 9, and 12, and Tables 1 – 3) at different temperatures and interpretations thereof led to assignments of all resonances, including those from the diastereotopic C(2)H₂ protons, and to determination of the conformationally averaged dihedral angles ϕ₂ and ϕ₃ (Figs. 2, 7, and 8) in the chain of the oligoester. The conclusions are: all but five or six terminal residues adopt the same conformation; the 2₁ helix is not the predominant secondary structure; the structure of the HB chain is averaged, even at –30°. Our investigation confirms the high flexibility of the polyester chain, a property that has been deduced previously from biological studies of PHB in membranes, in ion channels, and as appendage of proteins.     

Stereocontrolled Synthesis of (2R,3S)‐2‐Methylisocitrate,a Central Intermediate in the Methylcitrate Cycle

2‐Methylisocitrate (=3‐hydroxybutane‐1,2,3‐tricarboxylic acid) is an intermediate in the oxidation of propanoate to pyruvate (=2‐oxopropanoate) via the methylcitrate cycle in both bacteria and fungi (Scheme 1). Stereocontrolled syntheses of (2R,3S)‐ and (2S,3R)‐2‐methylisocitrate (98% e.e.) were achieved starting from (R)‐ and (S)‐lactic acid (=(2R)‐ and (2S)‐2‐hydroxypropanoic acid), respectively. The dispiroketal (6S,7S,15R)‐15‐methyl‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadecan‐14‐one ( 2a ) derived from (R)‐lactic acid was deprotonated with lithium diisopropylamide to give a carbanion that was condensed with diethyl fumarate (Scheme 3). The configuration of the adduct diethyl (2S)‐2‐[(6S,7S,14R)‐14‐methyl‐15‐oxo‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadec‐14‐yl]butanedioate ( 3a ) was assigned by consideration of possible transition states for the fumarate condensation (cf. Scheme 2), and this was confirmed by a crystal‐structure analysis. The adduct was subjected to acid hydrolysis to afford the lactone 4a of (2R,3S)‐2‐methylisocitrate and hence (2R,3S)‐2‐methylisocitrate. Similarly, (S)‐lactic acid led to (2S,3R)‐2‐methylisocitrate. Comparison of 2‐methylisocitrate produced enzymatically with the synthetic enantiomers established that the biologically active isomer is (2R,3S)‐2‐methylisocitrate.     

Synthesis of the Enantiomerically Enriched Macrocyclic Lactones (+)-(S)- and (−)-(R)-Phoracantholide I and (+)-(S)-Tetradecan-13-olide

Synthesis of two naturally occurring macrocyclic lactones is described. (?)-(R)-Phoracantholide I ((?)- 1 ; Scheme 2) was synthesized by asymmetric and chemoselective reduction of the side-chain C?O group of (?)4-(1-nitro-2-oxocyclohexyl)butan-2-one ((?)- 6 ) with (R)-Alpine-Hydride (47% ee). It was shown that the formation of only one diastereoisomer of the hemiacetal 5 , by methylation with (i-PrO)₂TiMe₂ of ketoaldehyde (?)- 2 is thermodynamically controlled. (+)-(S)-Tetradecan-13-olide ((+)- 10 ) was obtained by reduction of diketone (±)- 11 with optically active borohydrides followed by denitration (Scheme 3).     

Synthesis of New Chiral Bidentate (Phosphinophenyl)benzoxazine P,N-Ligands

Two new chiral bidentate (phosphinophenyl)benzoxazine P,N-ligands 2a and 2b were synthesized from highly enantiomer-enriched 2-(1-aminoalkyl)phenols 4 . Ligand rac- 2a was obtained on refluxing the t-Bu-substituted (aminomethyl)phenol 4a with 2-(diphenylphosphino)benzonitrile in chlorobenzene in the presence of anhydrous ZnCl₂ followed by decomplexation (Scheme 2). This reaction, when carried out with (+)-(S)- 4a , was accompanied by racemization at the stereogenic center of the alkyl side chain. The enantiomerically pure ligands (+)-(R)- 2a and (−)-(S)- 2a were obtained using a stepwise procedure via the amides (−)-(R)- and (+)-(S)- 5b , respectively, followed by cyclization to benzoxazines (+)-(R)- and (−)-(S)- 7b , respectively, with triflic anhydride and by F-atom substitution by diphenylphosphide (Schemes 3 and 5). In the case of the i-Pr analogue 2b , this last step resulted in racemization (Scheme 6). This was overcome by preparing the bromo derivative and introducing the diphenylphosphine group via Br/Li exchange and reaction with chlorodiphenylphosphine (Scheme 7). The first application of (+)-(R)- 2a in an asymmetric Heck reaction showed high enantioselectivity (91%) (Scheme 8).