Solid-phase synthesis and stereochemical assignments of tenuecyclamides A-D employing heterocyclic amino acids derived from commercially available Fmoc alpha-amino acids

[reaction: see text] The solid-phase assembly of heterocyclic amino acids enabled the total synthesis of numerous diastereoisomers of tenuecyclamides A-D, establishing or correcting the stereochemistry of each natural product. This strategy provides a very efficient route to synthesize thiazole- and oxazole-containing macrolactams from heterocyclic amino acids that are readily prepared from Fmoc-alpha-amino acids. This methodology appears to be broadly applicable to the synthesis of natural product libraries incorporating unnatural heterocyclic amino acid residues for the purpose of drug discovery.     

Synthesis and Conformational Preferences of a Potential beta-Sheet Nucleator Based on the 9,9-Dimethylxanthene Skeleton

A 4,5-disubstituted-9,9-dimethylxanthene-based amino acid (10) has been synthesized for incorporation into peptide sequences which have a propensity to adopt beta-sheet structure. Molecular dynamics studies support the FT-IR and NMR results which demonstrate that amides based on this residue utilize the NH and the C=O from the xanthene residue to form an intramolecular hydrogen bond (13-membered ring), unlike the previously studied dibenzofuran-based amino acid residues in which the NH and the C=O of the attached amide groups participate in intramolecular hydrogen bonding (15-membered ring). Interestingly, residue 10 derivatized as a simple amide prefers to adopt a trans conformation where the aliphatic side chains are placed on opposite sides of the plane of the 9,9-dimethylxanthene ring system. This is different than the conformational preferences of the dibenzofuran-based amino acids which adopt a cis conformation that is preorganized to nucleate beta-sheet formation. It will be interesting to see how these conformational differences effect nucleation in aqueous solution.     

Solid-phase synthesis of an A-B loop mimetic of the Cepsilon3 domain of human IgE: macrocyclization by Sonogashira coupling

The solid-phase synthesis of a cyclic peptide containing the 21-residue epitope found in the A-B loop of the Cepsilon3 domain of human immunoglobulin E has been carried out. The key macrocyclization step to form the 65-membered ring is achieved in approximately 15% yield via an "on-resin" Sonogashira coupling reaction which concomitantly installs a diphenylacetylene amino acid conformational constraint within the loop.     

Enantioselective total synthesis of clavirolide C. Applications of Cu-catalyzed asymmetric conjugate additions and Ru-catalyzed ring-closing metathesis

The first enantioselective total synthesis of clavirolide C, a member of the dolabellane family of diterpenes isolated from Pacific soft coral Clavularia viridis, is disclosed. The total synthesis features the application of chiral amino acid based ligands in Cu-catalyzed asymmetric conjugate addition (ACA) reactions and a relatively rare application of catalytic ring-closing metathesis to access an 11-membered ring structure. The total synthesis effort has spawned the development of a new protocol for NHC.Cu-catalyzed ACA of alkylaluminum reagents to beta-substituted cycloalkenones. The enantioselective clavirolide C synthesis requires 17 steps (longest linear sequence), affords the target molecule in 3.5% overall yield, and confirms the stereochemical assignment for the natural product.     

Synthesis of cyclic peptidomimetics from aldol building blocks

Aldol products (3-hydroxy acids) with an allyl-protected hydroxy group were converted to amino alcohols by Curtius rearrangement. Combination of the carboxylic acid with the amino alcohols gave the amides 10. Ring-closing metathesis led to the 12-membered lactams 12 as mixtures of E/Z-isomers. The scheme was also transferred to the solid-phase. In this case the macrolactams are formed via cyclorelease. For a pair of E/Z-isomers the solution conformation was determined by ROESY spectroscopy.     

1′-Acetylferrocene amino acid esters and amides. A simple model for parallel β-helical peptides

In this study we present a synthesis and conformational analysis of 1′-acetylferrocene amino acid derivatives of type Ac–Fn–CO–AA–Y (Fn=ferrocene-1,1-diyl; AA=Gly, Ala or Val; Y=OMe or NHMe) as a simple model for parallel β-helical peptides. Derivatives with only one amino acid adopt a reduced number of total conformations and allow a more exact analysis of intramolecular hydrogen bonds (IHB) close to the ferrocene unit. Conformational analysis of these bioconjugates was performed by a combination of spectroscopic techniques (IR, NMR and CD) and corroborated by solution-phase DFT calculations. The investigation of ester conjugates 1–3 indicates the coexistence of non-bonded (an open forms) and hydrogen bonded NH_a group forming a 7-membered ring (γ-turn). The amide derivatives 4–6 with an additional NH_b hydrogen bond donor are mostly constituted of conformers with a 10-membered ring (β-turn) as a single IHB pattern or the β-turn accompanied by a 7-membered ring (γ-turn) containing NH_a group. The exchange of the amino acid side-chain does not significantly affect the conformational properties and IHB pattern of the studied conjugates 1–6.     

Total synthesis of the ristocetin aglycon

The first total synthesis of the ristocetin aglycon is described employing a modular and highly convergent strategy. An effective 12-step (12% overall) synthesis of the ABCD ring system 3 from its amino acid subunits sequentially features an intramolecular aromatic nucleophilic substitution reaction for formation of the diaryl ether and closure of the 16-membered CD ring system (65%), a respectively diastereoselective (3:1, 86%) Suzuki coupling for installation of the AB biaryl linkage on which the atropisomer stereochemistry can be further thermally adjusted, and an effective macrolactamization (51%) for closure of the 12-membered AB ring system. A similarly effective 13-step (14% overall) synthesis of the 14-membered EFG ring system 4 was implemented employing a room-temperature intermolecular S(N)Ar reaction of an o-fluoronitroaromatic for formation of the FG diaryl ether (69%) and a key macrolactamization (92%) with formation of the amide linking residues 1 and 2. The two key fragments 3 and 4 were coupled, and the remaining 16-membered DE ring system was closed via diaryl ether formation to provide the ristocetin tetracyclic ring system (15 steps, 8% overall) enlisting an unusually facile (25 degrees C, 8 h, DMF, >/=95%) and diastereoselective (>/=15:1) aromatic nucleophilic substitution reaction that benefits from substrate preorganization.     

A reverse turn structure induced by a D,L-alpha-aminoxy acid dimer

Our previous work revealed that two adjacent D-alpha-aminoxy acids could form two homochiral N-O turns, with the backbone folding into an extended helical structure (1.8(8)-helix). Here, we report the conformational studies of linear peptides 3-6, which contain a D,L-alpha-aminoxy acid dimer segment. The NMR and X-ray analysis of 3 showed that it folded into a loop conformation with two heterochiral N-O turns. This loop segment can be used to constrain tetrapeptides 4 and 6 to form a reverse turn structure. (1)H NMR dilution studies, DMSO-d6 addition studies, and 2D-NOESY data indicated that tetrapeptides 4 and 6 folded into reverse turn conformations featured by a head-to-tail 16-membered-ring intramolecular hydrogen bond. In contrast, tetrapeptide 5 with L-Ala instead of Gly or D-Ala as the N-terminal amino acid could not form the desired reverse turn structure for steric reasons. Quantum mechanics calculations showed that model pentamide 7, with the same substitution pattern of 4, adopted a novel reverse turn conformation featuring two heterochiral N-O turns (each of an 8-membered ring hydrogen bond), a cross-strand 16-membered ring hydrogen bond, and a 7-membered ring gamma-turn.     

First and second generation total synthesis of the teicoplanin aglycon.

Full details of studies leading to the total synthesis of the teicoplanin aglycon are provided. Key elements of the first generation approach (26 steps from constituent amino acids, 1% overall) include the coupling of an EFG tripeptide precursor to the common vancomycin/teicoplanin ABCD ring system and sequential DE macrocyclization of the 16-membered ring with formation of the diaryl ether via a phenoxide nucleophilic aromatic substitution of an o-fluoronitroaromatic (80%, 3:1 atropisomer diastereoselection) followed by 14-membered FG ring closure by macrolactamization (66%). Subsequent studies have provided a second generation total synthesis which is shorter, more convergent, and highly diastereoselective (22 steps, 2% overall). This was accomplished by altering the order of ring closures such that FG macrolactamization (95%) preceded coupling of the EFG tripeptide to the ABCD ring system and subsequent DE ring closure. Notably, DE macrocyclization via diaryl ether formation on substrate 57, the key intermediate in the latter approach incorporating the intact FG ring system, occurred with exceptional diastereoselection for formation of the natural atropisomer (>10:1, 76%) without problematic C(2)(3) epimerization provided the basicity of the reaction is minimized.     

Diversity-oriented synthesis of 13- to 18-membered macrolactams via ring-closing metathesis

An efficient build/couple/pair approach to diversity-oriented synthesis was employed to access several structurally complex macrolactams. In this paper, we describe the successful evaluation of ring-closing metathesis toward the systematic generation of skeletal diversity. By appropriately varying the nature and chain length of the alkenol fragment, a diverse collection of 13- to 18-membered macrolactams were obtained.     

Alpha,alpha-disubstituted glycines bearing a large hydrocarbon ring: peptide self-assembly through hydrophobic recognition

A method was developed for synthesizing alpha,alpha-disubstituted glycine residues bearing a large (more than 15-membered) hydrophobic ring. The ring-closing metathesis reactions of the dialkenylated malonate precursors proceed efficiently, particularly when long methylene chains tether both terminal olefin groups. Surprisingly, the amino groups of these alpha,alpha-disubstituted glycines are inert to conventional protective reactions (e.g., N-tert-butoxycarbonyl (Boc) protection: Boc(2)O/4-dimethylaminopyridine (DMAP)/CH(2)Cl(2); N-benzyloxycarbonyl (Z) protection: Z-Cl/DMAP/CH(2)Cl(2)). Curtius rearrangement of the carboxylic acid functionality of the malonate derivative after ring-closing metathesis leads to formation of an amine functionality and can be catalyzed by diphenylphosphoryl azide. However, only the intermediate isocyanates can be isolated, even in the presence of alcohols such as benzyl alcohol. The isocyanates obtained by Curtius rearrangement in an aprotic solvent (benzene) were isolated in high yields and treated with 9-fluorenylmethanol in a high-boiling-point solvent (toluene) under reflux to give the N-9-fluorenylmethoxycarbonyl (Fmoc)-protected aminomalonate derivatives in high yield. These hydrophobic amino acids can be incorporated into a peptide by Fmoc solid-phase peptide synthesis and the acid fluoride activation method. The stability of the monomeric alpha-helical structure of a 17-amino-acid peptide was enhanced by replacement of two alanine residues with two hydrophobic amino acid residues bearing a cyclic 18-membered ring. The results of sedimentation equilibrium studies suggested that the peptide assembles into hexamers in the presence of 100 mM NaCl.     

Synthesis of a novel 14-membered highly constrained cyclic peptidic scaffold

The synthesis and NMR analysis of a novel highly constrained scaffold is described. The 14-membered macrocyclic ring structure was inspired by many medicinally relevant natural products that also contain the bi-aryl ether moiety. The synthesis required only commercially available starting materials and involved a base mediated S_NAr cyclization. A conformational search was performed, which indicated a strong preference for a single conformation, which was consistent with observed ROE signals by NMR.     

Anion and carboxylic acid binding to monotopic and ditopic amidopyridine macrocycles

Binding of inorganic anions, carboxylic acids, and tetraalkylammonium carboxylates by macrocyclic compounds of different size was studied by NMR in DMSO-d6. It has been shown that at least a 15-membered ring is necessary for successful recognition of fluoride. Larger macrocycles were shown to bind HSO4(-), H2PO4(-), Cl(-), and carboxylic acid salts. Effects of binding topicity are discussed. The 30-membered macrocycles 4 and 4m selectively bind substrates that are size- and shape-complementary: maximum binding is observed for dicarboxylic acids and dicarboxylates with four-carbon chains, and the binding constant for association of fumaric acid and 4 is ca. 5 orders of magnitude higher than that of maleic acid. The 30-membered macrocycle 4m showed selectivity toward alpha-ketocarboxylic acids. Secondary amino groups were not crucial for binding of fluoride to the macrocycles; however, they proved to be very important for selectivity and strength of carboxylic acid binding. The X-ray structure of the adduct of 4 and nitrobenzoic acid confirmed the guest H-bonding with both the amide and the secondary amino groups of the 30-membered macrocyclic host.     

Synthetic studies on stevastelins. 1. Total synthesis of stevastelins B and B3

[Chemical reaction: See text] The synthesis of stevastelin B3 (2) and B (5) are described. In a first approach, epoxy cyclodepsipeptide 8 was considered as a promising candidate for the synthesis of the [15]-membered ring members of the stevastelins; however, the oxirane ring opening, required for the completion of the natural stevastelin synthesis, failed. Thus, we synthesized stevastelin B (5), carrying out the oxirane ring opening earlier in the synthesis and following a synthetic scheme capable of delivering analogues. On the other hand, a translactonization reaction of the [15]-membered ring derivative 59 led to the total synthesis of the natural [13]-membered ring component of the stevastelins family, stevastelin B3 (2).     

The journey of total synthesis toward nannocystin Ax

Herein we describe the detail on our full investigations that led to the achievement of the total synthesis of nannocystin Ax, a 21-membered macrocyclic natural product composing of a tripeptide fragment and a polyketide fragment, which featured in 8 longest linear steps in with 13.9% total overall yield. The key synthetic strategy relied on the late-stage stille coupling for the macrolactonization to construct the 21-membered ring, while direct connection between the tripeptide fragment and the polyketide fragment failed. ¹H NMR experiments reveal that nannocystin Ax should exist as conformational mixtures in deuterated solvents.     

Broad substrate specificity of the amide synthase in S. hygroscopicus--new 20-membered macrolactones derived from geldanamycin

The amide synthase of the geldanamycin producer, Streptomyces hygroscopicus, shows a broader chemoselectivity than the corresponding amide synthase present in Actinosynnema pretiosum, the producer of the highly cytotoxic ansamycin antibiotics, the ansamitocins. This was demonstrated when blocked mutants of both strains incapable of biosynthesizing 3-amino-5-hydroxybenzoic acid (AHBA), the polyketide synthase starter unit of both natural products, were supplemented with 3-amino-5-hydroxymethylbenzoic acid instead. Unlike the ansamitocin producer A. pretiosum, S. hygroscopicus processed this modified starter unit not only to the expected 19-membered macrolactams but also to ring enlarged 20-membered macrolactones. The former mutaproducts revealed the sequence of transformations catalyzed by the post-PKS tailoring enzymes in geldanamycin biosynthesis. The unprecedented formation of the macrolactones together with molecular modeling studies shed light on the mode of action of the amide synthase responsible for macrocyclization. Obviously, the 3-hydroxymethyl substituent shows similar reactivity and accessibility toward C-1 of the seco-acid as the arylamino group, while phenolic hydroxyl groups lack this propensity to act as nucleophiles in the macrocyclization. The promiscuity of the amide synthase of S. hygroscopicus was further demonstrated by successful feeding of four other m-hydroxymethylbenzoic acids, leading to formation of the expected 20-membered macrocycles. Good to moderate antiproliferative activities were encountered for three of the five new geldanamycin derivatives, which matched well with a competition assay for Hsp90α.     

A quantum chemical investigation of pyrolysis reactions of glyoxylic acid ethylester

Two possible reaction paths for the pyrolysis of the ethylester of glyoxylic acid have been studied by ab initio molecular orbital calculations. The basis sets 3-21G and 6-31G * have been used, and electron correlation has been included by Møller–Plesset calculations up to fourth order. Our calculations indicate that the reaction leading to acid and ethylene through a 6-membered ring transition state is favored relative to a process involving a formyl hydrogen transfer via a 5-membered ring to the alkyl unit leading to ethane, CO, and CO₂. The predicted activation energies for these two reactions obtained at the highest level of calculation, MP 4(SDTQ )/6–31G *, are 50.4 and 71.7 kcal/mol, respectively. The transition states have RHF wave functions that are stable relative to UHF solutions using the 3–21G basis. The geometry of the transition states and IRC following indicate that both reactions are strongly asynchronous: The C? O bond rupture is virtually completed before hydrogen transfer occurs. For comparative purposes, analogous calculations have been performed for the ethylester of formic acid, where it is confirmed that a 6-membered ring transition state is preferred relative to a 4-membered one by around 42 kcal/mol at the highest level of calculation.     

Carbon-13 NMR spectra of saturated heterocycles: VIII—tetrahydrofurans and 1,3-dioxolanes

The ¹³C NMR spectra of 17 methyl- and ethyl-substituted tetrahydrofurans and 58 alkyl- and aryl- substituted 1,3-dioxolanes are reported. For the cases in which substituents recur, additive substituent parameters are calculated and compared with similar parameters in cyclopentanes and in 6-membered rings. Considering the conformational flexibility of 5-membered ring systems, additivity of the parameters is surprisingly good.     

Umamidierungsreaktionen an cyclischen Amino-amid-Systemen. 3. Mitteilung über Umamidierungsreaktionen

Transamidation Reactions with Cyclic Amino-amides Lactames which are substituted at the nitrogen atom by a 3-aminopropyl residue are transformed under base catalysis to cyclic amino-amides enlarged by 4 ring atoms. The formed ring must be at minimum 12-membered. Scheme 2 illustrates this result: the 8-membered 7 is transamidated in 96% yield to the 12-membered ring 8 (in the presence of potassium 3-aminopropylamid in 1, 3-propanediamine), the 9-membered 10 to the 13-membered ring 11 (97%) and the 11-membered 14 to the 15-membered ring 15 . Furthermore, the 13-membered ring 27 (Scheme 5) is transformed to the 17-membered 28 . In the case of the 15-membered lactame 15 it is demonstrated that 14 is not formed back under the conditions of the transamidation. Large ring lactames which are substituted at the nitrogen atom by a 3-(alkylamino) propyl group lead under base catalysis to an equilibrium mixture, e.g. the 17-membered 26 is in equilibrium with the 21-membered 29 . This result is similar to the behavior of the corresponding open-chain amino-amides [2]. Because of transannular interactions, the 11-membered ring 2 is not stable: transamidation of the 7-membered 1 (Scheme 1) doesn't give the expected 2 , but its water elimination product 3 in small yield. The N-tosyl derivative of 2 , namely 20 , is synthesized by an independent route (Scheme 3). Detosylation of 20 yields the 7-membered 1 instead of 2 . Concerning the mechanism of this interesting reaction see Scheme 4.     

Estimation on the intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies in glycine and alanine peptides

Computation of accurate intramolecular hydrogen-bonding energies for peptides is of great importance in understanding the conformational stabilities of peptides and developing a more accurate force field for proteins. We have proposed a method to determine the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies in glycine and alanine peptides. In this article, the method is further applied to evaluate the intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies in peptides. The optimal structures of the intramolecular 10-membered ring N-H...O=C hydrogen bonds in glycine and alanine tripetide molecules are obtained at the MP2 level with 6-31G(d), 6-31G(d,p), and 6-31+G(d,p) basis sets. The intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies are then evaluated based on our method at the MP2/6-311++G(3df,2p) level with basis set superposition error correction. The intramolecular 10-membered ring N-H...O=C hydrogen-bonding energies are calculated to be in the range of -6.84 to -7.66, -4.44 to -4.98, and -6.95 to -7.88 kcal/mol. The method is also applied to estimate the individual intermolecular hydrogen-bonding energies in the dimers of amino-acetaldehyde, 2-amino-acetamide, formamide, and oxalamide, each dimer having two identical intermolecular hydrogen bonds. According to our method, the individual intermolecular hydrogen-bonding energies in the four dimers are calculated to be -1.77, -1.67, -6.35, and -4.82 kcal/mol at the MP2/6-311++G(d,p) level, which are in good agreement with the values of -1.84, -1.72, -6.23, and -4.93 kcal/mol predicted by the supermolecular method.