Asymmetric synthesis of alpha,alpha-disubstituted alpha-amino acids using (S,S)-cyclohexane-1,2-diol as a chiral auxiliary

Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

1H NMR spectra of ethane-1,2-diol and other vicinal diols in benzene: GIAO/DFT shift calculations

The proton NMR spectra of several 1,2-diols in benzene have been analysed so as to associate each magnetically nonequivalent proton with its chemical shift. The shifts and coupling constants of the OH and methylene protons of ethane-1,2-diol have been determined in a wide range of solvents. The conformer distribution and the proton NMR shifts of these 1,2-diols in benzene have been computed on the basis of density functional theory. The solvent is included using the integral-equation-formalism polarizable continuum model implemented in Gaussian 09. Relative Gibbs energies for all stable conformers are calculated at the Perdew, Burke and Enzerhof (PBE)0/6-311 + G(d,p) level, and shifts are calculated using the gauge-including atomic orbital method with the PBE0/6-311 + G(d,p) geometry but using the cc-pVTZ basis set. Previous calculations on ethane-1,2-diol and propane-1,2-diol have been corrected and extended. New calculations on tert-butylethane-1,2-diol, phenylethane-1,2-diol, butane-2,3-diols (dl and meso) and cyclohexane-1,2-diols (cis and trans) are presented. Overall, the computed NMR shifts are in good agreement with experimental values for the OH protons but remain systematically high for CH protons. Some results based on the Gaussian 03 solvation model are included for comparison.

Catalytic asymmetric synthesis using feedstocks: an enantioselective route to 2-arylpropionic acids and 1-arylethyl amines via hydrovinylation of vinyl arenes

A three-step procedure for the synthesis of 2-arylpropionic acids (profens) from vinyl arenes in nearly enantiomerically pure form has been developed. Excellent yields (>97%), regioselectivities (>99%), and enantioselectivities (>97% ee) for the desired branched products were obtained in the asymmetric hydrovinylation reactions of vinyl arenes, and the products from these reactions were transformed into 2-arylpropionic acids via oxidative degradation. Subsequent Curtius or Schmidt rearrangements of these acids provided highly valued 1-arylethyl amines, including a prototypical primary amine with an alpha-chiral tertiary N-alkyl group, in very good yields.

Rearrangement of Indene Skeletons under Mild Conditions

Isomerization between two isomers of 1,2-disubstituted 3-aminoindenes occurs via the rearrangement of indene frameworks. In contrast to previous rearrangements of indene derivatives, which occur under high-temperature conditions or the irradiation of light, this rearrangement proceeds at room temperature without UV light irradiation. An amino group at the 3-position plays an important role to accelerate the rearrangement under mild conditions.

Design and synthesis of chiral alpha,alpha-disubstituted amino acids and conformational study of their oligopeptides

alpha,alpha-Disubstituted amino acids are alpha-amino acids in which the hydrogen atom at the alpha-position of the L-alpha-amino acid is replaced with an alkyl substituent. The introduction of an alpha-alkyl substituent changes the properties of amino acids, with the conformational freedom of the side chain in the amino acids and the secondary structure of their peptides being especially restricted. The author developed a synthetic route of optically active alpha-ethylated alpha,alpha-disubstituted amino acids using chiral cyclic 1,2-diol as a chiral auxiliary. It was found that the preferred secondary structure of peptides composed of chiral alpha-ethylated alpha,alpha-disubstituted amino acids is a fully extended C5-conformation, whereas that of peptides composed of chiral alpha-methylated alpha,alpha-disubstituted amino acids is a 3(10)-helical structure. Also, a new chiral cyclic amino acid; (3S,4S)-1-amino-3,4-di(methoxy)cyclopentanecarboxylic acid {(S,S)-Ac5c(dOM)}, and a bicyclic amino acid; (1R,6R)-8-aminobicyclo[4.3.0]non-3-ene-8-carboxylic acid {(R,R)-Ab5,6= c}, in which the alpha-carbon atom is not the chiral center but chiral centers exist at the side-chain cycloalkane skeleton, were designed and synthesized. The (S,S)-Ac5c(dOM) hexa- and octapeptides preferentially formed left-handed (M) helices, in which the helical-screw direction is exclusively controlled by the side-chain chiral centers. Contrary to the left-handed helices of (S,S)-Ac(5)c(dOM) peptides, the (R,R)-Ab5,6= c hexapeptide formed both diastereomeric right-handed (P) and left-handed (M) helices, and the twelve chiral centers at the side chain showed no preference for helical-screw direction. Thus, the chiral environment at the side chain is important for the control of helical-screw direction. Furthermore, the author designed a new class of chiral cyclic alpha,alpha-disubstituted amino acids that have pendant chiral centers at the substituent of the delta-nitrogen atom. The synthetic route would provide various optically-active cyclic alpha,alpha-disubstituted amino acids bearing a pendant chiral moiety.

Diastereoselective synthesis of quaternary α-amino acids from diketopiperazine templates

Sequential enolate alkylations of (S)-N(1)-methyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one and (S)-N(1)-p-methoxybenzyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one proceed with excellent levels of diastereoselectivity (>90% de) affording quaternary alpha-amino acids in high enantiomeric excess (>98% ee) after deprotection and hydrolysis.

Recent approaches towards the asymmetric synthesis of α,α-disubstituted α-amino acids

The class of alpha,alpha-disubstituted alpha-amino acids has gained considerable attention in the past decades and continues doing so. The ongoing interest in biological and chemical properties of the substance class has inspired the development of many new methodologies for their asymmetric construction, which have not found their way into the general focus of organic chemistry yet. The aim of this review is to provide an overview of the developments in the field since 1998.

Oxaziridine-Mediated Amination of Branched Allylic Sulfides: Stereospecific Formation of Allylic Amine Derivatives via [2,3]-Sigmatropic Rearrangement

Reaction of branched allylic sulfides with the N-Boc-oxaziridine 1 results in [2,3]-sigmatropic rearrangement of the intermediate allylic N-Boc-sulfimides with a high level of chirality transfer. The first example of formation of a quaternary stereocenter using this transformation is reported.

Design and Synthesis of Non-proteinogenic Amino Acids and Secondary Structures of Their Peptides

Replacement of the alpha-hydrogen atom of L-alpha-amino acids with an alkyl substituent results in alpha,alpha-disubstituted amino acids. The modification changes the properties of the amino acids. Incorporation of alpha,alpha-disubstituted amino acids into oligopeptides restricts the conformational freedom of their peptides. The author developed a synthetic route for optically active alpha-ethylated alpha,alpha-disubstituted amino acids using chiral cyclic 1,2-diol, and disclosed that the preferred conformation of peptides composed of chiral alpha-ethylated disubstituted amino acids is a fully planar conformation, whereas that of chiral alpha-methylated disubstituted amino acids is a 3(10)-helical structure. Furthermore, the author designed and synthesized two chiral cyclic alpha,alpha-disubstituted amino acids, i.e., (3S,4S)-1-amino-3,4-di(methoxy)cyclopentanecarboxylic acid {(S,S)-Ac5cdOM}, and (1R,6R)-8-aminobicyclo-[4.3.0]non-3-ene-8-carboxylic acid {(R,R)-Ab5,6=c}. They do not have a chiral center at the alpha-position, but do have chiral centers on the side-chain cyclopentane or the bicyclic skeleton. The preferred secondary structure of the (S,S)-Ac5cdOM homopeptides was the left-handed (M) 3(10)-helical structure (hexapeptide) and the left-handed (M) alpha-helical structure (octa- and decapeptides), while that of the (R,R)-Ab5,6=c hexapeptide was both the right-handed (P) and left-handed (M) 3(10)-helices. These results indicate that the side-chain chiral centers affect the secondary structure of their peptides, and the side-chain chiral environment is important for the control of the helical-screw direction of peptides.

Tin(II) chloride catalyzed reactions of diazodiphenylmethane with vicinal diols in an aprotic solvent. The reactions with cis- and trans-1,2-cyclohexanediols and 1,2-propanediol

The paper reports the tin(II) chloride catalyzed reactions of diazodiphenylmethane with the cis- and trans-1,2-cyclohexanediols and R,S-1,2-propanediol in 1,2-dimethoxyethane and the identification of the monodiphenylmethyl ethers formed. The catalyst is shown to work for both the cis- and trans-cyclohexanediols, but the catalyst is unstable at high reagent concentrations, especially in the case of the trans-isomer. Conditions where catalyst destruction is negligible show that the rate of the reaction with the trans-isomer is larger than with the cis-isomer. The reactions with 1,2-propanediol show small difference between the selectivity for the primary and secondary hydroxyl groups. This is in contrast with the tin(II) chloride catalyzed reactions of diazomethane and diazophenylmethane in methanol with carbohydrates, glycerol and ribonucleosides, where the primary hydroxyl group does not react.

Solid-state conformation of a hybrid tripeptide between beta-amino acid; 8-aminocyclooct-4-enecarboxylic acid and 2-aminoisobutyric acid

An eight-membered cyclic beta-amino acid, 8-aminocyclooct-4-enecarboxylic acid, was designed as a conformationally restricted non-proteinogenic amino acid. A hybrid tripeptide containing this eight-membered cyclic beta-amino acid and 2-aminoisobutyric acids was synthesized by conventional solution methods. The conformation of the tripeptide was studied using X-ray analysis and was shown to form an eleven-membered hydrogen-bonded turn (3(11)-helical structure) in the solid state.