A simple synthesis of (−)-(1S,2R)-allocoronamic acid in its enantiomerically pure form

Asymmetric synthesis of (1R,2S)-1-amino-2-vinylcyclopropanecarboxylic acid by sequential SN2–SN2′ dialkylation of (R)-N-(benzyl)proline-derived glycine Schiff base Ni(ii) complex

The synthesis of the title compound, a key structural fragment of several hepatitis C virus inhibitors, is described.

An Overview of Stereoselective Synthesis of α-Aminophosphonic Acids and Derivatives

An overview of all methodologies published during the last few years focused to the stereoselective (diastereoselective or enantioselective) synthesis of α-aminophosphonic acids and derivatives is reported. The procedures have been classified according a retrosynthetic strategy and taking into account the formation of each one of the bonds connected to the chiral centre.

Synthesis and HPLC enantioseparation of the cyclopropane analogue of valine (c3Val)

A new and efficient method is presented for the preparation of the N-Boc-protected cyclopropane analogue of valine, 1-(N-tert-butoxycarbonyl)amino-2,2-dimethylcyclopropanecarboxylic acid, both in racemic and enantiomerically pure forms. Cyclopropanation of the exocyclic double bond of 2-phenyl-4-isopropylidene-5(4H)-oxazolone with diazomethane followed by elaboration of the heterocyclic moiety provided multigram quantities of the racemic target compound. Subsequent HPLC resolution of a racemic precursor on a noncommercial chiral stationary phase has given access to enantiomerically pure products. Almost 1.5 g of the first-eluted enantiomer and 1.0 g of the second-eluted enantiomer have been isolated in optically pure form using a 150 x 20 mm ID column containing mixed 10-undecenoate/3,5-dimethylphenylcarbamate of cellulose covalently bonded to allylsilica gel with a mixture of hexanes/tert-butyl methyl ether/ethyl acetate as the mobile phase.

Synthesis of (1R,2S)-1-Amino-2-vinylcyclopropanecarboxylic Acid Vinyl-ACCA) Derivatives: Key Intermediates for the Preparation of Inhibitors of the Hepatitis C Virus NS3 Protease

(1R,2S)-1-Amino-2-vinylcyclopropanecarboxylic acid (vinyl-ACCA) is a key building block in the synthesis of potent inhibitors of the hepatitis C virus NS3 protease such as BILN 2061, which was recently shown to dramatically reduce viral load after administration to patients infected with HCV genotype 1. We have developed a scalable process that delivers derivatives of this unusual amino acid in >99% ee. The strategy was based on the dialkylation of a glycine Schiff base using trans-1,4-dibromo-2-butene as an electrophile to produce racemic vinyl-ACCA, which was subsequently resolved using a readily available, inexpensive esterase enzyme (Alcalase 2.4L). Factors that affect diastereoselection in the initial dialkylation steps were examined and the conditions optimized to deliver the desired diastereomer selectively. Product inhibition, which was encountered during the enzymatic resolution step, initially resulted in prolonged cycle times. Enrichment of racemic vinyl-ACCA through a chemical resolution via diastereomeric salt formation or the use of forcing conditions in the enzymatic reaction both led to improvements in throughput and the development of a viable process. The chemistry described herein was scaled up to produce multikilogram quantities of this building block.

Folding of peptides characterized by c3Val, a highly constrained analogue of valine

Using a combined chemical/chiral chromatographic approach we synthesized an N-protected derivative of (R)-c(3)Val, a severely conformationally restricted C(alpha)-tetrasubstituted alpha-amino acid characterized by a C(beta,beta)-dimethylated cyclopropane system. A set of terminally protected derivatives and model peptides (to the heptamer level), containing one or two (R)-c(3)Val residues in combination with either Aib or Gly residues, was prepared by solution methods. A detailed solution and crystal-state conformational investigation, based on Fourier transform infrared (FTIR) absorption, (1)H-NMR, and x-ray diffraction techniques, performed in comparison with a similar study on related derivatives and peptides rich in (alphaMe)Val, the prototype of C(alpha)-tetrasubstituted alpha-amino acids of this subfamily, allowed us to conclude the following: (a) c(3)Val is a good beta-bend and helix former, although less efficient than (alphaMe)Val. (b) The relationship between alpha-carbon chirality and screw sense of the folded structure formed is the same as that of (alphaMe)Val, i.e., the (R)-enantiomer has a strong left-handed bias. (c) c(3)Val seems more prone than (alphaMe)Val to fold into a gamma-bend conformation. The conformational propensities of C(beta,beta)-disubstituted Ac(3)c residues are also discussed in comparison with those of the parent cyclopropane residue.

Design of peptides, proteins, and peptidomimetics in chi space

Peptide and protein biological activities depend on their three dimensionals structures in the free state and when interacting with their receptors/acceptors. The backbone conformations such as alpha-helix, beta-sheet, beta-turn, and so forth provide critical templates for the three-dimensional structure, but the overall shape and intrinsic stereoelectronic properties of the peptide or protein important for molecular recognition, signal transduction, enzymatic specificity, immunomodulation, and other biological effects depend on arrangement of the side chain groups in three-dimensional chi space (their chi 1, chi 2, etc. torsional angles). In this paper we explore approaches to the de novo design of polypeptides and peptidomimetics with biased or specific conformational/topographical properties in chi space. We consider computational and experimental methods that can be used to examine the effects of specific structural modifications in constraining side chain groups of amino acid residues and their similarities in chi space to the natural amino acids to evaluate what sort of mimetics are likely to mimic normal amino acids. We then examine some of the asymmetric synthetic methods that are being developed to obtain the amino acid mimetics. Finally, we consider selected examples in the literature where these specialized amino acids have been incorporated in biologically active peptides and the specific insights they have provided regarding the topographical requirements for bioactive peptide potency, selectivity, and other biochemical and pharmacological properties. Constraints in chi space show great promise as useful tools in peptide, protein, and peptidomimetic de novo design of structures and pharmacophores with specific stereostructural, biochemical and biological properties.

New chiral didehydroamino acid derivatives from a cyclic glycine template with 3,6-dihydro-2H-1,4-oxazin-2-one structure: applications to the asymmetric synthesis of nonproteinogenic alpha-amino acids

New chiral (Z)-alpha,beta-didehydroamino acid (DDAA) derivatives with 3,5-dihydro-2H-1,4-oxazin-2-one structure 11a-f have been stereoselectively prepared after condensation of chiral glycine equivalent 7 with aldehydes in the presence of K(2)CO(3) under mild solid-liquid phase-transfer catalysis reaction conditions. These new systems have been used in diastereoselective cyclopropanation reactions using Corey's ylide for the asymmetric synthesis of 1-aminocyclopropane-1-carboxylic acids (ACCs) such as allo-corononamic and allo-norcoronamic acids. The hydrogenation reaction of these systems at ambient pressure in the presence of formaldehyde affords saturated oxazinones and N-methylated oxazinones which have been transformed into the N-methyl-alpha-amino acids (N-MAAs) (S)-2-(methylamino)butanoic acid and (S)-N-methylleucine. In addition, the parent alpha, beta-didehydroalanine derivative 11g has been prepared by a direct aminomethylation-elimination sequence from 7 and Eschenmoser's salt and has been used in Diels-Alder cycloaddition with endo selectivity for the synthesis of the enantiomerically pure bicyclic alpha-amino acids (-)-2-aminobicyclo[2.2.1]heptane-2-carboxylic and (-)-2-aminobicyclo[2.2.2]octane-2-carboxylic acids.

Atmospheric pressure chemical ionization multi-stage mass spectrometry in the characterization of stereoisomeric synthons of cyclopropane amino acids

The mass spectrometric behaviour of (1S,2R)-, (1R,2R)-, (1R,2S)- and (1S,2S)-2-[(S)-2,2-dimethyl-1, 3-dioxolan-4-yl]-1-spiro-¿4'[2'-phenyl-5'(4'H)-oxazolone]¿ cyclopropane (2) and (1S,2R)-, (1R,2R)-, (1R,2S)- and (1S, 2S)-methyl-1-benzamido-2-[(S)-2,2-dimethyl-1, 3-dioxolan-4-yl]cyclopropanecarboxylate (3) was studied under atmospheric pressure ionization conditions and by multi-stage mass spectrometric (MS(n)) experiments performed with an ion trap. Interestingly, by using methanol as solvent, compounds 2 lead to [M + H + CH(3)OH](+) ions which, as proved by collisional experiments, exhibit the same structure of the corresponding compound 3. MS/MS of [MH](+) ions allows a clear characterization of the different stereoisomers, which give rise to specific fragmentation pathways, rationalized with respect to the structure of the neutral molecules.