Search

We describe the synthesis of trihydroxylated cyclohexane β-amino acids from (-)-shikimic acid, in their cis and trans configuration, and the incorporation of the trans isomer into a trans -2-aminocyclohexanecarboxylic acid peptide chain. Subsequently, the hydroxyl groups were partially or totally deprotected. The structural study of the new peptides by FTIR, CD, solution NMR and DFT calculations revealed that they all fold into a 14-helix secondary structure, similarly to the homooligomer of trans -2-aminocyclohexanecarboxylic acid. This means that the high degree of substitution of the cyclohexane ring of the new residue is compatible with the adoption of a stable helical secondary structure and opens opportunities for the design of more elaborate peptidic foldamers with oriented polar substituents at selected positions of the cycloalkane residues.

Enantiorecognition between a racemic reagent and a racemic substrate can be a valuable process in organic synthesis. This review highlights representative examples of this phenomenon and the use of mutual kinetic resolution as a method for screening of kinetic and/or parallel kinetic resolutions.

The SuperQuat (4-substituted 5,5-dimethyloxazolidine-2-one) family of chiral auxiliaries was first developed by us in the 1990s to address the shortcomings of the Evans (4-substituted oxazolidin-2-one) family of chiral auxiliaries. The incorporation of geminal dimethyl substitution at C(5) has two effects: (i) it induces a conformational bias on an adjacent, otherwise conformationally labile C(4)-substituent so that it projects towards the N-acyl fragment, thus offering superior diastereofacial selectivity in a range of transformations; and (ii) it hinders nucleophilic attack at the endocyclic carbonyl group, facilitating recovery and recyclability of the auxiliary, with enhanced cleavage properties. This review summarises the development and some of the most common uses of the SuperQuat family of chiral auxiliaries, particularly in the synthesis of natural products or other targets having bioloigcal interest. Where possible, comparisons with the performances of the corresponding Evans auxiliaries are presented.

Concise asymmetric syntheses of (-)-lupinine, (+)-isoretronecanol, (+)-5-epi-tashiromine and (R,R)-1-(hydroxymethyl)octahydroindolizine (the azabicyclic core within stellettamides A-C) have been achieved in 8 steps or fewer from commercially available starting materials. The key steps in these syntheses involved the preparation of enantiopure β-amino esters, upon conjugate addition of lithium (R)-N-(p-methoxybenzyl)-N-(α-methyl-p-methoxybenzyl)amide to either ζ-chloro or ζ-hydroxy substituted tert-butyl (E)-hept-2-enoate, or ε-chloro or ε-hydroxy substituted tert-butyl (E)-hex-2-enoate. Activation of the ω-substituent as a leaving group led to SN2-type ring-closure, which occurred with concomitant N-debenzylation via an E1-type deprotection step, to give the corresponding pyrrolidine or piperidine in good yield. Subsequent alkylation of these enantiopure azacycles, followed by a second ring-closure/concomitant N-debenzylation step formed the pyrrolizidine, indolizidine or quinolizidine motif, and reduction with LiAlH4 gave the target compounds in diastereoisomerically and enantiomerically pure form.

This article compares the diastereoselective epoxidation of acyclic and cyclic allylic alcohols, with the chemo- and diastereoselective olefinic oxidation of a range of acyclic and cyclic allylic amines. The diastereoselectivity in these systems is compared and a discussion about the origin of this high diastereocontrol is also presented. The ammonium directed epoxidation methodology has been extended to more complex substrates and representative applications of this protocol in natural product synthesis are also summarised.

The diastereoselective Ireland-Claisen rearrangement of a range of substituted allyl β-amino esters gave the corresponding enantiopure α-substituted-β-amino esters with good diastereoselectivity. The application of this methodology in the asymmetric synthesis of a range of C(5)-substituted 1,2-anti-1,5-syn-transpentacins was demonstrated by the rearrangement of a range of β-amino esters derived from sorbic acid, followed by esterification, ring-closing metathesis, hydrogenolytic deprotection/reduction, and hydrolysis, which gave the C(5)-substituted transpentacins in only 9 steps from commercially available starting materials.