Edward D. Savory

Kinetic resolution and parallel kinetic resolution of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates for the asymmetric synthesis of 5-alkyl-cispentacin derivatives.

Conjugate addition of lithium dibenzylamide to methyl 5-isopropyl, 5-phenyl- and 5-tert-butyl-cyclopentene-1-carboxylates occurs with high levels of substrate control (>88% de), with preferential addition to the face of the cyclic alpha,beta-unsaturated acceptor anti- to the stereodirecting 5-alkyl substituent. Treatment of a range of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates with both lithium (+/-)-N-benzyl-N-alpha-methylbenzylamide and lithium (+/-)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide indicates significant enantiorecognition in their mutual kinetic resolutions, with preferential addition anti- to the 5-alkyl substituent, giving the 1,2-syn-1,5-anti-arrangement (E >16) after enolate protonation anti- to the amino functionality. The kinetic resolution of a range of methyl (+/-)-5-alkyl-cyclopentene-1-carboxylates with lithium (S)-N-benzyl-N-alpha-methylbenzylamide, and their efficient parallel kinetic resolution with a pseudoenantiomeric mixture of lithium (S)-N-benzyl-N-alpha-methylbenzylamide and lithium (R)-N-3,4-dimethoxybenzyl-N-alpha-methylbenzylamide are also demonstrated, giving a range of 5-alkyl-cispentacin derivatives in >98% de and high ee after N-deprotection.

Pure by NMR

Integration of a (13)C-(1)H satellite peak of a given (12)C-(1)H parent resonance within a quantitative (1)H NMR spectrum and comparison to the minor component represents a simple protocol for the accurate determination of diastereoisomeric ratios of up to 1000:1 (i.e., 99.8% de).

Rearrangements and racemisation during the synthesis of L-serine derived oxazolidin-2-ones

Abstract The propensity for N-Boc-4-hydroxymethyl-oxazolidin-2-ones to undergo rapid O–O and N–O carbonyl transfer makes these l -serine derived chiral auxiliaries unsuitable for attachment to polymers.

Asymmetric synthesis of homochiral differentially protected bis-β-amino acid scaffolds

Abstract A strategy for the asymmetric synthesis of homochiral [(R,R)- or (S,S)-], or meso-(R,S) bis-β-amino acid scaffolds, formally resulting from the stepwise conjugate addition of two differentially protected homochiral lithium amides to two α,β-unsaturated esters attached to a central arene, is demonstrated. Further manipulation enables the efficient synthesis of orthogonally protected pseudo-meso or pseudo-C2 symmetric scaffolds via selective N-benzyl or N-allyl deprotection, enabling regio-, stereo- and chemoselective functionalisation.

Conformational control in the SuperQuat chiral auxiliary 5,5-dimethyl-4-iso-propyloxazolidin-2-one induces the iso-propyl group to mimic a tert-butyl group

1 H NMR nOe spectroscopic studies reveal that conformational control in the enolates of N-acyl-5,5-dimethyl-4-iso-propyloxazolidin-2-ones ensures that the stereodirecting effect of its 4-iso-propyl-5,5-dimethyl functional group affords superior levels of facial selectivity normally associated with enolates derived from N-acyl-4-tert-butyloxazolidin-2-ones.

Double diastereoselective [3,3]-sigmatropic aza-Claisen rearrangements.

Asymmetric [3,3]-sigmatropic aza-Claisen rearrangement of the (Z)-N-allyl-N,O-silylketene aminal of (3S,4E,alphaR)-1-benzyloxy-3-(N-propionyl-N-alpha-methylbenzylamino)hex-4-ene furnishes (2S,3R,4E,alphaR)-N-alpha-methylbenzyl-2,3-dimethyl-7-benzyloxyhept-4-enamide in > 92% d.e.; rearrangement of the diastereomeric (3R,4E,alphaR)-(Z)-N,O-silylketene aminal proceeds with low diastereoselectivity.

Asymmetric synthesis of ??-pyridyl-??-amino acid derivatives

The conjugate additions of homochiral lithium (R)-N-benzyl-N-α-methyl-4-methoxybenzylamide to tert-butyl 3-(3-pyridyl)- and tert-butyl 3-(4-pyridyl)-prop-2-enoates proceed in 84% de, and after subsequent recrystallisation and oxidative N-deprotection furnish the (S)-3-(3-pyridyl)- and (S)-3-(4-pyridyl)-β-amino acid derivatives in 97% ee and 98% ee respectively. Conjugate additions of lithium N-benzyl-N-α-methyl-4-methoxybenzylamide to tert-butyl 3-(2-pyridyl)prop-2-enoates proceed with low levels of diastereoselectivity unless the 3-(2-pyridyl) ring is substituted. Application of this methodology allows the asymmetric synthesis of (R)-tert-butyl 3-(2-chloro-3-methoxymethoxy-6-pyridyl)-3-aminopropanoate, the protected β-amino ester component of kedarcidin, in 97% ee.

Chiral glycine cation equivalents: N-acyliminium species derived from diketopiperazines

Studies towards a N,N′-bis(p-methoxybenzyl)diketopiperazine asymmetric glycine cation equivalent for the synthesis of homochiral α-amino acids are described. The oxidation of enolate 3 with molecular oxygen provides either a mixture of hydroxylated diketopiperazines 7 and 8 or trione 10 depending upon the reaction conditions. The nucleophilic reduction of trione 10 and the reaction of acetoxy N-acyliminium ion precursors 5 and 6, derived from 7 and 8, with allyltrimethylsilane and boron trifluoride etherate is examined and a model for the stereoselectivity observed in these additions is presented.