Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

Validation of the distal effect of electron-withdrawing groups on the stability of peptide enolates and its exploitation in the controlled stereochemical inversion of amino acid derivatives

Theoretical studies had predicted that N-electron-withdrawing substituents, hydrogen bonding, and protonation at amide nitrogen selectively increase the acidity of a distal proton adjacent to the amide carbonyl to the extent that the α-carbonyl acidity of some N-substituted amides exceeds that of typical ketones. Now, in the present work, competitive, base-catalyzed hydrogen-deuterium exchange experiments, with diisopropyl ketone and a series of N-substituted acetamides and diketopiperazines, have established that there is a strong correlation between the calculated acidities and the experimental rates of deprotonation in these systems. The results show that the rates of exchange of the distal protons of N-acylated and N-sulfonylated amides are more than 4 orders of magnitude faster than those of the N-methylated analogues and that the acylated and sulfonylated amides are much more reactive in this regard than diisopropyl ketone. The magnitude and regioselectivity of the distal effect is sufficient for practical applications and has been exploited in the manipulation of N-acetyl α,α'-disubstituted diketopiperazines for the controlled α-deuteration and stereochemical inversion of N-methylamino acids, and in the production of α-deuterated (2R,3S)-N-methylalloisoleucine from the nondeuterated (2S,3S)-isoleucine diastereomer.

The distal effect of electron-withdrawing groups and hydrogen bonding on the stability of peptide enolates

Relative gas-phase carbon acidities have been computed for a series of acetamides, diketopiperazines, and linear dipeptides. The results show that N-electron-withdrawing substituents, protonation, and hydrogen bonding at amide nitrogen in these systems increase the acidity of both a C-H proton adjacent to the amide carbonyl and that of one proximal to the amide nitrogen. There is a good correlation between the magnitudes of the increases at the two positions, but the extent of the increase for the distal C-H adjacent to the carbonyl is greater than that for the proximal C-H, in most cases by a factor of about two. The effects on the stability of the distal enolate are shown to result from predominantly inductive affects. The size of these effects is such that protonation and hydrogen bonding at nitrogen increase the acidity of the distal C-H to almost the same extent as seen for the analogous interactions at the carbonyl oxygen. The effect is also seen in solution, where the computed aqueous pK(a) values are greater for the C-H adjacent to the amide carbonyl, by up to 13 units, and where preliminary experimental studies have shown that N-acetylation of an amide increases the rate of hydrogen-deuterium exchange via formation of the corresponding distal enolate by more than 3 orders of magnitude above the rates of exchange via the proximal enolate, of the nonacetylated amide and of diisopropylketone. The results also indicate that hydrogen bonding to amide nitrogen could be as important as bonding to oxygen in enzyme-catalyzed cleavage of alpha-C-H bonds.

Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study

The alpha-alkylation of amide enolates by using a pseudo-C(2) symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (>99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F...metal and pi...metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 kcal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F...metal and pi...metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 kcal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.

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.