Folding strain stereocontrol in cyclohexane ring formation by means of an intramolecular ester enolate alkylation reaction

Abstract

Diastereoselectivity in the cyclization of ethyl 7-bromo-2-methylheptanoates with an additional substituent at various positions in the chain, by LDA treatment, was investigated in connection with the concept of folding strain stereocontrol. Cyclization of 3-, 4-, and 6-methyl-substituted substrates revealed high selectivity, which demonstrates the prevalence of folding strain stereocontrol and the usefulness of this approach for stereoselective ring construction. In particular, reactions of the latter two substrates resulted in the stereodivergent preparation of diastereomeric 1,3-dimethylcyclohexanecarboxylates. In the case of the 5-mefhyl-substituted substrate, the selectivity of ring closure was only moderate.1H and13C NMR spectroscopic data were useful for determining the conformation of 1 -methylcyclohexanecarboxylate derivatives. The origin of the diastereoselectivity was examined through the qualitative comparison of the strain in the diastereomeric folding in the transition state. Various factors that might affect stereoselectivity were examined in the cyclization of 5-substituted substrates to better understand this concept. As predicted, the selectivity increased as the substituent became bulkier: Ph < Me ≈ Et < i-Pr < t-Bu. The effects of other factors — solvent, base counter cation, and leaving group — on selectivity agree with results predicted from the reactivity–selectivity relationship. Key words: folding strain stereocontrol; diastereoselectivity in ring-closure reaction; remote asymmetric induction; ethyl 2-methylcyclohexanecarboxylate derivatives,1H and13C NMR; stereoselective synthesis of substituted cyclohexane derivatives.