Determination of absolute configuration of the alkyl group at the α-position in the acyclic α-alkyl-( E)-β,γ-enoates by circular dichroism

Abstract

The absolute configuration at the α-alkylated carbon center in γ-oxygenated or γ-aminated α,β-unsaturated esters can be determined by circular dichroism measurement with confidence. Whereas (2 S)-compounds show a positive n → π ∗ Cotton effect around 220 nm, (2 R)-series of compounds exhibit a negative n → π ∗ Cotton effect near 220 nm. Thus, given the sign of the n → π ∗ Cotton effect one can determine the absolute configuration at the α-position in the α-alkyl-( E)-α,β-unsaturated ester. The presence of oxygentated or aminated group at the δ-position does not exert any influence on the sign of n → π ∗ Cotton effect.