DETERMINATION OF ABSOLUTE-CONFIGURATION OF THE ALKYL GROUP AT THE ALPHA-POSITION IN THE ACYCLIC ALPHA-ALKYL-(E)-BETA,GAMMA-ENOATES BY CIRCULAR-DICHROISM

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 (2S)-compounds show a positive n → π* Cotton effect around 220 nm, (2R)-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. © 1990.