A Theoretical Study on the Origin of pi-Facial Stereoselectivity in the Alkylation of Enolates Derived from 4-Substituted gamma-Butyrolactones

摘要

The alkylation of 4-methoxymethyl-gamma-butyrolactone enolate with methyl chloride was studied at the B3LYP/6-31+G~* level.Conformer search of the free enolate gave 15 unique conformers within 5.39 kcal/mol.The transition structures for both anti- and syn-attacks of methyl chloride on these 15 conformers were located.In all cases,the anti-transition structures are more stable than the corresponding syn-ones.The alkylation of gamma-valerolactone was studied at the MP2,B3LYP,and HF levels of theory with the 6-31+G~* basis set in the presence of Li~+ and dimethyl ether molecules.Basis set effects were also examined by the comparison of the MP2 results with the 6-31+G~*,6-31+G~(**),and 6-311+G~(**) basis sets in one case.This study shows that the main source of the anti-selectivity of 4-substituted gamma-butyrolactones is eclipsing strain in the syn-transition structures.