Asymmetric syntheses via heterocyclic intermediates, XL. Studies on the acylation of lithiated bislactim ethers of cyclo(‐L‐Val‐Ala‐) and cyclo(‐L‐Val‐Gly‐) Asymmetric synthesis of (R)‐α‐alkenyl and (R)‐α‐ethinyl alanine methyl esters by the bislactim ether method