Alkylation of Imidazolidinone Dipeptide Derivatives: Preparation of Enantiomerically Pure Di‐ and Tripeptides by ‘Chirality Transfer’ via a Pivalaldehyde N,N‐Acetal Center. Preliminary Communication

Glycylglycine, glycyl‐( S )‐alanine, and ( S )‐alanylglycine esters are cyclized through pivalaldehyde imines to give dipeptide‐derived 3‐(benzyloxycarbonyl)‐2‐( tert ‐butyl)‐5‐oxoimidazolidine‐1‐acetates 1 – 3 . These are alkylated diastereoselectively by Li‐enolate formation and addition of alkyl bromides or iodides (products 4 – 6 ). Starting from ( S )‐alanine and glycine, ( S )‐alanyl‐( S )‐alanine or ( R )‐alanyl‐( R )‐alanine, and ( R )‐alanyl‐( R )alanyl‐( S )‐alanine‐ have thus been prepared, with the ( tert ‐butyl)‐substituted N,N‐acetal center playing the role of a pivot or lever for diastereoselective formation of new stereogenic centers under kinetic or thermodynamic control.