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.