On the Question of the Diastereoselective Alkylation of 4-Unsubstituted 3-Amino β-Lactams. A Concise Synthesis of α-Branched α-Amino β-Lactams and their Coupling with α-Amino Acid Esters

The reaction of [(4S)-2-oxo-4-phenyloxazolidin-3-yl]acetic acid chloride with N-methylidene-bis-[(trimethylsilyl)methyl]amine and triethylamine in refluxing chloroform leads to a 4-unsubstituted β-lactam able to undergo highly asymmetric alkylations at the α-position of the β-lactam ring. The resulting adducts can be efficiently transformed into N-unsubstituted α-branched 3-amino β-lactams as the cyclisized forms of α-branched β-aminoalanines or transformed into their imide N-Boc derivatives, which are suitable substrates for non-proteinogenic peptide synthesis, by reaction with α-amino esters.