β-Lactams with contiguous tetra- and trisubstituted carbon centers were prepared in a highly enantioselective manner through 4-exo-trig cyclization of axially chiral enolates generated from readily available α-amino acids. Use of a weak base (metal carbonate) in a protic solvent (EtOH) is the key to the smooth production of β-lactams. Use of the weak base is expected to generate the axially chiral enolates in a very low concentration, which undergo intramolecular conjugate addition without suffering intermolecular side reactions. Highly strained β-lactam enolates thus formed through reversible intramolecular conjugate addition (4-exo-trig cyclization) of axially chiral enolates undergo prompt protonation by EtOH in the reaction media (not during the work-up procedure) to give β-lactams in up to 97 % ee. Inherently chiral enolates: β-Lactams with contiguous tetra- and trisubstituted carbon centers were prepared in a highly enantioselective manner starting from α-amino acids through axially chiral enolates (see scheme; PMB=p-methoxybenzyl). The use of a weak base (metal carbonate) in a protic solvent (EtOH) is the key to smooth production of β-lactams.
All Science Journal Classification (ASJC) codes
- Organic Chemistry