Synthesis of β-lactams by condensation of titanium enolates of 2-pyridylthioesters with imines. Influence of the imine structure on the trans/cis stereoselectivity
The condensation of the titanium enolates of C-2 alkyl substituted 2-pyridylthioesters with imines affords beta-lactams in trans/cis ratios that largely depend on the structure of the C-imine residue. Bulky and non-chelating heteroatom-containing groups lead to the formation of trans p-lactams, while sterically non-requiring or chelating groups favour the formation of the cia-products. On the basis of NMR evidences a rationale is proposed to account for the observed stereoselectivity.
Asymmetric synthesis of (1'R,3R,4R)-4-acetoxy-3-[1'-((tert-butyldimethylsilyl)oxy)ethyl]-2-azetidinone and other 3-(1'-hydroxyethyl)-2-azetidinones from (S)-(+)-ethyl 3-hydroxybutanoate: formal total synthesis of (+)-thienamycin