Stereocontrol in organic synthesis using silicon-containing compounds. A formal synthesis of (±)-thienamycin
The lithium enolate Z-11 derived from methyl 3-dimethyl(phenyl)silylbutanoate reacts with the N-silylimines of cinnamaldehyde 12, of 3-trimethylsilylpropynal 18, and of 3-trimethylsilylpropenal 19 to give β-lactams with a high level of stereoselection in favour of the cis isomers 13, 20 and 21, respectively. The dimethyl(phenyl)silyl group in the N-benzyldihydro derivative of the β-lactam 13 was converted into a hydroxy group by protodesilylation followed by peracid oxidation. The aldol product 24 of acetaldehyde with the same enolate Z-11 was converted into the corresponding O-benzyl hydroxamate 25, which gave the trans-β-lactam 26 by a Mitsunobu reaction. A similar aldol reaction using 3-dimethyl(phenyl)silylpropanal and the lithium Z-enolate derived from benzyl 3-dimethyl(phenyl)silylbutanoate gave the aldol 32, which was converted successively by way of the O-benzyl hydroxamate 33 and the trans-β-lactam 34 into the disilylated trans-β-lactam 35. Silyl-to-hydroxy conversion of both silyl groups and N,O-acetonide formation gave the known intermediate 36. Another Mitsunobu reaction with formic acid gave the C-8 epimer 2, which has previously been converted into (±)-thienamycin 1.