The reaction of acetaldehyde with enolates of N-protected 4-tritylthio-2-azetidinones 4b and 4c was studied. While the lithium enolate of N-methoxymethylazetidinone 4b gave only transS* 3-(1′-hydroxyethyl)azetidinone 8 the corresponding tetrabutylammonium enolate gave a mixture of transR* and transS* azetidinones 8 and 9. The metal enolate of N-(tert-butyldimethylsilyl)-4-tritylthio-2-azetidinone 4c gave all four possible isomers.