ADDITION TO ACTIVATED IMINES OF ENOLATES FROM CHIRAL N-ACYLOXAZOLIDINONES

The lithium and titanium enolates of N-acyloxazolidinones 1a-c add in their chelated forms to PhCH=NTs, to give stereoselectively the beta-amino acid derivatives 3a-c and 4a-c. The relative configurations of the newly created chiral centres were established by conversion to the trans (8a-c) or cis (9a-c) beta-lactams. Absolute stereochemistry was assigned by correlation with known compounds 10a adn ent-11a, together with an X-ray crystal structure determination on 9a. The use of the lithium enolate of 1a at -78 degrees C gave only a 1.4:1 ratio of anti product 3a to syn product 4a, but this ratio was improved to >5:1 by the use of the chlorotitanium enolate of 1a at -55 degrees C or below. Similar results were obtained by using the chlorotitanium enolate of 1b, but for the chlorotitanium enolate of the N-(phenylacetyl)oxazolidinone 1c the proportion of syn product 4c to anti product was significantly greater, consistent with a kinetically controlled reaction involving the si face of the chelated (Z)-enolate and the competing alternative transition states TS3 (reaction on si face of imine) and TS7 (reaction on re face of imine).