SYNTHESIS OF A PEPTIDYL DIFLUORO KETONE BEARING THE ASPARTIC-ACID SIDE-CHAIN - AN INHIBITOR OF INTERLEUKIN-1-BETA CONVERTING ENZYME

The synthesis of the peptidyl difluoro ketone 1, an inhibitor of interleukin-1beta converting enzyme (ICE), was accomplished by a route beginning with the reaction of BrZnCF2CO2Et (2) with (2S)-1-(tert-butyldimethyl-silyl)-4-oxo-2-azetidinecarboxaldehyde (3) to give a mixture of epimeric alcohols 4 and 5. Conversion of alcohol 5 to 1 was carried out by a sequence including the novel coupling of beta-lactam 17 and BOC-Ala-O(succinimide) to form lactone 18. An early attempt to synthesize 1 utilized the beta-lactam ketone 10 obtained in two steps from either 4 or 5. This underwent addition of ethanol and stereoselective migration of the tert-butyldimethylsilyl group to afford the mixed silyl ethyl ketal 12. Unfortunately, efforts to open the beta-lactam ring of 12 and couple the intermediate beta-amino ester 14 to BOC-Ala-O(succinimide) were complicated by an unexpected cyclization reaction. The difluoro ketone 1 was found to exist as a mixture of diastereomeric gamma-hydroxy lactone tautomers in chloroform solution.