Substitution with sodium [N-(allyloxycarbonyl)amino]thioacetate of the tert-butylsulfonyl group of the (3S,4R)-azetidinone (I) (obtained in several steps from L threonine), with overall retention of configuration at C-4, gives the azetidinone (II), wnich is converted with allyloxalyl chloride to the oxamide (III). Cyclization of (III) with triethylphosphite yields the (5R,6S,1'R)-allyl-2-(allyloxycarbonylaminomethyl)-6-(1'-allyloxycarbonyloxyethyl)penem-3-carboxylate (IV), which is deprotected to yield crystalline CGP-31608.

Substitution with sodium [N-(allyloxycarbonyl)amino]thioacetate of the tert-butylsulfonyl group of the (3S,4R)-azetidinone (I) (obtained in several steps from L threonine), with overall retention of configuration at C-4, gives the azetidinone (II), wnich is converted with allyloxalyl chloride to the oxamide (III). Cyclization of (III) with triethylphosphite yields the (5R,6S,1'R)-allyl-2-(allyloxycarbonylaminomethyl)-6-(1'-allyloxycarbonyloxyethyl)penem-3-carboxylate (IV), which is deprotected to yield crystalline CGP-31608.