1-Oxacephem skeletons were constructed in a convergent manner starting from building blocks B and C, which were easily prepared from penicillin and diketene, respectively, followed by intramolecular carbene insertion reaction of the resulting intermediate $. Reported methods' for the construction of the 1-oxacephem skeleton involve oxazinering cyclization either between CS and 01, or C3 and C4, as the key steps, as originally reported by Wolfe' and by the Merck group,3 respectively. Our highly stereocontrolled and industrially feasible synthesis4 also involves intramolecular etherification between CB and Ol as the key synthetic step. Our continuing studies on 1-oxacephem syntheses led us to an alternative, useful method featuring a convergent-type synthesis from two building blocks, g and C, to form intermediate A, followed by an intramolecular carbene insertion reaction5 between C4 and N5. The synthesis is described here. Building block B (3) was easily prepared from +-oxazolinoazetidinone6 by ozonolytic removal [03, (CH3)2S, then catalytic amounts of NaOCH3] of the 3-methyl-2butenoate side chain followed by Nl-H silylation (eSi-Cl, Et3N-CH2C12) in 90% overall 2 R'=H O+N * 2 l\R' 2 R' = -Si+ yield. Building block C was prepared from diphenylmethyl 4-bromo-acetoacetate 4, which was readily derived from diketene. 7 4-Hydroxyacetoacetate 5 was obtained directly when 6.