NEW MATERIAL:The compound of formula I [R is (substituted) benzyl].

EXAMPLE: (5R,6R)-p-Nitrobenzyl 6-(1-hydroxy-1-methylethyl)-1-azabicyclo-[3.2.0] heptane-3,7-dione-2-carboxylate.

USE: An intermediate for the preparation of an antibacterial agent, carpetimycin A.

PROCESS: The compound of formula I can be prepared e.g. by (1) oxidizing the compound of formula II (R1 is OH-protecting group) with chromic acid, etc., (2) reacting the obtained carboxylic acid derivative with malonic acid monoester or its reactive derivative to obtain the ketoester compound of formula III, (3) diazotizing the compound with p-toluenesulfonyl azide, etc., (4) removing the OH- protecting group of the resultant diazoester of formula IV, and (5) heating and cyclizing the product in the presence of rhodium acetate, etc.