NEW MATERIAL:The 1-methylcarbapenem derivative of formula 1 [R1 and R2 are H, halogen, nitro, alkyl, allyl, alkoxycarbonyl, cyano, alkoxy, alkylsulfonyl or R1 and R2 together form -CH=CH-CH=CH-, -(CH2)3- or -(CH2)4-; R3 is H or carboxyl-protecting group; R4 is H or OH-protecting group; n is 0, 1 or 2].

EXAMPLE: (1R, 5R,6S)-[(R)-1-t-butyldimethylsilyloxyethyl]-1-methyl-2-phenylthio-1- carbadethiapen-2-em-3-carboxylic acid 4-nitrobenzyl ester.

USE: A synthetic intermediate for (1R)-1-methylcarbapenem derivative having excellent antibacterial activity.

PREPARATION: The compound of formula I, i.e. the compound of formula 1a or formula 1b (m is 1 or 2) can be produced from the compound of formula 2 produced by the reaction of the compound of formula 3 with the compound of formula 4.