NEW MATERIAL: Compounds of formula I {R1 is H, amino protector; R2 is H, (substd.) alkyl, alkenyl, cycloalkyl, aryl, etc.; R is CHRaOCORb [Ra is H, C1-6 alkyl, C3-7 cycloalkyl, methyl, phenyl; Rb is C1-16 alkyl, C1-16 alkoxy, (substd.) phenyl, benzyl, etc.]}.

EXAMPLE: 6β-[Z-[2-(2-aminothiazol-4-yl)-3-phenyl]propeneamido]penycillanic acid 1-acetoxyethyl.

USE: Treatment of bacterial infectious diseases.

PROCESS: A compd. (salt) of formula II is treated with a compd. of formula XCHRaOCORb (X is a removable group).