6-Acylaminopenicillanoyloxymethyl esters and 1-(6-acylaminopenicillanoyloxy)ethyl esters of 2-beta-acetoxymethyl-2-alpha-methyl-(5R)penam-3-alpha-carboxylic acid 1,1-dioxide are useful as antibacterial agents. The 6-aminopenicillanoyloxymethyl ester, the 1-(6-aminopenicillanoyloxy)ethyl ester and certain other mono-substituted methyl and ethyl esters of 2-beta-acetoxymethyl-2-alpha-methyl-(5R)penam-3-alpha-carboxylic acid 1,1-dioxide are useful intermediates to the aforesaid antibacterial agents.