Novel 2-substituted methyl penam derivatives include the formula (I), their analogs, their tautomeric forms, their stereoisomers, their polymorphs, their solvates, their pharmaceutically acceptable salts, and pharmaceutical compositions containing them; wherein A=C or N; Het is a three- to seven-membered heterocyclic ring; R1 represents carboxylate anion, or —COOR4 where R4 represents hydrogen, carboxylic acid protecting group or a pharmaceutically acceptable salt; R2 and R3 may be same or different and independently represent hydrogen, halogen, amino, alkyl, protected amino, optionally substituted alkyl, alkenyl, alkynyl and the like; R represents substituted or unsubstituted alkyl, alkenyl, aryl, aralkyl, cycloalkyl, heterocyclyl or heterocyclylalkyl.