NEW MATERIAL: β-lactam (salt) represented by formula I {wherein R1 is a group represented by formula II [wherein X is a (substd.) monocyclic group or a fused polycyclic aromatic group and OH bonded in this case is conjugated with a group represented by formula III and n is 1 or 2]; R2 is H or (aryl) 1-6C alkyl; R3 is H or a carboxy protecting group and Y is groups represented by formulae IV, V (Y0 is S, SO or SO2)}.

EXAMPLE: t-butyl 7β-[(3,5-di-t-butyl-4-hydroxy)benzylideneamino]-7- formamidecepharosporanate.

USE: An antibiotic.

PROCESS: A compd. (salt) represented by formula VI [wherein R(OX)1 is quinone corresponding to an oxidized form of R1] is reacted with a nucleophilic derive. of formamide or (protected)ammonia and subsequently reacted with a formylating agent.