Treatment of p-methoxybenzyl 6β-(triphenylmethylamino)penicillanate (Ia) with methyl iodide and strong anhydrous base caused selective cleavage of the thiazolidine ring to give 1-(1-p-methoxybenzyloxycarbonyl-2-methylprop-1-enyl)-4R-methylthio-3R-(triphenylmethylamino)azetidin-2-one (IIa) which was converted into analogues of pencillins containing a non-fused β-lactam ring.