STUDIES ON β-LACTAM SYNTHESIS. V. VERSATILE SYNTHESIS OF N-PROTECTED ALKYL 2-(TRANS-4-ALKYLTHIO-3-AMINO-2-OXO-1-AZETIDINYL)ACRYLATES (TRANS-7-AMINO-1,2-SECOCEPH-3-EM-4-CARBOXYLATES) WITH MODIFIED SIDE CHAINS