A sulfonyl promoted synthetic protocol for the C-3 alkylation of trans-3-phenylsulfonyl-β-lactams 6(a-e) with active organic halides in presence of K2CO3 as mild base and DMF as solvent is described. This protocol furnished cis- and trans-β-lactams as major and minor isomers respectively with alkyl halides while arylalkyl/unsaturated halides yield only cis-β-lactams exclusively. Further, the effect of sterically bulky group on the C-3 substitution was investigated by the reaction of 3-phenylsulfonyl-β-lactams 6(d-e) with crotyl chloride (predominantly E) to achieve diastereomeric mixture of β-lactams 7/7ʹ and 8. This strategy reveals advantages in terms of cost effectiveness, functional group tolerance and ease of operation.