This invention relates to a process for preparing novel penicillins and to penicillin antibiotic compounds.  In particular, it relates to an enzymatic process for converting an N-terminal vinyl-substituted tripeptide to a 2-methyl-2-vinylpenam with isopenicillin N synthetase and to 6-acylamino-2-methyl-2-vinylpenam-4-carboxylic acids prepared with the product of the process. 
The enzyme isopenicillin N synthetase is well known for its ability to cyclize the tripeptide .delta.-(L-.alpha.-aminoadipoyl)-L-cysteinyl-D-valine to isopenicillin N. The enzyme is obtained from numerous sources such as Penicillium chrysogenum, Cephalosporium acremonium and Streptomyces clavuligerus cultures and is used as a cell-free extract.  Because of the importance of the penicillins and other .beta.-lactam antibiotics in therapy, considerable effort has been undertaken to explore the use of this synthetase in the preparation of .beta.-lactam compounds.  As described herein, the isopenicillin N synthetase is employed in a process for converting an N-terminal vinyl-substituted tripeptide to a 2-methyl-2-vinylpenam.