Penicillin synthesis

A process for the preparation of a compound of formula (I):

• ##STR9## wherein R.sub.1 is a group ##STR10## wherein R.sub.3 is phenyl, p-hydroxyphenyl, cyclohexadienyl, or 2-or 3-thienyl, R.sub.4 is an amino or conventional penicillin protected amino group, R.sub.5 and R.sub.6 are both methyl or, together with the carbon atom to which they are joined, represent a cycloheptyl ring;
• and R.sub.2 is a group of formula (II) or (IIA):
• ##STR11## wherein R.sub.7 and R.sub.8 are the same or different and are hydrogen, methyl or methoxy, R.sub.9 is acetoxy, pivaloyloxy or ethoxycarbonyloxy, and R is hydrogen or methyl;
• which process comprises reacting an aqueous solution of a compound of formula (III):
• ##STR12## wherein R.sub.1 has the above meaning and A is a salt-forming ion, with a solution in a water-immiscible organic solvent of a compound R.sub.2 OH or a reactive esterifying derivative thereof, in the presence of a phase transfer catalyst which is a compound which increases the solubility of the salt form of the parent penicillin in the water-immiscible solvent.