[Ingeniosuccinimide]

Is there some method I could use to directly convert penicillin-K salt  (the cation on the scheme is wrong, if it matters at all) to alkyl ester? We have quite a good amount of salt but I have to prepare an ester.

If I have to hydrolize the salt first, which acid or reagent should I use to push the salt out and get the acid (the acidic form of this penicillin has pK_a≈2.8 )?

[phth]

Any acid with lower pka will work.  I don't like using acid to make the ester.  It will cause more side reactions on the amides than base catalysis will.  I think reaction with alkyl halides or diazo compounds will give you the best results.  A lewis acid combined with the electrophile could increase yield (like crown ether with cation but opposite).  LA effectiveness may depend on the lability of the substrate though.  I am not positive, so I would like be corrected if wrong.

[Ingeniosuccinimide]

I think reaction with alkyl halides or diazo compounds will give you the best results.

 

Diazo? Are you thinking of TMS-diazomethane, ethyl diazoacetate or something third?

A lewis acid combined with the electrophile could increase yield (like crown ether with cation but opposite).

Can you give some example?

[phth]

Yes, TMS-CH-N2 etc.  Here are some examples using LA (mild condition) to form the ester http://www.organic-chemistry.org/synthesis/C1O/esters/transesterifications.shtm; I haven't looked into the lability of amides though, but I'm guessing some LA's will be better than others.

[phth]

Oops, there's an extra semicolon on the end of the link