The S-benZYL group has been widely used for protecting the thiol group of
cysteine in BOC chemistry, but I am not aware of the the use of S-benZOYL
(which does not mean that it hasn't been used--just my ignorance).
S-Benzoylmercaptoacetic acid is a thioester and is therefore very unstable
under alkaline conditions or in the presence of nucleophiles. In fact
thioesters were used in the olden days for making peptide bonds. I would
worry about it benzoylating the alpha-amino group of your peptide when you
couple S-benzoylmercaptoacetic acid to it. On the other hand, if you
activate the carboxyl group of S-benzoylmercaptoacetic acid sufficiently
(carbodiimide or symmetrical anhydride methods? maybe even the acid
chloride) it might outcompete the benzoylation reaction. I would avoid
activation with reagents that use hydroxybenzotriazole, since HOBt probably
would react with the thioester making the benzoyl group even more reactive.
I believe that the thioester would survive the acidic conditions needed to
cleave the peptide from the resin.
Maybe someone out there has some firsthand experience.
Richard Laursen
>Dear ABRFers,
>
>I am preparing peptides using a benzoylated derivative of mercaptoacetic
>acid, a cyseine analog. I have heard rumous that s-benzoyl protected
>cysteine was used as a protecting group for boc chemistry in the distant
>past. Are there any old-timers out there who confirm or deny this? I
>would appreciate any references if you have some handy.
>
>Sincerely,
>
>Gary Van Domselaar
>
>Gary Van Domselaar gvd@redpoll.pharmacy.ualberta.ca
>Faculty of Pharmacy Phone: (403) 492-4493
>University of Alberta FAX: (403) 492-5305
>Edmonton, Alberta, Canada
Richard A. Laursen
Department of Chemistry
Boston University
590 Commonwealth Ave.
Boston, MA 02215
Tel (617) 353-2491; FAX (617) 353-6466
email: <laursen@bu.edu>