ON THE ALLYL PROTECTION OF THE IMIDAZOLE RING OF HISTIDINE

The regiospecific synthesis of the N(pi)-allyl and the N(tau)-allyl de rivatives of N(alpha)-tert-butoxycarbonyl-histidine is described. The DCC-mediated coupling of pha)-tert-butoxycarbonyl-N(pi)-allyl-(L)-hist idine with (L)-prolinamide, used as a test reaction to probe the exten t of racemization during coupling processes, was found to occur with g ood (ca. 97%) conservation of enantiomeric purity. Three methods have been devised, all of them based on catalytic palladium pi-allyl chemis try,for the selective removal under mild conditions of the N-imidazoli c allyl groups. Finally it is shown that the allyl group at N(tau)-pos ition may be used as a temporary protection in the synthesis of N(pi)- substituted derivatives of histidine. (C) 1997 Elsevier Science Ltd.