The synthesis of a d'carbapenem and two ,?-lactams possessing a Br-atom at the N-substituting center not involved in the lactam ring and bearing the carboxyl group is described. The ,?-lactams having this kind of Br-substitution are more susceptible to nucleophilic attack than those having a conjugated double bond with the N-atom of theb-lactam ring. DBU is found to be an excellent reagent for the elimination of the silyloxy function. Moreover, a simple method for the addition of diethyl phosphite to an a$-unsaturated double bond using a catalytic amount of NaH is described.
As part of a continuing program to prepare nonclassical B-lactam antibiotics, we synthesized /?-lactams 9,15, and 16. The method used to prepare the monocyclic precursors 4 derives from that developed by Doyle et al. [l] and by ourselves [2-61.
L-Serine (la) and L-threonine (lb) were converted to their esters 2a and 2b, respectively (100 %). Treatment of 2a,b with (tert-buty1)dimethylsilyl or trimethylsilyl chloride gave compounds 3a, 3b, and 3'a in excellent yield. Reactions of 3a,b with cinnamaldehyde
=Phthalirnido. X-H 15 R'=MeO. R'=PhC(OMe)=CH. R'=Phthalimida. X=Br 9 R=PO(OEt),. Fl=Phlbalirnido 14 Ft=PhliIalirnido 16 FI=Phthalimido 12 fl=(t~Bu)Me,SiO. Ft=Phthalirnido