ALKYL (E)-2,3-DIBROMOPROPENOATES AND (Z)-2,3-DIBROMOPROPENOATES AS PRECURSORS TO 3-SUBSTITUTED ALKYL (E)-2-BROMOPROPENOATES AND (Z)-2-BROMOPROPENOATES, 2,3-DISUBSTITUTED ALKYL (Z)-PROPENOATES AND (E)-PROPENOATES AND SOME HETEROCYCLIC-COMPOUNDS

3-(1-alkynyl)- and 3-aryl-substituted alkyl (Z)- and (E)-2-bromopropen oates have been stereospecifically and regioselectively synthesized by Pd-catalyzed cross-coupling reactions between 1-alkynyl-zinc chloride s, arylzinc chlorides or aryltributylstannanes and alkyl (Z)- and (E)- 2,3-dibromopropenoates, (Z)- and (E)-2, respectively. The catalyst pre cursor consisting of a mixture of Pd on carbon and 3.9 equivalents of AsPh(3) as well as that obtained by treatment of Pd(OAc)(2) with 4 equ ivalents of AsPh, in THF at 60 degrees C can conveniently replace Pd(P Ph(3))4 in the reactions between (Z)- or (E)-2 and aryl or 1-alkynylzi nc halides. On the other hand, a representative alkyl (Z)-3-alkyl-2-br omopropenoate has been prepared by reaction of a 9-alkyl-9-BBN derivat ive with (Z)-2a in DMF solution, in the presence of K2CO3 and a cataly tic quantity of PdCl2(dppf). Some applications demonstrate the synthet ic utility of 3-aryl-substituted alkyl (Z)- and (E)-propenoates, (Z)- and (E)-4. Thus, ethyl E)-2-bromo-3-[(2-methoxymethoxy)phenyl]propenoa te, (E)-4e, represents a direct precursor to 3-bromocoumarin, 7, and c ompound (Z)-8, which is obtained by acidic hydrolysis of (Z)-4e, under goes a Pd-catalyzed intramolecular carbonylation reaction, which affor ds 3-ethoxycarbonylcoumarin, 9, in high yield. Moreover, treatment of compounds (2)- and (E)-4 with organo-zinc or organotin derivatives, in the presence of catalytic amounts of a suitable Pd catalyst, affords, stereospecifically and in satisfactory to excellent yields, trisubsti tuted alpha,beta-unsaturated esters, (E)- and (Z)-10, respectively, wh ich cannot be easily prepared in stereoisomerically pure form by class ical procedures. One of these esters, i.e. compound (E)-10c, represent s a direct precursor to an isoaurone, i.e. (E)-3-benzylidene benzofura n-2-one, (E)-11. Finally, a representative stereodefined unsymmetrical ly 2,3-diaryl-disubstituted alkyl propenoate has been synthetized by a one-pot procedure involving two sequential palladium-catalyzed arylat ions of (Z)-2a.