3-(1-alkynyl)- and 3-aryl-substituted alkyl (Z)- and (E)-2-bromopropenoates have been stereospecifically and regioselectively synthesized by Pd-catalyzed cross-coupling reactions between 1-alkynyl-zinc chlorides, arylzinc chlorides or aryltributylstannanes and alkyl (Z)- and (E)-2,3-dibromopropenoates, (Z)- and (E)-2, respectively. The catalyst precursor 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 equivalents of AsPh, in THF at 60 degrees C can conveniently replace Pd(PPh(3))4 in the reactions between (Z)- or (E)-2 and aryl or 1-alkynylzinc halides. On the other hand, a representative alkyl (Z)-3-alkyl-2-bromopropenoate has been prepared by reaction of a 9-alkyl-9-BBN derivative with (Z)-2a in DMF solution, in the presence of K2CO3 and a catalytic quantity of PdCl2(dppf). Some applications demonstrate the synthetic utility of 3-aryl-substituted alkyl (Z)- and (E)-propenoates, (Z)- and (E)-4. Thus, ethyl (E)-2-bromo-3-[(2-methoxymethoxy)phenyl]propenoate, (E)-4e, represents a direct precursor to 3-bromocoumarin, 7, and compound (Z)-8, which is obtained by acidic hydrolysis of (Z)-4e, undergoes a Pd-catalyzed intramolecular carbonylation reaction, which affords 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, trisubstituted alpha,beta-unsaturated esters, (E)- and (Z)-10, respectively, which cannot be easily prepared in stereoisomerically pure form by classical procedures. One of these esters, i.e. compound (E)-10c, represents a direct precursor to an isoaurone, i.e. (E)-3-benzylidene benzofuran-2-one, (E)-11. Finally, a representative stereodefined unsymmetrically 2,3-diaryl-disubstituted alkyl propenoate has been synthetized by a one-pot procedure involving two sequential palladium-catalyzed arylations of (Z)-2a.
|Autori:||Rossi, R; Bellina, Fabio; Carpita, Adriano; Gori, R.|
|Titolo:||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|
|Anno del prodotto:||1995|
|Appare nelle tipologie:||1.1 Articolo in rivista|