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, resp. The catalyst precursor consisting of a mixt. of Pd on carbon and 3.9 equiv of AsPh3 as well as that obtained by treatment of Pd(OAc)2 with 4 equiv of AsPh3 in THF at 60° can conveniently replace Pd(PPh3)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 prepd. by reaction of a 9-alkyl-9-BBN deriv. with (Z)-2 in DMF soln., 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, Et (E)-2-bromo-3-[(2-methoxymethoxy)phenyl]propenoate, (E)-4e, represents a direct precursor to 3-bromocoumarin, 7, and compd. (Z)-8, which is obtained by acidic hydrolysis of (Z)4e, undergoes a Pd-catalyzed intramol. carbonylation reaction, which affords 3-ethoxycarbonylcoumarin, 9, in high yield. Moreover, treatment of compds. (Z)- and (E)-4 with organozinc or organotin derivs., in the presence of catalytic amts. of a suitable Pd catalyst, affords, stereospecifically and in satisfactory to excellent yields,(E)- and (Z)- trisubstituted α,β-unsatd. esters,resp.( e.g.(E)- I), which cannot be easily prepd. in stereoisomerically pure form by classical procedures. One of these esters, i.e. compd. (E)-I, represents a direct precursor to an isoaurone, i.e. (E)-3-benzylidene-benzofuran-2-one, (E)-II. Finally, a representative stereo-defined unsym. 2,3-diaryl-disubstituted alkyl propenoate has been synthesized by a one-pot procedure involving two sequential palladium-catalyzed arylations of (Z)-2.
|Autori:||ROSSI R; BELLINA F; CARPITA A; GORI R|
|Titolo:||Alkyl (E)- and (Z)-2,3-dibromopropenoates as precursors to 3-substituted alkyl (E)- and (Z)-2-bromopropenoates, 2,3-disubstituted alkyl (Z)- and (E)-propenoates and some heterocyclic compounds|
|Anno del prodotto:||1995|
|Appare nelle tipologie:||1.1 Articolo in rivista|