Synthesis of trisubstituted furans from epoxypropargyl esters by sequential SmI2-promoted reduction-elimination and Pd(II)-catalyzed cycloisomerization.
A two-step one-pot synthesis of 2,3,5-trisubstituted furans 8 from 4,5-epoxyalk-2-ynyl esters 6 is described. The sequence is initiated by a SmI2-promoted reduction that takes advantage of the ability of the alkynyloxirane moiety present in 6 to accept electrons from SmI2. The resulting organosamarium species then eliminates an adjacent acetate or benzoate leaving group, leading to the formation of unstable 2,3,4-trien-1-ols 7. Without isolation, these are cycloisomerized to furans 8 by treatment with a catalytic amount of a Pd(II) complex and a proton source. The whole sequence takes place under mild reaction conditions. Some useful functional groups such as cyano and alpha,beta-unsaturated esters are tolerated, but benzyl- and silyl-protected hydroxyl groups are deprotected to some extent. Starting materials can be easily assembled using reliable reactions from acetylene, an aldehyde or ketone, and a vinyl halide fragment. This offers the possibility of introducing branched substituents at C-5 of the furan ring.
Aurrecoechea
J M
JM
Departamento de Química Orgánica, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain.
Pérez
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Solay
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