Conjugate addition of organozinc reagents to enones linked to a tricarbonyl iron diene unit. Asymmetric synthesis of a building block of amphotericin B

Conjugate addition of organometallics to enones linked by the ketocarbonyl to tricarbonyliron diene units can be successfully achieved with dialkylzinc reagents, when used in the presence of trimethylsilyl triflate. Organocopper reagents are not well suited, giving only incomplete and partially destructive reactions. The enone complexes are easily obtained by dehydration of aldol condensation products, which is particularly interesting when the aldol reaction is also a resolution (chiral aldehydes). The conjugate addition is not highly diastereoselective, unless the enones are substituted with polar groups (de up to 88% for γ-alkoxylated enones). This conjugate addition reaction was used to synthesize, in two practical steps from the enone 14 (two asymmetric centers), the chlorohydrin 25 (54%, five asymmetric centers), an elaborate intermediate for the synthesis of amphotericin B.