Synthesis of coruscanones A and B, metabolites of piper coruscans, and related compounds

Abstract

Base-catalyzed rearrangements of both individual 4-(acylmethylidene) butenolides and their mixtures prepared by condensation of citraconic anhydride with various phosphoranes occur successfully only in the presence of 5.2% MeONa in MeOH (molar ratio MeONa: substrate ≤ 10:1, room temperature, 1-2 h). Under these conditions, the yields of 2-cinnamoyl-4-methylcyclopent-4-ene-1,3-dione (coruscanone B) and 2-acetyl-4-methylcyclopent-4-ene-1,3-dione are 56 and 65%, respectively. With a considerable increase in the reaction temperature or the molar ratio MeONa: substrate, formal addition of MeOH to the C(4)=C(5) double bond of these triketones becomes an appreciable (or predominant) process. A reaction of coruscanone B with CH2N2 in ether gives coruscanone A as a ∼3:2 mixture of (Z)-and (E)-methyl enolates (43%); other products (10%) result from the expansion and aromatization of the five-membered ring of the triketone. The simplest analog of coruscanone B, 2-acetyl-4-methylcyclopent-4-ene-1,3-dione, reacts with CH2N 2 in a similar way. © 2010 Springer Science+Business Media, Inc.