Cucurbitacins fromCucurbita texana: Evidence for the role of isocucurbitacins

In addition to cucurbitacins E andI, cucurbitacins D,1, 3-epi-isocucurbitacin D,2, and B,3, were isolated from the fruits ofCucurbita texana and structurally identified by UV, IR,¹H NMR,¹³C NMR, and MS, and 2-O--glucopyranosylcucurbitacin I was identified. These compounds have not been reported previously as constituents of this species. The isolation of 3-epi-isocucurbitacin D2 together with normal cucurbitacins suggests that isocucurbitacins occur naturally. Evidence is also discussed that isocucurbitacins are biosynthesized one step ahead of normal cucurbitacin.