Biosynthesis of 24β-ethylsterols in cultured cells of Trichosanthes kirilowii(cucurbitaceae) fed with [1,2-13C2]- and [2-13C2H3]-acetate: reinvestigation of the stereochemistry at C-25

Abstract

[¹³C]- and [¹³C²H]-Enriched 24β-ethylsterols, 24β-ethyl-25-dehydrolophenol (14) and (18), 22-dihydro-25-dehydrochondrillasterol (15) and (19), 22-dihydrochondrillasterol (16) and (20), and chondrillasterol (17) and (21) have been isolated from cultured cells of Trichosanthes kirilowii Maxim. var. japonica fed with [1,2-¹³C₂]- and [2-¹³C²H₃]-acetate. The most probable biosynthetic mechanism of the 24β-ethyl side-chain (27) from the 24-methylene side-chain (24) is that (i) the second methyl group attacks the C-28 of 24-methylenecycloartanol (24), (ii) 25-H migrates back to the 24-Re-face, (iii) 26-H is eliminated to form the 25(26) double bond (26), and (iv) the protonation at C-25 of the Δ²⁵-sterols takes place from the Si-face to form the 24β-ethyl-25-saturated side-chain (27). The labelling patterns also indicate that reduction of the 22(23) double bond is not a primary pathway in the formation of 22-saturated sterols and that 24β-ethylsterols are formed via cycloartenol (1A).