Sterols of cucurbitaceae: The configurations at C‐24 of 24‐Alkyl‐Δ5‐,Δ7‐ and Δ8‐sterols

Abstract

The major sterols of the seeds ofBenincasa cerifera, Cucumis sativus, Cucurbita maxima, C. pepo andTrichosanthes japonica and of the mature plant tissues (leaves and stems) ofCitrullus battich, Cucumis sativus andGynostemma pentaphyllum of the family Cucurbitaceae were 24-ethyl-Δ⁷-sterols which were accompanied by small amounts of saturated and Δ⁵-and Δ⁸-sterols. The 24-ethyl-Δ^7,22,Δ^7,25(27) and Δ^7,22,25(27)-sterols constituted the predominant sterols for the seed materials, whereas the 24-ethyl-Δ⁷ and Δ^7,22-sterols were the major ones for the mature plant tissues. The configurations of C-24 of the alkylsterols were examined by high resolution¹H NMR and¹³C NMR spectroscopy. Most of the 24-methyl- and 24-ethylsterols examined which lack a Δ²⁵⁽²⁷⁾-bond (i.e., 24-methyl-, 24-methyl-Δ²²-, 24-ethyl- and 24-ethyl-Δ²² sterols) were shown to occur as the C-24 epimeric mixtures in which the 24α-epimers predominated in most cases. The 24-ethylsterols which possess a Δ²⁵⁽²⁷⁾ (i.e., 24-ethyl-Δ²⁵⁽²⁷⁾-and 24-ethyl-Δ^7,22,25(27)-sterols) were, on the other hand, composed of only 24β-epimers. The Δ⁸-sterols identified and characterized were four 24-ethyl-sterols: 24α-and 24β-ethyl-5α-cholesta-8,22-dien-3β-ol, 24β-ethyl-5α-cholesta-8,25(27)-dien-3β-ol and 24β-ethyl-5α-cholesta-8,22,25(27)-trien-3β-ol. This seems to be the first case of the detection of Δ⁸-sterols lacking a 4-methyl group in higher plants, and among the four Δ⁸-sterols the latter two are considered to be new sterols. The probable biogenetic role of the Δ⁸-sterols and the possible biosynthetic pathways leading to the 24α- and 24β-alkylsterols in Cucurbitaceae are discussed.