Sterols of Cucurbitaceae: the configurations at C-24 of 24-alkyl-delta 5-, delta 7- and delta 8-sterols

The major sterols of the seeds of Benincasa cerifera, Cucumis sativus, Cucurbita maxima, C. pepo and Trichosanthes japonica and of the mature plant tissues (leaves and stems) of Citrullus battich, Cucumis sativus and Gynostemma pentaphyllum of the family Cucurbitaceae were 24‐ethyl‐Δ 7 ‐sterols which were accompanied by small amounts of saturated and Δ 5 ‐and Δ 8 ‐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‐Δ 7 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 resolution 1 H NMR and 13 C NMR spectroscopy. Most of the 24‐methyl‐ and 24‐ethylsterols examined which lack a Δ 25(27) ‐bond (i.e., 24‐methyl‐, 24‐methyl‐Δ 22 ‐, 24‐ethyl‐ and 24‐ethyl‐Δ 22 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 Δ 25(27) (i.e., 24‐ethyl‐Δ 25(27) ‐and 24‐ethyl‐Δ 7,22,25(27) ‐sterols) were, on the other hand, composed of only 24β‐epimers. The Δ 8 ‐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 Δ 8 ‐sterols lacking a 4‐methyl group in higher plants, and among the four Δ 8 ‐sterols the latter two are considered to be new sterols. The probable biogenetic role of the Δ 8 ‐sterols and the possible biosynthetic pathways leading to the 24α‐ and 24β‐alkylsterols in Cucurbitaceae are discussed.