Synthesis and stereochemistry of 23,24-dihydrocalysterol: implications for marine sterols of a unified biosynthetic scheme involving protonated cyclopropanes

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Abstract
The partial synthesis of the marine cyclopropyl-sterol dihydrocalysterol (1), which has allowed the confirmation of the originally postulated absolute stereochemistry around the cyclopropane ring, is described. The firm knowledge of this stereochemistry and that of the other marine sterols petrosterol (7), dehydroaplysterol (21), ficisterol (22), nicasterol (8), and hebesterol (23) has allowed us to construct a unified biosynthetic scheme for these sterols of diverse side-chain structure. Our central postulate, that the intermediate in sterol side biomethylation may be described in terms of a protonated cyclopropane species [i.e. protonated dihydrocalysterol (1)], enables us to interrelate the absolute stereochemistries of the above sterols and predict the biosynthetic origin of the side-chain carbon atoms.