Studies in steroids and tetracyclic triterpenoids

The triterpenoid group of naturally occurring compounds may be classified according to the number of rings contained in their carbocylic structures. There are no known mono or dicylic triterpenoids, but acyclic and tricyclic triterpenoids are represented by aqualene (I) and ambrein (II) respectively. The pentacyclic triterpenoids include the alpha-amyrin, beta-amyrin, lupcol, taraxasterol and taraxerol series, and form the largest group. Individual members of this group possess structures which are divisible into six isopentane units and thus they conform to the "isoprene rule". In contrast, members of the smaller tetracyclic group of triterpenoide do not formally conform to the empirical isoprene rule and moreover, some members of the group have recently been shown to contain thirty one carbon atoms. Consequently the tetracyclic triterpenoid tripencide can be regarded as tri or tetramethyl steroids. The widely occurring sterol, cholesterol (III), is synthesized in vivo from acetic acid through the intermediation of swualene (I). The biosynthesis of the tetracyclic triterpenoids may be considered to involve the cyclisation of equalene, motivated by attack of the cation OH+ at the position C(3) and proceeding synchronously by the formation of the intermediates (Ia), (Ib) and (Ic). Lanosterol (IV, R - H) is obtained from (Ic) by movement of the C(13)-hydrogen atom to C(20), accompanied by the simultaneous migrations of the beta-methyl group at C(14) to position C(13), and of the alpha-methyl group at C(8) to position C(14). Cholesterol is formed biogenetically from lanosterol by loss of three carbon atoms. Other ionic mechanisms have been proposed for the biosynthesis of the pentacyclic group of triterpenoids from aqualene. The work described in Part I of this thesis is concerned solely with the tetracyclic group of triterpenoids