Study of stereoselectivity of reduction of 18-oxo des-E triterpenoids by sodium borohydride in the presence of cerium chloride

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• 作者：Miroslav Kvasnica^a ; Jan Sarek^b ; ^{jan.sarek@gmail.com"" rel=""nofollow} ; Martin Vlk^b ; ^c ; Milos Budesinsky^a ; Ondrej Stepanek^a ; Tomas Kubelka^a ; Iva Plutnarova^d
• 刊名：Tetrahedron: Asymmetry
• 出版年：2011
• 出版时间：15 May 2011
• 年：2011
• 卷：22
• 期：9
• 页码：1011-1020
• 全文大小：643 K

文摘

Herein, we studied the stereoselectivity of the reduction of the 18-oxo group in cytotoxically active des-E lupane derivatives with respect to a dependence on the substituent type at the 17-position by means of sodium borohydride and also in the presence of cerium chloride. It was found that the stereoselectivity of the reduction of the 18-keto group both depended on the type and length of the substituent chain at the 17-position. In most cases, 18α-hydroxyderivatives were obtained by the assumed attack from the upper side. However, in the case of 18-oxo-17α, 17β-disubstituted derivatives and a 17-methylene derivative, a mixture of both epimeric 18α and 18β alcohols was obtained by the reduction with borohydride itself. However, if the reduction of the 18-oxo-17α, 17β-disubstituted derivatives was carried out in the presence of cerium chloride, the corresponding 18α-hydroxyalcohols were obtained exclusively; conversely, if the 17-methylene derivative was reduced, the 18β-epimer prevails.