期刊
TETRAHEDRON
卷 61, 期 11, 页码 2803-2814出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.tet.2005.01.077
关键词
cephalostatin; radical reaction; hydrogen abstraction; alkoxyl radical; steroid; spirostan sapogenin
A simple synthesis of all eight C-22, C-23, and C-25 diastereoisomers, of the cephalostatin north 1 side chain has been accomplished from (25R)-5 alpha-spirostan-3 beta-ol (tigogenin). The synthesis involves selective hydroxylations at C-23 and C-25 and reductive opening of the 1,6-dioxaspiro[4.5]decane spirostan system to give a conveniently protected 5 alpha-furostan-3 beta,23,25,26-tetrol. The construction of the required 1,6-dioxaspiro[4.4]nonane system entailed an intramolecular hydrogen abstraction reaction promoted by the C-25 alkoxyl radical as the key step. Acid-catalyzed isomerization of the spiroketal unit suggested that 22R isomers are the thermodynamic products while the 22S isomers are the result of kinetic control. The acid-catalyzed equilibrium between 1,6-dioxaspiro[4.4]nonane and 1,6-dioxaspiro[4.5]decane systems was also studied. In the 1,6-dioxaspiro[4.4]nonane units, the observed (3)J(23,24) coupling constants suggest that the five-membered puckered ring-F undergoes substantial conformational changes on going from 22S to 22R isomers. (c) 2005 Elsevier Ltd. All rights reserved.
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