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Title of article :
Hydrogen atom transfer methodology for the synthesis of C-22, C-23, and C-25 stereoisomers of cephalostatin north 1 side chain from spirostan sapogenins
Author/Authors :
Carmen Betancor، نويسنده , , Raimundo Freire، نويسنده , , Inés Pérez-Mart?n، نويسنده , , Eric de la Fortelle and Thierry Prangé، نويسنده , , Ernesto Su?rez، نويسنده ,
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هفته نامه با شماره پیاپی سال 2005
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Abstract :
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)-5α-spirostan-3β-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 5α-furostan-3β,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 3J23,24 coupling constants suggest that the five-membered puckered ring-F undergoes substantial conformational changes on going from 22S to 22R isomers.
Keywords :
Steroid , Spirostan sapogenin , Cephalostatin , radical reaction , Alkoxyl radical , hydrogen abstraction
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