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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

AutorBetancor, Carmen; Freire, Raimundo ; Pérez-Martín, Inés ; Prangé, Thierry; Suárez, Ernesto
Palabras claveCephalostatin
Radical reaction
Hydrogen abstraction
Alkoxyl radical
Spirostan sapogenin
Fecha de publicación9-feb-2005
CitaciónTetrahedron 61(11): 2803-2814 (2005)
ResumenA 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.
Descripción12 pages, 2 figures, 1 table, 2 schemes.-- Printed version published Mar 14, 2005.-- Supporting information (14-page Word file) available at: http://dx.doi.org/10.1016/j.tet.2005.01.077
Versión del editorhttp://dx.doi.org/10.1016/j.tet.2005.01.077
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