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ELECTROPHILIC ADDITIONS TO 10β-VINYL CHOLESTANE DERIVATIVES
Kocovsky, Pavel,Stary, Ivo,Turecek, Frantisek,Hanus, Vladimir
, p. 2994 - 3019 (2007/10/02)
Hypobromous acid addition to steroid dienes I-VI proceeds in four steps.The reaction commences by the attack on more reactive endocyclic double bond from the α-side yielding intermediary diaxial bromohydrins XXVIII, XXXIV, XL, XLIV, L and LVI via corresponding α-bromonium ions.The 10β-vinyl group of the bromohydrins then reacts with a second equivalent of the reagent forming transient 19-epimeric bromonium ions.The ions generated from I,II,V and VI are cleaved intramolecularly by the hydroxyl group, in accordance with the Markovnikov rule, giving rise to 19-epimeric dibromo epoxides XXXIa and XXXIIa, XXXVII and XXXVIII, LIIIa and LIVa , LIX and LX.By contrast, the ions generated from III and IV are cleaved in an anti-Markovnikov manner to yield dibromoepoxides XLII, XLVII and XLVIII.Products due to formation of a new C-C bond were not observed.The reaction mechanism and differences in the behavior of the dienes I-VI are discussed.
