Photooxygenation of Pregnanes

A. PONCE, MARIA; ANDREA BRUTTOMESSO,; G. GROS, EDUARDO; ERRA BALSELLS, ROSA

HELVETICA CHIMICA ACTA

Elsevier.

Lugar: Amsterdam.; Año: 2004 vol. 87 p. 2987 - 3003

0018-019X

The course of the singlet-oxygen reaction with 17-C(20)-ene and C5,19-C(20)-diene  pregnanes  was studied. In order to compare the reactivity of two alkene moieties present in some steroid families, a serie of new 3b-ter-butyldimethylsilyloxy-pregnanes were synthesised, in which the allylic double bond unities are present at 5-C and/or at 17-C(20). The former allylic moiety was previously shown to be quite reactive in the stigmastanes family. The latter is an exocyclic alkene system with allylic character too. From commercially available 3b-hydroxy-5a-androstan-17-one and 3b-hydroxyandrosta-5-en-17-one the exocyclic double bond at 17-C(20) was introduced preparing the 3b-terbutyldimethylsilyloxy-5a-21-norpregn-17(20)-ene (1), the 3b-terbutyldimethylsilyloxy-21-norpregna-5,17(20)-dien-16a-ol (6), the (17E) 3b-terbutyldimethylsilyloxy-pregna-5,17(20)-dien-16a-ol (7), the (17E) 3b-terbutyldimethylsilyloxy-5a-pregn-17(20)-en-16b-ol (4), the (17E) 3b-terbutyldimethylsilyloxy-5a-pregn-17(20)-en-16a-ol (3), the (17E) 3b-terbutyldimethylsilyloxy-5a-pregn-17(20)-en-16b-yl acetate (5), the (17Z) 3b-terbutyldimethylsilyloxy-5a-pregn-17(20)-ene (2) and the (17Z) 3b-terbutyldimethylsilyloxy-pregna-5,17(20)-diene (8), the (17E) 3b-terbutyldimethylsilyloxy-pregna-5,17(20)-dien-21-ol (9) and the (17E) 3b-terbutyldimethylsilyloxy-5a-pregn-17(20)-en-21-yl acetate (10), among others. The photoproducts obtained by irradiation of the pregnanes and Rose Bengal in oxygen saturated pyridine solution were isolated and characterized by 1H-, 13C-NMR and MS (EI; FAB; EIHR; ESI- and UV-MALDI-TOF) data. In all the examples studied clearly was observed that the exocyclic double bond at 17-C(20) reacted.  Among the products isolated majority were those formed through the ene rection involving as intermediates the hydroperoxydes and the cyclic tautomers of the allylic hydroperoxydes called dioxolanes (allylic rearrangement).