The syntheses of 24-methylidene[24-14C]cholesterol (7a) and of 24-methylidene[7-3H]cholesterol (7b) from commercially available (20S)-3-oxopregn-4-ene-20-carbaldehyde (1) are described. The method also provides simple preparations of 3P-aceto~y[24-'~C]chol-5-en-24-oic acid (4) and 24-oxocholest-5-en-3~-yl acetate (6b).
Introduction. -In connection with our studies on the biosynthesis of withanolides in Acrzistus breviflorus [ 11, we required 24-methylidenecholesterol labelled with I4C at C(24) (see 7a) and with 3H at C(7) (see 7b) in order to carry out double-labelling experiments.
Previous syntheses of labelled 24-methylidenecholesterol afforded the sterol labelled with 14C or 3H at position 28 by a Wittig reaction between 24-oxocholest-5-en-3~-yl acetate (6b) and appropriately labelled methylidene(tripheny1)phosphorane [2]. We now describe a synthesis of the I4C-labelled sterol from (20S)-3-oxopregn-4-ene-20-carbaldehyde (1) via 3~-aceto~y[24-~~C]choI-5-en-24-oic acid (4) and 24-0~0[24-~~C]choIest-5-en-3p-yl acetate (6a), the latter being converted into 24-methylidene[24-14C]cholesterol (7a) by an improved procedure (for previous preparations of labelled 4, see [3]). The 3H-label was introduced on unlabelled 24-oxocholest-5-en-3P-yl acetate (6b) which was then converted to the 3H-labelled 24-methylidenecholesterol7b.
Results and
Discussion. --Reaction of (20S)-3-oxopregn-4-ene-20-carbaldehyde (1) with [(ethoxy['4C]carbonyl)methylidene]triphenylphosphorane in dry MeCN afforded the (E)-ester 2 in high yield. The latter was converted to the 3-en01 acetate by treatment with AcCl/Ac,O and reduced to the 3P-hydroxy-5-ene steroid with NaBH, in 70 %I EtOH. Saponification of the ester group (-+3), acetylation, and hydrogenation afforded 3P-ace-'H-NMR, MS) to 7a obtained above.