A Study of the Reactivity of Polyhydroxylated Sterol Derivatives

Abstract

Ad etailed study of regiospecific silylation, oxidation, and reductiono fs terols is reported. The differentr eactivity of 2a and 3a hydroxy groupsi sd emonstrated on a2 a,3a-dihydroxy-5a-pregnane-6,20-dionea samodel substrate. Ar egiospecific silylationo ft he 2a-alcohol occurred when silyl chloride was used under mild reaction conditions.The selective oxidation of one of the 2a,3a-diols by dimethyldioxirane or the Dess-Martin periodinaneled to amixture of a-hydroxyketones with high prevalence of 3,6,20-trione over 2,6,20-trione in the ratio 10:1. Ar egiospecific reduction of am odel substrateb yl ithium tri-tert-butoxyaluminium hydride gives the C6 alcohol.

Enantiospecific tritium labeling of 28-homocastasterone

“…Acetonide 4 was further oxidized at the position of C(3)‐OH by freshly synthesized DMD . The reaction performed in CH 2 Cl 2 overnight at 4°C in dark afforded the desired α‐hydroxy ketone 6 (63%) accompanied by its regioisomer 7 (7%), which is in agreement with our previous results obtained on BRs . In general, α‐hydroxy ketone 6 is supposed to be isolated in a higher yield when the oxidation of 2,3‐unprotected diols takes place; selective deprotection of acetonide by Ce(NH 4 ) 2 (NO 3 ) 6 in borate buffer followed by the DMD oxidation of the vicinal 2,3‐diol group used to provide a higher overall yield of about 5% .…”

“…We have recently reported the synthetic transformation of the vicinal 2α,3α‐diols of BRs to appropriate α‐hydroxy ketones by oxidation with a freshly generated dimethyldioxirane (DMD) . Such a α‐hydroxy ketone moiety proved to be an excellent substrate for high‐yield enantiospecific formation of 3β‐chloro‐2,3‐carbonates by a reaction with easy‐to‐handle triphosgene .…”

The labeling of brassinosteroids by tritium

“…[12][13][14] An endiol form of such an a-hydroxy ketone is a suitable substrate for synthetic transformations leading to an appropriate bis-substituted endiol 11 or chlorocarbonate. [12][13][14] An endiol form of such an a-hydroxy ketone is a suitable substrate for synthetic transformations leading to an appropriate bis-substituted endiol 11 or chlorocarbonate.…”

Recent Developments in a Radio-labeling of Brassinosteroids

“…The reaction carried out in CH 2 Cl 2 overnight at 4°C in dark afforded the desired α-hydroxy ketone 13 accompanied by its regioisomer 3-hydroxy ketone, in a ratio of 10:1 in favor of isomer 13. In general, α-hydroxy ketone 13 is supposed to be isolated in a higher yield when the oxidation of 2,3-unprotected diols takes place; a selective deprotection of acetonide by Ce(NH 4 ) 2 (NO 3 ) 6 in borate buffer followed by the DMD oxidation of the vicinal 2,3-diol group used to provide a higher overall yield of about 5% [21]. However, in the case of the 28-HCS derivative, a partial deprotectionoxidation reaction sequence afforded a drop of an isolated yield of about 15% compared to direct oxidation of the protected derivative.…”