Review of chemical syntheses of 7‐keto‐Δ⁵‐sterols

Abstract

Steroids bearing ketone functionality at carbon-7 are found commonly in nature, and the most prevalent of these are the 7-keto-delta5-sterols. These substances have diverse biological properties and are present in biological samples and food products. For the purpose of studying this class of oxysterols, many chemical methods, involving the chemical oxidation of delta5-sterols to the corresponding 7-keto-delta5-sterol derivatives have been developed to produce these compounds. We have undertaken a review and evaluation of chemical methods for the synthesis of these compounds and have endeavored to enhance one of these procedures to yield products for chemical and biological investigations.

Bismuth(III) Reagents in Steroid and Terpene Chemistry

“…The allylic oxidation of Δ 5 -steroids [ 42 , 43 ] using several homogeneous or heterogeneous bismuth catalysts in combination with t -BuOOH has been recently reported [ 44 , 45 ]. BiCl 3 was found to be the best catalyst and several Δ 5 -steroids were converted into the corresponding Δ 5 -7-oxosteroids in good to high yields ( Scheme 5 ).…”

An Effective Method for Allylic Oxidation of Δ5-Steroids Using tert-Butyl Hydroperoxide

“…Because of the biological and physiological properties of their 7-keto-Δ 5 -steroid oxidation products, Δ 5 -steroids have attracted attention as highly useful synthetic building blocks. − Easy access to the 7-keto-Δ 5 -steroids can be achieved via allylic oxidation of Δ 5 -steroids. Although a variety of chromium(VI) compounds have been used in the synthetic modifications of steroids, ,− complications in applying these methods remain because of the harsh reaction conditions required and difficult workup and/or purification procedures. In addition, the accumulations of chromic acid or chromium salt wastes that are the side products of these reactions are of great environmental concern .…”

Metal‐Free Allylic Oxidation of Steroids Using TBAI/TBHP Organocatalytic Protocol

“…Next, cholesterol ( 1 b ) was subjected to the investigation. Many of the allylic oxidation methods are not compatible with unprotected hydroxyl group at the C(3) carbon of cholesterol since it can be oxidized to ketone readily. Surprisingly, the TBAI/TPHP catalytic protocol could oxidize 1 b to yield 3β‐hydroxycholest‐5‐en‐7‐one ( 2 b ) as the major product in 37 % yield, suggesting that the allylic oxidation protocol has high functional group compatibility (Scheme , Eq.…”