(23S,25R)-1-alpha,25-Dihydroxyvitamin D3 26,23-lactone (1a), a major metabolite of 1-alpha,25-dihydroxyvitamin D3 2, was synthesized efficiently and stereoselectively from 1-alpha-hydroxydehydroepiandrosterone (3). The 17-oxosteroid 3 was first converted to C(22)-steroid aldehyde 9 with the natural stereochemistry at C(17) and C(20) using a stereoselective ene reaction as the key step. Then it was combined with the chiral C5 sulfone 4 having the correct stereochemistry for the lactone in 1a. Sulfone 4 was readily obtained from commercially available (R)-citramalic acid. The side-chain lactone with the natural stereochemistry at C(23) was constructed with high stereoselectivity (84%) by iodo lactonization of the DELTA(22)-26-carboxylic acid 16b under kinetically controlled conditions in the presence of gamma-collidine. The stereoselectivity of the iodo lactonization of steroidal DELTA(22)-25-hydroxy-26-carboxylic acids 16b, 24, and 25 was studied in detail, and a mechanism is proposed in which the configuration at C(25) and an added pyridine base play an important role.
(23S,25R)-1-α,25-二羟基
维生素D3 26,23-内酯(1a)是1-α,25-二羟基
维生素D3的主要代谢物之一,它通过从1-α-羟基脱氢
表雄酮(3)高效且有选择性地合成而得。首先,17-氧类
固醇3通过以具有立体选择性的烯反应(ene reaction)为关键步骤,被转化为C(22)类
固醇醛9,该反应保留了C(17)和C(20)位的天然立体
化学。随后,9与具有正确立体
化学以形成内酯(1a)的非对映异构C5
硫酮4相结合。
硫酮4可从商品化的(R)-柠檬
苹果酸(R)-citramalic acid)中方便地制备。在具有γ-蜂碱(γ-collidine)的动态控制条件下,通过
碘化内酯法,将Δ(22)-26-
羧酸(16b)转化为侧链内酯,该内酯在C(23)位具有天然立体
化学,且立体选择性高达84%。对类
固醇Δ(22)-25-羟基-26-
羧酸(16b、24和25)的
碘化内酯化反应进行了详细研究,并提出了一种机制,其中C(25)位的构型和加入的
吡啶碱起着重要作用。