Novel versatile approach to an enantiopure 19-nor, des-C,D vitamin D3 derivative

A short and efficient de novo route to the des-C,D vitamin D3 derivative 3 (Ro 65-2299), a potential antipsoriatic, has been developed. This route features an assembly strategy so far unexplored in vitamin D chemistry involving a modified Julia olefination of the A-ring ketone 30 and the 2-benzothiazolyl sulfone 60. Construction of the A-ring building block was accomplished by an efficient three-step route starting from the meso trans-1,3,5-cyclohexane triol (26), which was desymmetrized by a highly selective enzymatic mono-hydrolysis of the corresponding triacetate 27 followed by oxidation of the alcohol 29 to give the homochiral diacetoxy ketone 30 (ee=99.5%) in 83% overall yield. Furthermore, we found efficient and practical syntheses of the 5-acetoxy-2-cyclohexenone (31) and its enantiomer 32, both new building blocks useful for natural product synthesis.