Parallel synthesis of a vitamin D₃ library in the solid-phase

A highly efficient synthesis of the vitamin D₃ system on solid support is described. Two synthetic strategies for the solid-phase synthesis of vitamin D₃ were developed. One is for 11-hydroxy analogues, and the other is for most other synthetic analogues. In the latter strategy, the sulfonate-linked CD-ring 58 was initially immobilized on PS-DES resin to give solid-supported CD-ring 63 (Scheme 10). Similarly, solid-supported CD-ring 63 was prepared by attachment of the CD-ring 10 to the chlorosulfonate resin 64. The vitamin D₃ system was synthesized by Horner-Wadsworth-Emmons reaction of the A-ring phosphine oxide to a solid-supported CD-ring, followed by simultaneous introduction of the side chain and cleavage from resin with a Cu¹-catalyzed Grignard reagent. Parallel synthesis of the vitamin D₃ analogues was accomplished by a split and pool methodology utilizing radio frequency encoded combinatorial chemistry, and a manual parallel synthesizer for side chain diversification and deprotection. Additionally, we demonstrated the synthesis of various A-rings in a similar protocol for efficient preparation of building blocks.