Convergent Total Synthesis of (+)-Calcipotriol: A Scalable, Modular Approach to Vitamin D Analogs

Vitamin D is a group of seco-steroids with diverse bioactivities. An enormous amount of effort was expended by medicinal chemists to search for Vitamin D analogs that could exhibit pro-differentiating and antiproliferative effects on normal and cancer cells as well as immunomodulatory effects without causing hypercalcemia. A convergent approach for the total synthesis of calcipotriol (brand name: Dovonex), a proven Vitamin D analog used for the treatment of psoriasis, and medicinally relevant synthetic analogs is described. Given the rich synthetic history of the Vitamin D family, a complete novel approach towards both the A-ring and CD-ring is reported. From a retrosynthetic standpoint, hidden symmetry within the decorated A-ring is disclosed, which allowed for scalable quantities of this advanced intermediate. In addition, a radical retrosynthetic approach is described, which highlights an electrochemical reductive coupling as well as an intramolecular hydrogen atom transfer (HAT)-Giese addition to establish the 6,5-trans-carbon skeleton found in the Vitamin D family. Lastly, a late-stage decarboxylative cross-coupling approach allowed for the facile preparation of various C20-arylated derivatives which show promising biological activity in an early bioassay.