Abstract
The vitamin D3 structure consists of the A-ring, a linker originating from the B-ring, C-ring, D-ring, and side-chain moieties. Each unit has its unique role in expressing the biological activities of vitamin D3. Many efforts have been made to date to assess the possible clinical use of vitamin D. Some organic chemists focused on the D-ring structure of vitamin D and synthesized D-ring-modified vitamin D analogues, and their biological activities were studied. This review summarizes the synthetic methodologies of D-ring-modified vitamin D analogues, except for seco-D, and their preliminary biological profiles.
Highlights
We focus on the D-ring, which is one of the essential sites forming the basic framework of the vitamin D3 molecule
We describe the synthetic approach to vitamin D3 analogues with substituents or structural transformation on the D-ring, except for seco-D analogues, and their preliminary biological activities
The 16-ene structure of the CD-ring part of vitamin D3 first appeared in the total synthesis of 1α,25(OH)2 D3 (2) reported by the Hoffmann-La Roche group in 1982 [51]
Summary
Each ketone (22,23) and the racemic A-ring moiety (24) were coupled using PhLi, and protecting groups of the coupling products were removed in the presence of TBAF to afford 16-ene analogues (9–12) (Scheme 3). A Wittig–Horner coupling reaction between a racemic phosphine oxide (24) and the 8-keto-CD-rings (40,41), followed by deprotection of the silyl protecting groups, produced the target analogues (30–33) (Scheme 5). Desilylation of 52 and 53, followed by oxidation at the C8-OH group, afforded ketones (54,55) These ketones (54,55) were converted into the desired vitamin D3 analogues (45–48) using the convergent coupling reaction with a lithium anion of the racemic A-ring moiety (56) (Scheme 7). In 2019, Okamoto and coworkers reported the synthesis of 16-oxa-vitamin D3 analogues (169,170) [29] They synthesized the 16-oxa-CD-ring moiety (171) starting from.
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