Synthesis of 24(28)-Methylene-1α-Hydroxyvitamin D3, a Novel Vitamin D3 Analogue:

24(28)-Methylene-1α-hydroxyvitamin D3 was synthesised in 13 steps from vitamin D2. The key step of the synthesis involved the Wittig–Horner olefination of a nor-vitamin D2 aldehyde with diethylphosphono-3-methyl-2-butanone. The resulting enone was followed by reduced methylenation, photoisomerisation and deprotection then gave the target compound.