19-Methyl Analogs of Vitamin D3: Synthesis and Structure Elucidation by 1HNMR

Synthesis of three isomeric 19-methylated vitamin D3 derivatives is described. Two different synthetic paths were used, both involving 3,5-cyclovitamin D precursors easily obtainable from commercial vitamin D3. The crucial step of the first route consisted of an acid-catalyzed cycloreversion of isomeric 19-methyl-3,5-cyclovitamin D3 compounds, whereas in the other it involved a Wittig reaction of 10-oxo-19-norvitamin analogs. Neither iodine-catalyzed nor thermal isomerization of the 5Z,10E-isomer provided detectable quantities of the fourth possible isomeric vitamin with a 5Z,10Z-configuration. Structures, stereochemistries and A-ring conformations of the final 19-methyl vitamin D3 analogs were tentatively established on the basis of their 500-MHz 1H NMR spectra and conformational analysis. Application of 1H NOE difference spectroscopy andmolecular modeling studies allowed for the assignment of preferred solution conformations of the 3,5-cyclovitamin D3 intermediates.