Synthesis of novel vitamin D3 analog with an additional ring annulated to A and seco-B rings

A simple method for the synthesis of yet unknown 5E-vitamin D3 analogs with an additional six-membered ring connecting C-6 and C-19 was developed. Ring-closing metathesis (RCM) was used for efficient formation thereof from the corresponding 5E-isomers of 6-alkenyl vitamin D3 compounds which in turn were prepared from the 6-oxo-3,5-cyclovitamin D3. Reinvestigation of the Grignard reactions of this latter compound as well as the following acid-catalyzed cycloreversions showed discrepancies with the literature data describing the course of such processes.