An alkylative cycloaddition method is provided that is particularly useful for the synthesis of many of the Vitamin D analogues with differing side chains. Thus, a preferred synthesis is of Vitamin D analogues having a side chain R1 where a substantially geometrically pure first precursor having structure (I), and a second precursor are provided, the second precursor being a 1,7 enyne. These precursors are reacted in the presence of a palladium catalyst to form compounds having structure (II), where R2 is hydrogen, hydroxyl, lower alkoxy, fluorine, or a protecting group, and R3 is hydrogen, hydroxyl, lower alkoxy, fluorine, or a protecting group.