Studies on vitamin D (calciferol) and its analogs. 23. Effect of 3-methyl substituents on the thermal [1,5]- and [1,7]-sigmatropic hydrogen shifts of vinylallenes and other seco steroids related to vitamin D: synthesis of 3-methyl- and 3,3-dimethyl-substituted analogs of 3-deoxy-1.alpha.,25-dihydrox