Enzymatic Desymmetrization of 19‐ nor ‐Vitamin D ₃ A‐Ring Synthon Precursor: Synthesis, Structure Elucidation, and Biological Activity of 1α,25‐Dihydroxy‐3‐ epi ‐19‐ nor ‐vitamin D ₃ and 1β,25‐Dihydroxy‐19‐ nor ‐vitamin D ₃