Structural resolution of the stereochemistry of a spirooxirane derived from an α‐arylidene heterocyclic carbonyl. The crystal and molecular structure of 2‐( p ‐chlorophenyl)‐5‐phenyl‐7‐methyl‐1‐oxa‐5,6‐diazaspiro[2.4]hept‐6‐en‐4‐one