When applying the principles of Restricted Rotatory Systems to Spiranes and Alkylidene cycloalkanes, — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

In topological stereochemistry, a rigid ring is treated mechanically identically to a $\pi$ bond. An alkylidene cycloalkane (1 ring + 1 $\pi$ bond) has 2 stereogenic units, making it an 'even' system with perpendicular ends (no GI). A standard two-ring spirane has 2 units, making it 'even' with perpendicular ends (no GI). However, a three-ring spirane has 3 units, making it an 'odd' system with a strictly co-planar environment capable of showing GI.