A Peculiar Substituent Effect Operative in the Ring Inversion of trans ‐3,3,7,11‐Tetramethyl‐1,5‐dioxa‐8,9,10‐trithiaspiro[5.5]undecane

The synthesis of the novel bicyclic spiranes 3,3‐diMe‐ and trans ‐3,3,7,11‐tetraMe‐1,5‐dioxa‐8,9,10‐trithiaspiro[5.5]undecane ( 1 and 2 ) is reported. In 1 , there are four degenerate chair/chair conformers. The rate of the chair‐to‐chair inversion of the constituent 1,3‐dioxane and 1,2,3‐trithiane rings can be measured separately, yielding barriers of Δ G * 1.70 = 34.7 and Δ G * 2.50 = 52.7 kJ. mol −1 , respectively. In 2 , however, only one pair of degenerate chair‐chair conformers is observed, and the rate of the chair/chair to inverted chair/inverted chair process yields Δ G * 2.50 = 76.5 kJ. mol −1 . A mechanism rationalizing the high barrier in 2 is proposed. A geometric definition of “coupled ring inversion” is proposed.