From Quinidine to new Enantiopure Materials–Tricyclic Allylic N,O‐Acetals and a Stereospecific, Onepot Conversion of 1,2‐Secondary, Tertiary Diols into Spiroepoxides

Abstract

AbstractHydrobromination of quinidine (1) with fuming HBr furnished diastereomeric secondary bromides 2a and 2b in 82% yield. After acetylation the resulting bromides 2a–Ac and 2b–Ac could be separated and converted stereospecifically into ethylidene rubanes Z)‐4 and (E)‐4, respectively. cis‐Dihydroxylation of (Z)‐olefin 4 with OsO4 was shown to be feasible by two catalytic variants, giving the two diastereomeric diols 5a and 5b, separable by chromatography. A simple onepot procedure was developed for converting the sterically hindered 1,2‐secondary, tertiary diols stereospecifically into spiroepoxides (5a 6a‐Ac; 5b 6b‐Ac). Our procedure involves overall inversion of configuration. The procedure complements the Kolb–Sharpless route to epoxides from 1,2‐disecondary diols with overall retention of configuration. The other two diastereomeric spiroepoxides 6c and 6d were prepared in one pot under different conditions (chloramine T, then alkali). Two unprecedented tricyclic allylic N,O‐acetals (Z)‐7 and (E)‐7 were also obtained. The structure of spiroepoxide 6c (as a CH2Cl2 monosolvate) and of tricyclic olefinic N,O‐acetal (E)‐7 was corroborated by X‐ray crystallography.