THE INFLUENCE OF REMOTE HETEROATOM SUBSTITUENTS ON THE STEREOSELECTIVITY OF CYCLOPENTENE OZONOLYSIS

The stereoselectivity for ozonide formation from cyclopentenes with allylic heteroatom substituents has been examined. Silyl ethers give high selectivity in favor of the exo-substituted ozonide, while esters of 2-cyclopentenol form ozonide mixtures with little stereoselection. Trapping experiments establish that fragmentation of the primary ozonides is highly regioselective to give only one of the isomeric omega-oxo carbonyl oxide intermediates and that the variation in final ozonide stereochemistry results from differing interactions between the remote substituent groups and the carbonyl oxide system during cyclization of this intermediate. The effect of substituent and solvent on ozonide stereochemistry suggests a dominant role for electrostatic repulsion between carbonyl oxide and the heteroatom substituent during intramolecular cycloaddition. A chair-like transition state is proposed for this process and is in accord with the increase in exo selectivity for derivatives of 2-methyl-2-cyclopentenol.