Ozonolyses of 1-Alkyl-Substituted 3-Methylindenes. Remarkable Effects of the Substituent Steric Bulk and the Stereochemistry of the Carbonyl Oxide Intermediates on the Efficiency of Ozonide Formation

Ozonolyses of 3-methyl- and 3-isopropyl-l-methylindenes (1a,b) in ether gave mainly the corresponding oligomers 3a,b, while in the case of 3-phenyl- and 3-tert-butyl-1-methylindenes (1c,d), the corresponding exo-endo mixtures of ozonides 2c,d were obtained in good yield, the exo isomer being the major one. The ozonolyses of two stereoisomeric keto vinyl ethers, (E)-16 and (Z)-16, demonstrated that the stereochemistry of the derived carbonyl oxide 5c exerts a remarkable influence on the course of the reaction. Particularly interesting is the fact that syn-5c having a proper geometry for the concerted cycloaddition provided ozonide 2c in high yield even in the presence of trifluoroacetophenone. The high degree of similarity in the nature and distribution of the products from the ozonolyses of keto olefin (E)-16 and 1-methyl-3-phenylindene (1c) would be consistent with their respective reactions proceeding predominantly through a common carbonyl oxide intermediate syn-5c.