Synthesis, X-ray analysis, and acidolysis of exo- and endo-1-methylindene ozonides

Ozonolyses of 2,3-disubstituted 1-methylindenes gave mixtures of the corresponding exo and endo ozonides. The structures of endo-1-methyl-3-phenylindene ozonide (3a) and exo-1-methyl-2,3-diphenylindene ozonide (2b) were determined by the X-ray analysis. Acidolysis of exo- and endo-1-methylindene ozonides 2a-d and 3a-d under several conditions revealed some characteristic features. (a) Antimony pentachloride or chlorosulfonic acid catalyzed the interconversion of the exo-endo ozonide isomers, the ratio in equilibrium being ca. 7:3 for all the pairs. (b) In the reaction of 3-methyl-substituted indene ozonides 2c,d and 3c,d in acetic-d₃ acid-d a rapid hydrogen-deuterium exchange on the bridgehead methyl was observed. (c) In methylene chloride or acetic acid the ozonides decomposed very slowly, yielding the mixtures of rearranged products 4 and 5, carboxylic acid 6, and diketone 7; both the rate of decomposition and the product distribution being a marked function of the structure of the ozonides. (d) Methanol accelerated the decomposition of ozonides, the disappearance following pseudo-first-order kinetics. The rate decreased in the order 3a > 2a > 2b > 2c ? 3c > 3b. This order was significantly different from the order observed in the reaction in acetic acid: 3b > 3c > 2b > 2c > 3a > 2a. (e) Reduction by AlHCl₂ gave a mixture of 3,4-dihydro-1H-2-benzopyrans 19-22, the composition being significantly varied depending on the stereochemistry of the ozonides. (f) Treatment of 2a or 3a with bis(a-hydroxy-4-methylbenzyl) peroxide (31) gave two stereoisomeric peroxides 32a and 33a in roughly equal amounts. In contrast, the reaction of an exo ozonide 2d with a mixture of p-tolualdehyde and 30% H₂O₂ (an equivalent of 31) afforded exclusively the corresponding exo peroxide 32d, while the endo isomer 3d gave predominantly the endo peroxide 33d. © 1983 American Chemical Society.