Ozonolyses of 1,2,3,3,4,4,5,5-octamethyl-(la), 1,2,3,3,4,4,5-heptamethyl-(lb), and 1,2,3,3,4,4-hexamethyl cyclopentenes (IC) in methanol did not yield the ordinary hemiperacetals but gave the corresponding ozonides 6a-c instead. "he ozonides 6a,b were extremely stable and remained intact even when refluxed with triphenylphosphine in tetrahydrofuran. Cycloreversion of the primary ozonides from unsymmetrically substituted 1,2,3,3-tetramethylcyclopentene (la) and 1,5,5-trimethylcyclopentene (lg) was highly regioselective to yield the intermediate w-oxo carbonyl oxides with the geminal methyl groups remote from the carbonyl oxide groups.The basic mechanism that describes the ozonolysis of a n alkene to produce a 172,4-trioxolane (secondary ozonide) evolved during the 1950's and is known as the Criegee mechanism.l It consists of three steps. The first step is a [3 + 21 cycloaddition reaction of ozone with the alkene leading to formation of a primary ozonide (1,2,3-trioxolane). The second is a cycloreversion process of the primary ozonide to provide the transient carbonyl oxide and a stable carbonyl compound, which may proceed in two different ways in the case of unsymmetrically substituted alkenes. Finally, recombination of the carbonyl oxide and the carbonyl compound gives the 1,2,4-trioxolane. In alcohols, however, the carbonyl oxide is generally captured by the solvent to yield the corresponding hemiperacetals. ' We report now that in ozonolyses of highly methylated cyclopentenes the methyl substituents exert a remarkable influence on the course of the reaction and also on the reactivity of the resulting ozonides toward triphenylphosphine.
Results and Discussion
Ozonolysis of Highly Methylated Cyclopentenes.Ozonolyses of octamethylcyclopentene (la) in ether as well as in CF3CH20WCH2C12 (1:4, v/v) or MeOWether (1:2, v/v) gave the corresponding ozonide 6a in isolated yields of 68-72%; no evidence for the formation of the corresponding epoxide 3aZa or its sequential products such as odamethylcyclopentanone2b was obtained (Scheme 1). The structure of the monoozonide 6a was unambiguously determined by X-ray a n a l y~i s .~ In analogy to the ozonolysis of compound la, Criegee and co-workers4 reported that the reaction of 1,2-dimethylcyclopentene (le) with ozone in pentane results in + Technische Hochachule Darmstadt.
