The reaction of 3-methylcyclopentene
with mchloroperbenzoic acid provides a mixture of epoxides in which the ci.r-isbmer predominates. Alternatively, treatment of the olefm with N-bromosuccinimide (NBS) and alkali provides a mixture rich in the trans-epoxide. The isomeric epoxides were characterized by their NMR spectra, including measurements of europium induced shifts, and their stereochemistry proven by examination of their lithium aluminium hydride reduction products.
A simple conformational argument is presented to account for the isomer distribution in the epoxidations IN CONNECTION with studies on a new approach to iridoid synthesis,' we required, as starting materials, the isomeric 3-methylcyclopentene oxides, II and III. While these could be obtained conveniently by oxidation of 3-methylcyclopentene (I), their planned use in stereospecific reaction sequences required a rigorous determination of their stereochemistry.
The presumption that the Me group in I would exert a simple steric hindrance effect24 on the approach of the co-reactant formed the basis for our initial structure assignmenta Thus, the major isomer obtained by treatment of I with m-chloroperbenzoic acid was assigned the trans structure III and, corroboratively, the major isomer resulting from the two stage (NBS/OH-) treatment of I was alloted the cis structure II. These arguments exactly parallel those used by Henbest and McCullough2 to describe the results obtained on epoxidation of 4-methylcycl+ pentene (IV). However, the next step in our synthetic sequence' afforded results difficult to rationalize on the basis of these naive assignments Therefore., armed with authentic samples of all the isomeric 2-and 3-methylcyclopentanols, the epoxides II and III were subjected to lithium aluminum hydride reduction.2 The results of these experiments (Experimental) unambiguously required the reversal of our initial assignments. Accordingly, the correct formulation of the epoxidation results are illustrated in Fig. 1.7
The NMR data collected in the table below show that the methyl doublet in the c&isomer II appears at slightly lower field than the corresponding doublet in the trans-isomer III. These relative shifts constitute a reversal of the situation observed for 3-methylcyclohexene oxides4 The signals arising from the protons on the oxide l This article is considered Part V in the series "Studies on Terpenes." For Part IV, see R. A. Finnegan and P. L. Bachman, J. Org. Chem.. 36, 31% (1971). 7 Sometime after this work was completed, we noted a report' which included the epoxidation of I; however, no indication of the stereochemistry of the product was given.