The 92:8 equilibrium mixture of (±)-trans-methyl dihydrojasmonate (1) and its cis-isomer is transformed in 63% overall yield into the title compound 4 by epoxidation of the derived enol acetate 2 with peracetic acid/Na 2 CO 3 in toluene and heating the resulting a-acetoxy epoxide 3 in MeOH in the presence of catalytic amounts of methanesulfonic acid. This communication describes a new synthetic approach (see Scheme) to methyl 3-oxo-2-pentyl-1-cyclopentene-1-acetate (4), 1 a direct precursor of (+)-cis-methyl dihydrojasmonate, 2 an important perfumery ingredient. 3
SchemeOur starting material, the commercially available 92:8 mixture of racemic trans-methyl dihydrojasmonate (1) 4 and its cis-isomer, was first converted to the known enol acetate 2 in 90% yield following the reported procedure. 1dOptimum conditions for epoxidation were found to involve treatment of a toluene solution of 2 with peracetic acid (1.1 equiv) in the presence of catalytic amounts of Na 2 CO 3 , leading to formation of the unstable a-acetoxy epoxide 3 in 94% yield. 5 Within the limits of the 1 H NMR analysis (structure assignment by 2D 1 H NMR), only the cis-isomer 3 was formed, which is consistent with related work. 6Although the acid-catalysed rearrangement of a-acetoxy epoxides to a-acetoxy ketones is well precedented in the literature, 7 we were more interested in finding reaction conditions in which 3 was converted directly to 4. Indeed, after screening a variety of Brønsted and Lewis acids in protic and aprotic solvents, it was found that catalytic amounts of methanesulfonic acid in refluxing MeOH gave satisfactory results, converting 3 to 4 in 74% yield; 8 bicyclic lactone 5 (3% (GC)), and dihydrojasmone 6 (4%), were formed as minor side-products. 9 Under these conditions rearrangement of 3 to the a-acetoxy ketone 7a is believed to be the first step. 10 Lactone 5 is presumed to originate from lactonisation of the a-hydroxy ketone 7b, and dihydrojasmone 6 by decarboxylation of the acid corresponding to 4, a vinylogous b-keto acid.
Acknowledgement
The synthesis of al six isomers of 1,1′‐dimethylbipyrazolyl is reported. Their structure assignments were based on mnr data. Nmr spectra were determined in widely different solvents and structure assignments were based on these data, particularly on the found solvent effect. Uv spectral data are also reported.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.