The 'H NMR spectral parameters of all eight 1,2-diacetoxy-3,4-dethyleyclopentanes and 1,2-diacetoxy-4-t-butyl-3-methyleyclopentanes are discussed.All eight isomeric 1,2-diacetoxy-3,4-dimethylcyclopentanes (la-lh) and 1,2-diacetoxy-4-t-butyl-3-methylcyclopentanes (2a-2h) were ~ynthesized"~ in connection with studies on the stereochemical outcome of the lithium-liquid ammonia reduction of 2,3-dialkyl-5-hydroxy-2-cycl~pentenones~ and 2,3-dialkyl-4-hydroxy-2-cyclopentenones.2-4 In this paper we discuss their 'H NMR spectral parameters, which should be of interest for the configurational assignment of some modified prostaglandins.6The configurational assignment of the eight isomers in both series, on the basis of 'HNMR spectral data alone, could have proved impossible, or at least tentative, in view of the difficulties commonly encountered in configurational assignments in pentacyclic syst e m~. ' ,~ Fortunately, the different isomers could be synthesized by cis-and trans-l,2-hydroxylation of the corresponding pure cis-and trans -3,4-dialkylcyclopentenes, followed by e~terification.~ As a consequence, one needs only to differentiate between the configurational pairs a-d, b-c, e-h and f-g in both series. Tables 1 and 2 give the NMR parameters of la-lh and 2a-2h, respectively, obtained at 300 MHz in cara b bon tetrachloride and benzene solution (see Experimental section).
RESULTS
Configurational assignment of la-lhThe importance of chemical shift criteria for structural elucidations in cyclopentanes, especially the 1,2-(or 0) syn-upfield (and anti-downfield) effect of methyl and acetoxy groups on vicinal protons, has been clearly demonstrated.' [Syn and anti are used here to denote the spatial relationship between two substituents (or hydrogens).] Based thereon, a significantly larger shift difference between the two H-5 atoms is expected in the isomers possessing a syn relationship of the 1-acetoxy and 4-Me groups than in the isomers possessing these substituents in the anti relationship; accordingly, the small shift differences (0.1-0.37ppm in b, d, f and h) are assigned to the trans-1-acetoxy-4-methylcyclopentanes and the large shift differences (0.91-1.27 ppm in a, c, e, g) to the cis-1-acetoxy-4-methyl isomers (CCl, solution). The same large difference between the geminal protons in p-position to two substituents in cis-1,3-cyclopentanediol derivatives has been observed This effect, combined with the known5 relative orientation of the 1,2-acetoxy and of the 3,4-methyl groups establishes unambiguously the configuration of the eight isomers in this series.