General method of assignment of relative stereochemistry in C-1-substituted 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one by <SUP>1</SUP>H and <SUP>13</SUP>C NMR correlations

The ¹H and ¹³C NMR spectra of cis-endo (a) and cis-exo (b) diastereoisomeric pairs of five differently C-1-functionalized 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-ones were completely assigned. Several trends regarding the variation of chemical shifts and coupling constants of hydrogen and carbon atoms, on changing the configuration at C-2 and C-4 in both diastereoisomers, were observed by correlation of spectral data: methyl groups attached at C-2 and C-4 in a appear in ¹H NMR at higher field than in diastereoisomer b. Simultaneously, H-2 and H-4 result in a lower field in a than in b. Both effects are due to the different interactions of hydrogens H-2, H-4, H-9 and H-10 with the bridging oxygen. In ¹³C NMR spectra it is possible to observe an upfield shift of C-9, C-10 and C-3 in b versus a. The difference observed in chemical shifts of the aforementioned hydrogens and carbons, between both diastereoisomers, allows one to assign the configuration at C-2 and C-4 in such structures. This phenomenon has wide scope and validity and could be applied to the stereochemical determination of any pair of diastereoisomers (a and b), independently of the function attached to C-1 of the oxabicyclic system. ©1998 John Wiley & Sons, Ltd.