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 1H and 13C NMR correlations

✍ Scribed by Angel M. Montaña; Sandra Ribes; Pedro M. Grima; Francisca García

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 331 KB
• Volume: 36
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199803)36:3<174::aid-omr242>3.0.co;2-y

✦ Synopsis

The 1H and 13C NMR spectra of cis-endo (a) and cis-exo (b) diastereoisomeric pairs of Ðve di †erently 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 conÐguration 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 Ðeld than in diastereoisomer (b). Simultaneously, H-2 and H-4 result in a lower Ðeld in a than in b. Both e †ects are due to the di †erent 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 upÐeld shift of C-9, C-10 and C-3 in b versus a. The di †erence observed in chemical shifts of the aforementioned hydrogens and carbons, between both diastereoisomers, allows one to assign the conÐguration 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.