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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 1 H and 13 C NMR correlations
Author(s) -
Montaña Angel M.,
Ribes Sandra,
Grima Pedro M.,
García Francisca
Publication year - 1998
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199803)36:3<174::aid-omr242>3.0.co;2-y
Subject(s) - diastereomer , chemistry , chemical shift , nmr spectra database , carbon 13 nmr , stereochemistry , spectral line , proton nmr , physics , astronomy
The 1 H and 13 C 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 1 H 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 13 C 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.