13C Nuclear Magnetic Resonance Studies. Part 48. 13C Spectra of a Variety of Bicyclic Ketones

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Abstract
The 13C n.m.r. spectra of a series of 37 bicyclic ketones have been recorded to examine the variations in 13C shieldings with structure and with methyl substitution. Several examples of six skeletal types were included: bicyclo[4.4.0]decanones, bicyclo[4.1.0]heptanones, bicyclo-[3.1.0]hexanones, bicyclo[3.2.2]nonanones, bicyclo[3.2.1]- and -[3.3.0]octanones as well as bicyclo[2.1.1]hexan-2-one, nopinone (6,6-dimethylbicyclo[3.1.1]heptan-2-one), and two methyl derivatives of the latter. The observed trends associated with methyl substitution follow well-defined patterns and offer further support for the application of 13C shieldings as aids for stereochemical assignments especially through the well known γ effects. The variation in the shieldings of the carbonyl carbons in these systems suggested that bond eclipsing interactions have an important influence on this parameter. With this notion, the heretofore puzzling variations observed for the cycloalkanones are readily interprétable in terms of their favored conformations.

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