The ketonic "C NMR signals of a series of 6-hydroxy-5-oxobieyclo[2.2.2]octt-7-ene-2-carboxylic acid lactones and of haplophytine and related N-substituted 3-piperidinones occur at exceptionally high field; the structural factors which are responsible have been elucidated by comparisons with related compounds.Carbonyl carbon signals in the 13C NMR spectra of organic compounds are usually readily identified since they normally occur in a region of the spectrum (6 170-225 ppm) in which few other types of carbon atoms give rise to signals.' They are of importance not only in revealing the presence of one or more carbonyl groups, but also in providing information concerning the functional environment of the carbonyl groups. Thus, simple saturated ketones normally give signals at 6 205-225 ppm and simple conjugated ketones at 6 190-200 ppm, while simple acids, esters, lactones, amides and lactams have signals at 6 170-190 ppm.l Gross deviations from these normal values are clearly of importance to organic chemists using 13C NMR spectroscopy as a tool for structural elucidation. We report our observations on such deviations in the cases of two classes of cyclic ketones (chemical shift values are given in ppm relative to internal TMS for solutions in CDCI,; negative values of differential shifts denote shielding effects).The first class comprises keto lactones of type 1,' whose ketonic carbon chemical shifts are listed in s~b s t i t u e n t ,~ but its magnitude is extraordinarily large. It can be compared with the effect of -5.3 pprn resulting on substitution of a 3-acetoxy group in 4a to give 4c. The effect is attributable largely to dipole-dipole interaction and its magnitude is considered to reflect, first, increased electrostatic interaction due to the rigidly held orientation of the lactone relative to the ketonic carbonyl group and, second, distortion of the geometry of the bicyclo[2.2.2]oct-5-en-2-one system.' A second factor is the P,y-ethylenic double bond. Introduction of such a double bond into cyclic ketones can lead to upfield shifts of their carbonyl carbon signals, which for bicyclo[2.2.2]octan-2-ones lie in the range of -4 to -6 ~p m .~,~ Comparison of la and 2 shows that here the ethylenic double bond leads to a shift of -5.6ppm. Such shifts are due in part to homoconjugation, but other effects play a role.5 The third factor is the presence at C-7 in l&li of a syn-or anti-carbomethoxy or -bromo substituent, which leads to a shift of -2.5 to -4ppm. Such y shielding effects on carbon by both gauche and anti electronegative p substituents have been observed previously.6 The only structural factors which lead to deshielding are the (Y -methyl substituents; however, their p deshielding
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