Abstract-Vicinal C,H spin coupling ( 3 J~,~) in substituted alkenes has been investigated systematically. Emphasis is laid on the stereochemical significance (JLranr/Jcii;) and o n the various structural factors which influence 3J(!.H, such as r-bond order, torsional angle 4, bond angle 0, electronegativity of substituents and steric effects. A new type of y-effect is observed in 3J:,2s which appears to have the same origin as the y-shift effect. By comparison of 3JJc_rr,,,l and 3JH,11, it was found that the relation ' J c~,~ = 0.6 %J,,,H holds for both trans and cis coupling constants. Finally, it is concluded that 3 J~, f~ constitutes a valuable criterion to distinguish E-and Z-isomers, particularly in trisubstituted alkenes. Applications to natural products are presented.(1) INTRODUCTION THE STEREOCHEMICAL significance of vicinal H,H spin coupling in alkenes was recognised early and has ever since been utilised in numerous stereochemical assignments. This method, however, is not applicable to trisubstituted alkenes. In such cases, the configuration may be determined by proton NMR using differential chemical shifts,l allylic H,H spin coupling2 or nuclear Overhauser effect^,^ particularly when alkyl substituents are present. Allylic spin coupling though has proven much less reliable2 than vicinal spin coupling and certainly requires availability of both stereoisomers.With the advent of carbon magnetic resonance new criteria for stereochemical assignments became available. The increased sensitivity of the chemical shift parameter to structural and, in particular, to stereochemical changes, allows the assignment of cis-trans (or Z / E ) isomers by the differential shift m e t h~d .~ The two important criteria for assignment are the higher shielding of the olefinic carbons in the Z-isomers and the diamagnetic shift of cis-oriented substituents bearing hydrogen (y-effect5). This method also requires both stereoisomers for a reliable assignment, unless very close model compounds are available.It is the purpose of this paper to show that unambiguous stereochemical assignments in trisubstituted alkenes can be obtained from vicinal C,H spin coupling, i.e. from proton coupled 13C NMR spectra. It has previously been shown6 that in mono-and disubstituted ethylenes 3J6,H > 3J(1 U,H which, in principle, would offer an analogous criterion for cis-trans isomerism as in proton NMR. Applications to trisubstituted alkenes are as yet scarce. An example has been reported for the E-and 2-isomers of methylphosphoenol-u-ketobutyrate.7* Author to whom correspondence should be addressed. (2) ANALYSIS OF SPECTRA Chemical shift assignments are based upon offresonance decoupling experiments, a partial analysis of proton coupled spectra, selective 13C-{1H} decoupling experiments and spectral comparison. In the case of hydrocarbons with gem-dimethyl groups, the y-shift effect was also utilised. For the analysis of the proton coupled carbon spectra, a first order approach was justified in the majority of cases. The spectra of compounds 33, 44...

SummaryHeterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by 13C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C, H-spin coupling is again demonstrated in the present series.

Abstract-Carbon and proton NMR spectra of 3-hydroxypyridine and its 0-methyl and N-methyl derivatives have been analysed, and are compared with the spectra of the corresponding 2-and 4-substituted pyridines. It is shown, that contrary to quantum chemical predictions and in agreement with chemical experience, 3-hydroxy-pyridine has a phenolic structure in solution. For the N-methyl derivative, however, carbon and proton NMR data clearly demonstrate that this compound should be formulated as N-methyl-3-pyridone. The general problem of the 3-pyridones is discussed on the basis of calculated r-electron densities, IR and NMR data.

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SummaryReductive elimination at 1,2exo-diiodonorbornane (6) was induced by Collman ' s reagent. Surprisingly, 1,2endo-diiodonorbornane (9) and l-iodo-2endo-trifluoromethylsulfonyloxy-norbornane (lob) lead only to reactions of the substituent in 2-position. Mechanistic aspects are related to the reactions of monosubstituted iodonorbornanes with Collman's reagent.Einleitung. -Nach wie vor ist die Reaktivitat von d'.*-Norbornen (1) weitgehend unbekannt. Dies hangt vor allem damit zusammen, dass das hoch reaktive BredtOlefin 1 [l] im Gegensatz zu Bicyclo