Natural-Abundance (13)C Nuclear Magnetic Resonance Spectra of Terpenes and Carotenes

Natural-abundance (13)C nuclear magnetic resonance spectra are reported for some simple terpenes and carotenes. The techniques involved in the assignment of the resonances to specific carbons are outlined. The potential of this nondegradative procedure for structural analysis is demonstrated for the investigation of carbon atoms in chemical and biochemical systems

Conformational equilibria in cyclohexyltrimethylstannane and cyclohexyltrimethylplumbane by low temperature 13C NMR spectroscopy

The variable temperature carbon-13 NMR spectra of cyclohexyltrimethylstannane and cyclohexyltrimethylplumbane have been recorded, and at -69°C, the chair-chair conformational interconversion is "frozen". A values (A = -ΔG° = RTln K) for the (CH)Sn (1.06 ± 0.14) and (CH)Pb (0.67 ± 0.06) groups are derived

Nuclear magnetic resonance spectroscopy ^(13)C Spectrum of a Dimer of Cyclooatatetraene

The structure of the cyclooctatetraene dimer of m. p. 53° has been a matter of controversy. Structure I was proposed by Jones(3} and could be reconciled with a proposed mechanism of conversion of the dimer to bullvalene(4). Schröder(5) has proposed structure II because the material afforded the all cis-cyclobutanetetracarboxylic acid on ozonolysis and peracid oxidation. However, there is still a possibility that I could yield ciclobutanetetracarboxilic acid on oxidation by way of its valence tautomer III

Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts in Norbornyl Derivatives

The ^(13)C chemical shifts for a number of norbornyl derivatives have been obtained in natural abundance. The compounds studied were the series of methylnorbornanes, methylnorbornenes, methyl-2-norbornanones, methyl-2,2-difluoronorbornanes, and a variety of exo- and endo-2-substituted norbornanes. The observed shifts have been interpreted in terms of inductive, bond length, and steric effects. The ɑ- and β-substituent effects are shown to be dependent on the extent of substitution at both the α and β carbon atoms and changes in bond lengths have been used to rationalize the shift changes. Steric interactions between the endo-2 and endo-6 positions, and the exo-2 and syn-7 positions have been shown to generate important chemical- shift changes and these changes can be used as a semiquantitative measure of steric interactions in norbornyl derivatives. Studies with electron-withdrawing groups have revealed a new long-range cmr y effect produced by exo-2 substituents on C-6. These shifts appear to provide a sensitive and quantitative measure of the electronic demands of a neighboring group as a function of stereochemistry. The substituent shifts obtained from the monosubstituted derivatives have been used to predict the shifts of each of the 1- and 3-methyl-2-norbornanols. The agreement between observed and calculated values is generally good and supports the general approach. Several ^(13)C-^(13)C coupling constants for norbornane, nortricyclene, and quadricyclene are reported and interpreted in terms of the s character of the C-C bonds