ChemInform Abstract: 13C and 1H NMR Shielding Effects in Aliphatic gauche/trans Fragments.

Abstract

On the basis of improved calculation models, of new NMR measurements with bicyclo[2.2.1]heptyl and cyclohexyl compounds, and of literature data it is shown that classical mechanisms of sterically induced charge polarizations, of linear electric field, and of anisotropy effects can account for many substituent-induced shielding (SIS) differences. The r61e of steric distortions on a-and PSIS is discussed; bond angle variations generated by a methyl group introduction at C-a and subsequent hybridization changes are correlated with Ca-Me SIS values as well as with the eq/ax shielding in cyclohexanes.The significant shielding differences between gauche and [runs orientations in aliphatic frameworks have initiated many publications. particularly in the field of I3C-NMR spectroscopy". The advent of high-field and of 2D NMR makes 'H-NMR shifts a similarly promising tool even for complicated frameworks such as saturated steroids", and allows for the first time a mechanistic comparison of complementary proton and carbon shieldings.Stereochemical applications of 13C-NMR shifts have been largely based on the shielding of y-carbon atoms by a syn (gauche) alkyl substitutent I), which has been rationalized by Grant and Cheney3' with sterically induced charge polarization along a syn-axial C -H bond. On the basis of more realistic model geometries and of a different equation derived a recognized force-field parametrization we have shown4a' that these steric effects not only correctly predict the substituent induced shieldings (SIS) by X = Me on syny atoms such as c -3 in l a / l b, Id, c-7 in 2a (R = R' = H), or C-6 in 2b, or spn-methyl carbons in 3a, 3b (Table 1) and similarly in steroids4b), but also do allow for deshielding effects on 9-carbon atom^]^.^,'), such as on C-10 in 1c6' or C-8 in 2a7' (R = Me, Table 1). As a necessary consequence of the angular dependence of the polarizing gradient, deshielding at y-carbon atoms will occur for 6 < 90 , which is the case for several interactions in 9 groups such C-8 in 2a (R' = R = Me)4d). If one places the point of action for the steric forces not arbitrarily at the hydrogen end of the polarized C -H bond as it has been done earlier3,4d' (model 1 a), but more towards carbon (model 1 b) -which is a better representation of the polarized electron cloud -it becomes clear, why with Chem. Ber. 122 (1989) 321 -326

Bornyl- and Isobornyl-Δ8-tetrahydrocannabinols: A Novel Class of Cannabinergic Ligands

“…The mixture was then treated with p -toluenesulfonic acid monohydrate in CH 2 Cl 2 to afford an 8:1 mixture of alkenes 10 . Hydrogenation of the mixture gave a 1:3.4 mixture of endo -adduct 5c to the exo -adduct 5d , an observation that is consistent with literature precedents for endo approach of larger reagents to 2-methylidene-7,7-dimethylbicyclo[2.2.1]heptanes. − The mixture was inseparable by both flash chromatography and chiral HPLC and was used without further separation. Demethylation of the mixture with BBr 3 gave a corresponding mixture of bornylmethylresorcinol 6c and isobornylmethylresorcinol 6d that was also inseparable and was used directly in the Friedel−Crafts type reaction with (+)- trans - p -mentha-2,8-dien-1-ol ( 7 ).…”