Transformation reactions of α-pinene II magnetic anisotropy of the cyclobutane system

Non-equivalence of gem-dimethyl protons in some bi- and tricyclic compounds. Stereochemical assignments in the khusenic, isokhusenic, and chamigrene series

“…It is also noteworthy that the frequency of the equatorial gem-methyl protons in the cyclohexanone 5 (1.18 6) and of the C-2 endo-methyl protons in pinocamphone (9) (1.15 6, CC1,) (9) lies within the range (1.1 5-1.20 6, CC1,) reported for the quasi-equatorial, C-2 methyl protons of variously substituted (C-3) cyclobutanones (10). For the khusenic diene methyl ester 6, the additional unsaturation ( A~,~) results in no appreciablechangein the deshielding of the C-7 equatorial methyl protons from that observed for the methyl ester l a (Table I).…”

“…More recently, our elucidation of long-range couplings in the khusenic (1) and isokhusenic (2) series (4) has provided additional evidence in support of these tricyclic sesquiterpenic skeletons. We now wish to discuss examples of non-equivalence of gem-dimethyl protons found in the khusenic (1) and isokl~usenic (2) series in relation to similar phenomena in the monoterpenic compounds [8][9][10][11][12]. The chemical shift data and experience gained from structural analysis of the above compounds proved helpful in assigning the stereochemistry of some recently reported, unique, spiro-bicyclic sesquiterpenes (chamigrene series).…”

Anisotropy of the Induced Current Density (ACID), a General Method To Quantify and Visualize Electronic Delocalization

“…The current density plotted onto the ACID isosurface indicates a strong diamagnetic ring current in the ring plane and hence σ aromaticity. In contrast to cyclopropane, cyclobutane is antiaromatic. − The ACID plot and the current density vectors (Figure ) exhibit a strong paratropic ring current and thus antiaromaticity. However, the paramagnetic properties most probably are exaggerated for reasons that are due to an inadequate electronic description , by the DFT method and the magnetic properties that are calculated using the CSGT approach. , This is probably also true for magnetic susceptibilities and NICS values calculated at the same level of theory.…”

“…In contrast to cyclopropane, cyclobutane is antiaromatic. [108][109][110][111][112][113][114][115][116] The ACID plot and the current density vectors (Figure 14) exhibit a strong paratropic ring current and thus antiaromaticity. However, the paramagnetic properties most probably are exaggerated for reasons that are due to an inadequate electronic description 117,118 by the DFT method and the magnetic properties that are calculated using the CSGT approach.…”