Conformational analysis of six-membered cyclic carbonates

Computer simulation of conformational transformations of 1,3-dioxanes and their 2-sila and 2-bora analogs

“…The results of quantum chemical study of the system 5-methyl-1,3-dioxane (III) · 39 H 2 O using the approach based on specific solvation of substrate molecules also demonstrated an appreciable increase of the energy difference between the global minimum (C eq ) and other conformers (C ax , flexible conformers) of the water cluster relative to isolated molecule III [53]. Thus, computer simulation of conformational transformations of 1,3-dioxanes predict equilibrium between the chair conformers through flexible forms as intermediate energy minima, which is consistent with the experimental data.…”

“…The ratio of the equatorial and axial conformers of carbonate XXVII is 86 : 14 (88 : 12 according to the NMR data [72]), and ∆G°2 98 ≈ -1.1 kcal/mol. Presumably, the increase of the free conformational energy in the latter case is related to steric interactions of substituents in the carbon fragment of the ring [73].…”

“…This is the reason why cyclic carbonates are considered in a separate section of the present review. Ab initio calculations showed that minima on the PES of unsubstituted 1,3-dioxan-2-one (XXV) correspond to energy-degenerate sofa conformers and that the ring inversion involves 2,5-T transition state [73,74] (Scheme 13). The ring inversion of 5-methyl-and 4,4,5-trimethyl-substituted 1,3-dioxan-2-ones XXVI and XXVII involves the equatorial and axial sofa conformers (S eq and S ax ), and the global minimum belongs to the former (Scheme 14).…”

Conformational behavior of hexahydropyrimidin-2-one and its ammonium and oxonium ions

“…These molecules, similar to cyclic carbonates [7], display interconversions between two energydegenerate compressed chair forms through a transition state (TS) close to the 2,5-twist form (2,5-T This process has a relatively low activation barrier close to the value calculated for six-membered carbonate [7].…”

Unusual conformational isomerization of 2,2-dimethyl-1,3-dioxa-2-germacyclohexane

“…Besides the case of 1,3-dioxa-2-silacyclohexanes [4], such a single-barrier transition to an alternative form has been found previously only for cyclohexane heteroanalogs with an sp 2 atom at the second ring position, in particular, for 2-oxo-1,3-dioxanes [8] and substituted 1,3,2-dioxaborinanes [9,10]. _______ * To whom correspondence should be addressed, e-mail: kuzmaggy@mail.ru.…”