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Enthalpies of combustion of 2,2- trans -4,6- ( 1 ) and 4,4,6,6-tetramethyl- ( 2 ) and 2,4,4,6,6- ( 3 ) and 2,2,4,4,6-pentamethyl-1,3-dioxanes ( 4 ) were determined to estimate their enthalpies of formation in the gas phase. By comparing the latter with the corresponding enthalpies estimated based on the various bond-bond interactions allowed to determine the chair-2,5-twist energy difference (Δ H  CT = 29.8 kJ mol -1 ) for 1 since C-13 shift correlations indicate that it escapes to the 2,5-twist form where the 2-methyl groups are isoclinal and 4- and 6-methyl groups pseudoequatorial to avoid syn-axial interactions. Compounds 2 and 3 in turn give the values 21.0 and 21.6 kJ mol -1 for the 4,6-diaxial Me,Me-interaction. Finally compound 4 , which retains the chair conformation to avoid pseudoaxial interactions in the twist forms gives the value 19.5 kJ mol -1 for the 2,4-diaxial Me,Me-interaction indicating that its chair form appears to be somewhat deformed.

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Enthalpies of Combustion and Formation of Severely Crowded Methyl-Substituted 1,3-dioxanes. The Magnitudes of 2,4- and 4,6-diaxial Me,Me-Interactions and the Chair–2,5-twist Energy Difference