Structure and conformation of heterocycles-VIII. Peripherally substituted 1,4,5,8-tetraoxadecalins

The synthesis of the title compounds was examined and a mechanistic scheme was put forward, which accounts for their formation along with higher substituted by-products and with the corresponding bidioxolan-2-yl isomers. Chemical correlation between two members of the series, trans -2,3-dicarbomethoxy- and trans-2, 3 - dimethyl - tetraoxadecalin, was performed. All stereoisomeric 2,3 - dicarbomethoxy -1,4,5,8- tetraoxadecalins were prepared and chemically equilibrated. The free energy differences at 65° between them were thus obtained. This led to an evaluation of the conformational free energy of the carbomethoxy group in dioxanes, ΔG_CO2Me^o. = 0.38 kcal/mole. Dynamic conformational analysis by variable temperature NMR measurements on cis - 2, 3 -dicarbomethoxy - anti -cis - 1, 4, 5, 8- tetraoxadecalin led to a ring inversion barrier of AG₁₉₃^≠ = 10.0 kcal/mole.