1,4,5,8-Tetraacyl-1,4,5,8-tetraazadecalins: Configurational and conformational analysis in the crystal and in solution: Structure and conformation of heterocycles-12

A series of 1,4,5,8-tetraacyl-1,4,5,8-tetraazadecalins was studied and found to exist in trans form in analogy with the parent system₂. Only tetraacetyltetraazadecalin (TATAD) could be obtained in both trans and cis forms, which were investigated in detail by X-ray diffraction analysis and NMR techniques, trans -TATAD (8) occurs in the crystal in a centrosymmetric double twist-boat conformation whereas cis -TATAD (9) has one ring as a flattened chair and the other as a twist boat with all the amide CO's syn-oriented. Three out of seven possible rotameric forms of 9 occur at low temperature in solution. The free energy of activation of amide rotation in trans-TATAD (8) is 14.5 kcal/mol, in contrast to ca 12kcal/mol in the unique cis isomer (9).