CONFORMATIONAL STUDIES ON 1,3-DIOXEPANS .2. 1,3-2,5-4,6-TRI-O-ETHYLIDENE-D-MANNITOL AND SOME RELATED COMPOUNDS

1,3:2,5:4,6-Tri-O-ethylidene-D-mannitol and 2,5-O-ethylidene-1,3:4,6-di-O- methylene-D-mannitol exist in one stable configuration. On the basis of n.m.r. studies and the comparative stabilities of the twist-chair (TC), twist-boat (TB), and chair (C) conformations for a seven-membered ring, it is suggested that the 1,3-dioxepan rings exist predominantly in the TC form. The chemical shift data for the O-ethylidene methine and methyl protons in the 1,3-dioxan and dioxepan rings of the above-mentioned derivatives and of 1,3:4,6-di-O- ethylidene-2,5-O-methylene-D-mannitol are compared.