Conformational and dynamical study of squalene derivatives. II - Vynil oxidosqualenes

Five different squalene derivatives, (1) (18Z)-29-methylidene-2,3-oxidohexanor-squalene; (2) (18E)-29-methylidene-2,3-oxidohexanorsqualene; (3) 18-heptanor-2,3-oxido-squalene 3; (4) (18Z)-29-hydroxy-2,3-oxidosqualene; and (5) (18E)-29-hydroxy-2,3-oxido-squalene, were studied with both theoretical and experimental methods. Chloroform solutions of these compounds were studied with different mono-, bidimensional and relaxation NMR methods. Analysis of the NMR spectroscopic data of these molecules in solution indicates that their central portion is relatively rigid, while their terminal portions are rather mobile. Further, NMR data of compounds 1 and 2 show that the cis conformation of the butadiene-like terminal group is the most stable conformation. Molecular mechanics and dynamic calculations in vacuum indicate that the mobility of the long squalene-like chain in compounds 1-3 is almost equivalent in all isoprenic moieties and that the higher mobility of the tails seems to be due to a cooperative increase of mobility in going from the center to the end of the chain. Theoretical results show that in compounds 4 and especially 5 there is a strong evidence of folded structures in vacuum, while in chloroform small percentages of folded structures become evident even for compounds 1-3. Copyright (C) 1999 Elsevier Science Ireland Ltd.