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

Five different squalene derivatives, (1) (18Z)-29-methylidene-2,3-oxidohexa nor-squalene; (2) (18E)-29-methylidene2,3-oxidohexanovsqualene; (3) 18-hept anor-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 o f the NMR spectroscopic data of these molecules in solution indicates that their central portion is relatively rigid, while their terminal portions ar e rather mobile. Further, NMR data of compounds I and 2 show that the cis c onformation of the butadiene-like terminal group is the most stable conform ation. Molecular mechanics and dynamic calculations in vacuum indicate that the mobility of the long squalene-like chain in compounds 1-3 is almost eq uivalent in all isoprenic moieties and that the higher mobility of the tail s seems to be due to a cooperative increase of mobility in going from the c enter 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. (C) 1999 Published by Elsevier Science Ireland Ltd . All rights reserved.