Planar-chiral (2E,7Z)- and (2Z,7E)-cyclonona-2,7-dien-1-yl carbamates by asymmetric, bis-allyic, alpha, alpha'-cycloalkylation--studies on their conformational stability.
Enantiomerically enriched (>80 % ee) (M,1R,2Z,7E)- and (M,1R,2E,7Z)-cyclonona-2,7-dienyl carbamates have been synthesized by an intramolecular cycloalkylation of the corresponding 1-lithio-9-chloronona-2,7-dienyl carbamates. The enantioenriched precursors were generated by asymmetric deprotonation of the dienyl carbamates by means of n-BuLi/(-)-sparteine. The primarily obtained cyclononadienes, each bearing one element of planar and centre chirality, were formed by an , coupling of both allylic moieties. These are the thermodynamically less favoured epimers, which arise from those allyllithiums bearing the carbamoyloxy residue in a 1-endo position. Both (M,R)-cyclononadienes epimerize slowly (t(1/2) = 209 min and 328 min at 308 K) with inversion of the planar chirality to the corresponding, more stable (P)-epimers (ratios 3:97 and 30:70). The kinetics were measured by 1H NMR spectroscopy, the activation energies EA were found to be 26.4 and 27.5 kcal mol(-1). From quantum-chemical calculations with the B3LYP density functional, the epimerization process was shown to consist of two coupled major conformational changes with high energy barriers. The calculated values match well with the observed ones.
Deiters
Alexander
A
Organische-Chemisches Institut, Westfäliche Wilhelms-Universität Münster, Germany.
Mück-Lictenfeld
Christian
C
Frölich
Roland
R
Hoppe
Dieter
D