A series of 2-fluoro-substituted 19β,28-epoxy-18α-oleanane triterpenoids was prepared. Detailed NMR analysis (1H, 13C, 19F) was used for independent determination of the configuration of substituents. Conformation of the A-ring was derived from coupling constants J(H,H), J(F,H), J(F,C) and NOE-contacts observed in 2D-H,H-NOESY spectra. It was found that A-ring in all 2α-fluoro derivatives 2-5 and 2β-fluoro-3β-OR derivatives 10 and 11 adopts chair-form, while in 2β-fluoro ketone 9 a boat-form is preferred. The chair-boat equilibrium of A-ring was found in 2β-fluoro-3α-OR derivatives 7 and 8.
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Tišlerová et al. (2006) studied this question.
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