The anomeric effect in thio derivatives of benzocycloheptene

The conformational properties of six thio derivatives of benzocycloheptene were studied by 1H and 13C high-field NMR spectroscopy in solution at low temperature. Both parent compounds 2 and 6, with sulfur at positions 3 and 1, respectively, were found to exist in the chair conformation. The 2-substituted derivatives of 2 (3: Y = OCH3 and 4: Y = Cl) revealed the presence of the Ca form only. In contrast, the 2-substituted derivatives of 6 (7: Y = OCH3 and 8: Y = Cl), with sulfur at position 1, showed mixtures of three conformers, namely Ca:Ce:TB (Ca = axial chair, Ce = equatorial chair, and TB = twist-boat), whose populations in CHF2Cl were, respectively, 93:5:2 and 85:12:3. The predominance of the Ca form in both series argues in favour of a strong anomeric effect in spite of the large departure from coplanarity (~35°) in the:—S—C—Y fragment of both 7 and 8. This noncoplanarity appears to weaken only slightly the n–σ* overlap characteristic of the endo-anomeric effect, contrary to earlier observations for...