CONFORMATIONAL-ANALYSIS OF [3,3]CYCLOPHANES .5. A CONFORMATIONAL STUDY OF [3.3](2,6)PYRIDINOPHANE BY THE DYNAMIC NMR METHOD AND X-RAY STRUCTURAL-ANALYSIS

A variable-temperature H-1-NMR study and X-ray structural analysis sho w that the most stable conformational isomer of [3.3](2,6)pyridinophan e 2 is the syn(boat-boat) conformer, and the relative stability order of the three stable conformers is syn(boat-boat) > syn(chair-boat) > s yn(chair-chair). This is in sharp contrast to the relative stability o rder of the parent [3.3]metacyclophane 1: syn(chair-chair) > syn(chair -boat) > syn(boat-boat). The high stability of the syn(boat-boat) conf ormer 2c is primarily attributed to weak attractive interactions via i ntramolecular hydrogen bonds between nitrogen lone pairs (N-1, N-2) an d the axial hydrogen atoms (2A-H, 11A-H) on the central carbon atoms o f the bridges, as suggested by their short transannular distances (2.5 0 Angstrom).