UNUSUAL CONFORMATIONAL ASPECTS OF SOME NOVEL CHIRAL NON-RACEMIC PYRIDINYL-2-PHOSPHONATES

Reaction of pyridinyl-2-phosphonyl dichloride (6) with 1-phenyl-2,2-di methylpropane-1,3-diol (9) leads to the two epimeric 2-oxo-2-(2-pyridi nyl)-4-phenyl-5,5-dimethyl- 1,3,2-dioxaphosphorinanes (10a,b). These c an be separated and the stereochemistry assigned on the basis of P-31 NMR spectroscopy. For 10a the pyridinyl substituent is arranged axiall y at phosphorus. Arguments derived from 2D NMR experiments indicated t hat the nitrogen of pyridine is locked in a conformation whereby the p yridinyl nitrogen points over the six-membered ring; in other words it is locked between the two ring oxygen substituents. This conclusion i s substantiated by an X-ray crystal determination. Oxidation of 10a wi th hydrogen peroxide leads to the N-oxide (12). The crystal structure of 12 reveals that despite serious steric overcrowding the N-O bond is also oriented over the six-membered ring. Methylation of 10a with met hyl trifluoromethanesulfonate affords the N-methyl pyridinium salt(13) . NMR experiments indicate that in this case the methylated nitrogen h as turned ''outside'' of the six-membered ring. The borane adduct of 1 0a appears on the basis of MMR data to have a conformation wherein the complexed borane is located just outside of the six-membered ring. Al though crystal structures have not been obtained the pyridinyl-2-thiop hosphonates (15a,b) obtained from treatment of 10a and 10b with [(4-Me OC6H4)(2)PS](2) appear to have the same conformational properties as 1 0a and 10b. Restricted Hartree-Fock geometry optimizations have been c arried out to aid in clarifying this unexpected conformational behavio ur. These calculational results are in excellent accord with the exper imental observations, and provide insight into the reasons for the con formational behaviour. (C) 1998 Elsevier Science Ltd. All rights reser ved.