No abstract

. Can. J. Chem. 69,2024Chem. 69, (1991.The structures of 6-ethoxy-2,3,4,5-tetrahydropyrylium, 1, 6-eth0xy-2-methyl-2,3,4,5-tetrahydropyrylium, 2, and 2-ethoxy-3,4,4a,5,6,7,8,8a-octahydro-1-benzopyrylium, 3, hexachloroantimonates have been determined by X-ray crystallography. In each case the tetrahydropyrylium rings exist in shallow half-chair conformations with the cationic centers C(6), C(5), 0(1), and O(6) having a planar arrangement. The cations all have a Z conformation about the 0 ( 6 j C ( 7 ) bonds and it is shown that this conformation is also preferred in solution by comparison of narrow-line I3C NMR spectra in the solid state and solution. The O ( l W ( 2 ) and 0 ( 6 w ( 7 ) bond distances in 1-3 are significantly longer than those of comparable bonds in neutral esters. The length of the O ( l W ( 2 ) bond is very dependent on substitution at C(2) suggesting a considerable fraction of the positive charge resides on this carbon as well as C(7). The closest cationlanion interactions present in the crystal lattices of these salts are between the chlorine atoms of the anion and the cationic center, C(6). These interactions provide information on the origin of the stereoselectivity observed in nucleophilic attack on these cations. The conformations, details of the C-0 bonding, and closest contacts between cation and anion are discussed in terms of the concept of stereoelectronic control.Key words: 6-ethoxytetrahydropyrylium cations, lactonium salts, structure, X-ray, stereoelectronic control. Chem. 69, 2024Chem. 69, (1991.Les structures des hexachloroantimonates des 6-Cthoxy-2,3,4,5-tttrahydropyrylium (I), 6-tthoxy-2-mCthyl-2,3,4,5-tttrahydropyrylium (2) et 2-~thoxy-3,4,4a,5,6,7,8,8a-octahydro-l-benzopyrylium (3) ont Ct C dCterminCes par diffraction des rayons-X. Dans chaque cas, les cycles tCtrahydropyrylium existent dans des conformations demi-chaises peu profondes dans lesquelles les centres cationiques C(6), C(5), O(1) et O(6) sont dans un arrangement planaire. Les cations possbdent tous une conformation Z autour des liaisons 0 ( 6 w ( 7 ) ; par comparaison des bandes fines des spectres RMN du 13C i i 1'Ctat solide et en solution, on a aussi dtmontrC que cette conformation est aussi la conformation privilCgiCe en solution. Les longueurs des distances 0 ( 1 ) 4 ( 2 ) et O(6)-X(7) des composCs 1-3 sont beaucoup plus longues que celles des liaisons comparables des esters neutres. La longueur de la liaison O ( l W ( 2 ) dtpend beaucoup de la substitution en C(2); ce rCsultat suggkre qu'une fraction importante de la charge positive se trouve sur ce carbone ainsi que sur celui en C(7). Les interactions cation/anion le plus proches observCes dans les rCseaux cristallins de ces sels se retrouvent entre les atomes de chlore de l'anion et le centre cationique C(6). Ces interactions fournissent de l'information sur l'origine de la stCrCosClectivitC observCe lors des attaques nuclCophiles sur ces cations. On fait appel au concept de contr6le stCrCoClectronique pour discuter des conformations, des details de l...

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