1. A great deal of work on pyryliums, by Roumanian and Russian workers, is described in the Russian literature. This is well reviewed in `Pyrylium salts. Part I. Syntheses’, Balaban, A. T., Schroth, W., and Fischer, G., Adv. Heterocycl. Chem.,1969
10,241; `Pyrylium salts. Synthesis, reactions and physical properties’, Balaban, A. T., Dinculescu, A., Dorofeenko, G. N., Fischer, G. W., Koblik, A. V., Mezheritskii, V. V., and Schroth, W., Adv. Heterocycl. Chem., Suppl. 2,1982.
2. Gârd, E., Vasilescu, A., Mateescu, G. D., and Balaban, A. T., J. Labelled Cmpds, 1967, 3, 193; Farcasiu, D., Vasilescu, A., and Balaban, A. T., Tetrahedron, 1971, 27, 681.
3. Le Fèvre, C. G. and Le Fèvre, R. J. W., J. Chem. Soc., 1932, 2894.
4. Williams, A., J. Am. Chem. Soc., 1971, 93, 2733.
5. Katritzky, A. R., Brownlee, R. T. C., and Musumarra, G., Heterocycles, 1979, 12, 775.