1. V. M. Berezovskii, The Chemistry of Vitamins, [in Russian] Moscow (1959), p. 404.
2. L.O.Sneidman The Manufacture of Vitamins, [in Russian], Moscow (1958), p. 291.
3. A. M. Yurkevich, Uspekhi Khim.,33, 418 (1964).
4. G. Popp, Justus Liebigs Ann. Chem., Bd. 250, S. 273 (1889).
5. J. B. Hatcher, J. Am. Chem. Soc.,69, 465 (1947).