Ultraviolet irradiation product of vitamin D2. Isoln: Windaus et al., Ann. 483, 17 (1930). Structure and stereochemistry: Dauben, Baumann, Tetrahedron Lett. 1961, 565. Crystal structure: C. P. Saunderson et al., ibid. 573. Improved isoln and spectral data: T. Kobayashi et al., J. Nutr. Sci. Vitaminol. 23, 291 (1977).