Occurs in α- and β-forms. Prepn: Baumann, Fromm, Ber. 22, 2600 (1889); Fromm, Engler, Ber. 58, 1916 (1925). Molecular structure: Hassel, Viervoll, Acta Chem. Scand. 1, 164 (1947). Stereochemistry: Schnberg, Barakat, J. Chem. Soc. 1947, 693.