Structure and synthesis: Ladenburg, Ber. 16, 1408 (1883); Merling, Ber. 25, 3124 (1892); Willsttter, Iglauer, Ber. 30, 721 (1897); 33, 1170 (1900); eidem, Ann. 317, 315, 350 (1901); Robinson, J. Chem. Soc. 111, 762, 876 (1917); Hess, Ber. 51, 1007 (1918); Schpf, Angew. Chem. 50, 779, 797 (1937); Ruggli, Maeder, Helv. Chim. Acta 27, 436 (1944); Keagle, Hartung, J. Am. Chem. Soc. 68, 1608 (1946).