Prepn: Ruzicka et al., Helv. Chim. Acta 16, 838 (1933), cf. Kon, J. Chem. Soc. 1933, 1081; Cook, Hewett, Chem. Ind. (London) 52, 451 (1933); Hawthorne, Robinson, J. Chem. Soc. 1936, 763; Bachmann, Kloetzel, J. Am. Chem. Soc. 59, 2207 (1937).