Prepn of dl-form: Ley, Popow, Ann. 174, 61 (1874); Smissman, J. Am. Chem. Soc. 76, 5805 (1954); Marvel, Org. Synth. coll. vol. III, 848 (1955). Prepn of d- and l-forms: Berlingozzi, Furia, Gazz. Chim. Ital. 5,, 82 (1926); Berlingozzi, Lenoci, ibid. 68, 721 (1938). Configuration of d-form: P. Brewster et al., Nature 166, 179 (1950).