A facile aromatization of vitamin D.

DANNENBERG and HEBENBROCK obtained a fluorescent hydrocarbon, C27H34, by refluxing vitamin D3 with chloranil in anisole (bp 155.5•Ž) for 1 hr (35% yield), to which a structure possessing the styryl indene chromophore was assigned (1). During the course of our study on a fl uorometric determination of vitamin D, we found this reaction to be unsuited for analytical purposes mainly because of its drastic reaction condition. We wish to report a very easy method of aromatization of vitamin D under extremely mild reaction conditions, and to emphasize the biosynthetic significance of this reaction. Treatment of vitamin D2 (I) in acetone with an equimolar amount of N bromosuccinimide at room temperature for a period of 10min led to a complete disappearance of the initial vitamin. Extraction of the diluted aqueous acetone solution with n-hexane gave a reaction mixture consisting principally of the two fluorescent components A and B, in approximately 3:1 proportion (2) as estimated by NMR (3). No other reaction products were detected on gas chromatographic tracings (4). Successive column chromatographic separation on silica gel, using n-hexane, afforded the ingredient A in a pure state (5). On the basis of the following data and of a comparative examination with the known vitamin isomers such as isovitamin D2 (II) and isotachysterol2 (III), the structure (IV) has been determined unequivocally for the compound A, and (V) was assigned tentatively to the compound B.