Structure Determination of Brominated Morphinan‐6‐ones by 13 C‐NMR. Spectroscopy: A novel closure of the oxygen bridge using 4‐acetoxymorphinan‐6‐ones

Bromination of (−)‐4‐hydroxy‐ N ‐methylmorphinan‐6‐one ( 3 ), prepared from natural morphine, with 1 mol of bromine in acetic acid, afforded the 1‐bromo ketone 5 . The structure of 5 was assigned by 13 C‐NMR.spectroscopy, and confirmed by X‐ray diffraction analysis of its hydrobromide salt. It is suggested that monobromination of synthetic (±)‐2,4‐dihydroxy‐ N ‐formylmorphinan‐6‐one ( 7 ) takes in principle a similar course, although the 13 C‐NMR.spectrum of the primary reaction product 9 could not be measured because of insolubility in commonly used solvents. Monobromination of (−)‐4‐acetoxy‐ N ‐formylmorphinan‐6‐one ( 12 ) of the natural series, and of (±)‐2,4‐diacetoxy‐ N ‐formylmorphinan‐6‐one ( 8 ) of the synthetic series, followed by treatment of the monobrominated ketones with potassium carbonate in methanol resulted in closure of the O‐bridge, and afforded after acid hydrolysis, the corresponding 4,5‐epoxy‐morphinan‐6‐ones (−)‐ 16 and (±)‐ 17 respectively. This variation of the ring closure reaction represents a novel and convenient method to convert 4‐hydroxymorphinan‐6‐ones into their corresponding 4,5‐epoxymorphinan‐6‐ones, without involving aromatic bromination and with only 1 mol of bromine.