Rearrangement and orientation in citran synthesis. X-Ray crystal structures of (–)-bruceol and a (±)-deoxybruceol derivative

Despite their co-occurrence in Eriostemon brucei, and the similarity between their chiroptical properties, an X-ray study confirms the structure of bruceol to be (1) and shows that of deoxybruceol (studied as a dibromo-derivative) to be (3), i.e. their p-menthane segments are of opposite orientation relative to the coumarin ring. Strain in these structures is similar to that in the citrans described in the preceding paper.Two monochromens are formed when 5,7-dihydroxycoumarin is treated with citral and pyridine: one of these is a citran precursor and its orientation is shown by nuclear Overhauser effects. On heating this chromen, rearrangement occurs to give deoxybruceol as the major product; isodeoxybruceol, of the same orientation as the chromen, is the minor product. Reference is made to the curious biosynthetic situation in E. brucei.