Halohydrins of antirrhinoside - the correct structures of muralioside and epimuralioside.

Reaction of the iridoid glucoside antirrhinoside (1) with pyridinium chloride in dimethylformamide gave rise to the two possible trans-halohydrins linarioside (4) and isolinarioside (5). Pyridinium bromide gave the two analogous bromohydrins. It is shown that the iridoid glucosides 8-epi-muralioside from Linaria arcusangeli and 7,8-epi-antirrhinoside from L. dalmatica are both identical with isolinarioside and therefore these names are redundant. The structure of muralioside isolated from Cymbalaria muralis is revised to that of its 8-epimer (8) while the structure of an isomeric, new iridoid glucoside from Paulownia tomentosa has been elucidated to be 7beta-hydroxyharpagide (3), the structure originally assigned to muralioside. In addition, 7alpha-hydroxyharpagide (2), the known product from the base-catalyzed hydrolytic opening of 1, has been isolated from Antirrhinum majus and thus shown to be a natural product.