An unexpected reaction of phosphorous tribromide on chromanone, thiochromanone, 3,4-dihydro-2H-benzo[b]thiepin-5-one, 3,4-dihydro-2H-benzo[b]oxepin-5-one and tetralone derived allylic alcohols: a case study

Treatment of chromanone, thiochromanone, tetralone and benzo[b]thiepinone derived allylic alcohols with phosphorous tribromide furnished unexpected products instead of desired bromo derivatives. Reaction of 7-methoxy-2,2-dimethylchromenyl-aryl-methanols and 7-methoxy-2,2-dimethylthiochromenyl-aryl-methanols with PBr3 gave saturated carbonyls and exocyclic olefins depending on the substituents at R4, whereas allylic alcohols without 7-methoxy substituent furnished rearranged products. Several related analogs varying in ring size, heteroatom in the ring and substituents at various positions were synthesized and studied. Reactions of other phosphorous-based reagents like PPh3, PPh3/I2 and PPh3Br2 on allylic alcohols were also attempted. A case study is presented here.