Hexamethylene tetramine hydrotribromide is found to be superior to pyridine hydrotribromide when compared with the rate of reaction and chemoselectivity for the regioselective cyclisation of a number of ortho-cyclohexenyl phenols 1(a-j) to 3-bromo-2,4-propano-chromans 2(a-j).
A number of ortho-cyclohexenyl phenols 2(a-i) have been cyclised by treatment with one equivalent of m-chloroperoxybenzoic acid in refluxing benzene for 4 h to furnish linearly fused 1-hydroxy-1,2,3,4,4a,9a-hexahydrodibenzofurans 3 (a-i) (70-80%) and bicyclic 3-hydroxy-1,3-ethanochromans 4(a-f) (10-20%). Products 3(a-i) were oxidised with 2,3-dichloro-5,6-dicyano-1 ,4-benzoquinone (excess) in refluxing dry xylene for 6 h to give 2,3-dihydrodibenzofuran-1 (2H)-ones 6(a-i) (85-95%).