Annelated furans. XV. The autoxidation of tetrahydrodibenzofurans

The autoxidation of N-phenyl-1,2,3,9b-tetrahydrodibenzofuran-1,2-dicarboximide and the corresponding 4-methyl and 4-phenyl derivatives, in benzene solution, produced a mixture of the respective stereoisomeric 4-hydroperoxy-1,2,3,4-tetrahydrodibenzofuranisn each case. This reaction was shown to be a ground state process and resulted in the rearrangement of the 1,2,3,9b-tetrahydrodibenzofuran system to the thermodynamically more stable 1,2,3,4-tetrahydrodibenzofuran moiety. The introduction of a 4-acetoxy or 3-methoxycarbonyl substituent into the 1,2,3,Yb-tetrahydrodibenzofuran system appeared to facilitate the autoxidation process, and the initial N-phenyl-1,2,3,9b-tetrahydrodibenzofuran- 1,2-dicarboximides could not be isolated in these instances. Some reactions of c-4- and t-4-hydroperoxy-4-methyl-N-phenyl-1,2,3,4-tetrahydrodibenzofuran-r-1,c-2-dicarboximide have been investigated. These hydroperoxides decomposed on warming in chloroform solution to produce 4-methyl-N-phenyl-1,2-dihydrodibenzofuran-1,2-dicarboximide and 4-methyl-N-phenyldibenzofuran-1,2-dicarboximide In addition to the latter product, treatment of the mixture of hydroperoxides with p-toluenesulphonic acid in chloroform solution resulted in cleavage of the heterocyclic ring to give 5-ethoxy-3-(2'-hydroxyphenyl)-5-methyl-4-oxo-N-phenylcyclohex- 2-ene-1,2-dicarboximide (44). The autoxidation of 4-methyl-1,2,3,9b-tetrahydrodibenzofuran-1,2-dicarboxylic acid anhydride (48) proceeded by an alternative route and gave a novel peroxylactone, 6-methyl-3-oxo-3,3a,4,5- tetrahydrobenzofuro[2,3-h]-1,2-benzodioxoe-4-carboxylic acid (50), in addition to the fully aromatic species, 4-methyldibenzofuran-1,2-dicarboxylic acid anhydride (59) The synthesis and autoxidation of 1-acetyl-4-methyl-1,2,3,9b-tetrahydrodibenzofurawn as also studied. Finally the reaction of N-phenylmaleimide with 2-(3'-benzofuranyl)but-1-ene (72) was shown to give 1-ethyl-N-phenyl-2,3,4,4a-tetrahydrodibenzofuran-3,4-dicarboximide (73 ). Autoxidation of the latter compound proceeded with rearrangement to give r-1-ethyl-1-hydroperoxy-N-phenyl-1,2,3,4- tetrahydrodibenzofuran-t-3,t-4-dicarboximide (74).