Oxidative Coupling of 4-Substituted N,N-Dimethylanilines with Cyclic Vinyl Ethers in the Presence of Either Manganese(II) or Cobalt(II) Nitrate under Oxygen

The reaction of 4-substituted N,N-dimethylanilines with 2.3-dihydrofuran and 3,4-dihydro-m-pyran proceeds in the presence of a catalytic amount of manganese(l1) or cobalt(l1) nitrate under oxygen to give the corresponding hexahydrofuro-and hexahydro-a-pyranoL3.2-~lquinolines, respectively.The synthesis of polycyclic compounds involving aquinoline ring is of current interest, since they are expected to exhibit pharmacological effects.1-3The [4+2] cycloaddition of benzylidenanilines with 2,3-dihydrofurans using acid catalysts is a useful method for preparation of tricyclic hexahydrofuro[3,2-rlquinoline derivatives,C8 some of them being known to exhibit biological activities such as antimalarial effecL5x6 We have recently reported that N,N-dimethylmilines react with acyclic vinyl ethers in the presence of a catalytic amount of iron(II1) chloride under oxygen to give 4-alkoxy-l,2,3,4-tetrahydro-l-methylquinolines.9We now report the synthesis of N,O-containing tricyclic compounds, hexahydrofuro-and hexahydro-2H-pyrano[3,2- dquinolines (4) and (5), by the reactions of 4-substitw,N-dimethylanilines (1) with 2.3-dihydrofuran (2) and 3,4-dihydro-a-pyran (3).respectively, in the presence of various transition-metal salts.Table 1 shows the results for the reaction of N,N-dimethyl-g-toluidine ( l a ) (2 mmol) with 2 (10 mmol) in acetonitrile under oxygen (1 atm) at 50 OC for 20 h using a variety of metal chlorides and nitrates (0.02 mmol) as the catalysts; 6-28 equivalent of the coupling product, 2,3,3a,4,5,9b-hexahydro-S,8-dimethylfuro[3,2clquinoline (4a).per equivalent of catalyst used was produced.Among the metal species tested, -Mn(N03)26H20 gave the most satisfactory result in terms of both the product selectivity (93 %)and yield (28 %)which are defined as the yield of 4a based on l a consumed and charged, respectively.Cobalt(ll) nitrate