Synthesis of Chromane Derivatives via Indium-mediated Intramolecular Allenylation and Allylation to Imines

Indium-mediated intramolecular allylation and allenylation to carbonyl groups have been good methods to prepare chromane structures. There are, however, still some limitations in these transformations. Especially, in the case of allylation mixtures of cis and trans isomers are always produced in about 2 : 1 ratio (cis/trans). The ratio was not improved under the various reaction conditions we attempted. Since the indium-mediated addition to carbonyl groups has been successful, it occurred to us that it would be worthwhile to test the addition to carbon-nitrogen double bonds, that is, imine groups. We wish to report here the results of the investigations on allylation and allenylation to C=N bond to provide the chromane structures. The whole transformations are shown below (Eq. 1 and 2).