Iodine-catalyzed stepwise [4+2] cycloaddition of phenothiazine- and ferrocene-containing Schiff bases with DHP promoted by microwave irradiation

On simultaneous effect of iodine-catalysis and microwave irradiation Schiff bases deactivated by electron-donating C-phenothiazinyl- and ferrocenyl-substituents, respectively, underwent formal inverse electron demand aza-Diels-Alder (DA) cycloaddition with 3,4-dihydro-2H-pyrane (DHP) employed as donor component. Depending on the substitution pattern of the N-phenyl group the reactions of phenothiazine-containing imines afforded 2H-pyrano[3,2-c]quinolines or 3-(3-hydroxypropyl)quinolines. Irrespective of the electronic properties of the N-phenyl substituent the less reactive ferrocene-based imines were directly converted into quinoline products. The intermediate iodoiminium ions were analysed by B3LYP/DGZVP calculations suggesting stepwise mode for the cycloaddition process. In one case the regioselectivity of the second step of cycloaddition was also interpreted by DFT analysis of the alternative rotamers of the primarily formed DHP adduct.