Synthesis of 1,2-Dihydroquinolines by Reduction of Quinolines with Sodium Cyanoborohydride

An efficient conversion of quinolines to 1,2-dihydroquinolines (50%～96% yield) was developed via the modification of the known methods.It was found that using sodium cyanoborohydride as a reductant would overcome the low conversion often encountered in previous studies.A series of N-alkoxycarbonyl-1,2-dihydroquinolines were obtained through reduction of activated quinolium salts.Notably, with the exception of the 3-and 4-substituted substrates, a mixture of 1,2-dihydroquinolines and the over reduced tetrahydroquinolines was obtained with the ratio over 4∶1.Besides, compared to the established methods, an easy operation without using large excess of chloroformate further enhances practicability of the methodology.