A new approach to the synthesis of substituted isoquinolines

Abstract

Isoquinoline-N-borane was used as an intermediate to synthesize 1-substituted and 1,4-disubstituted 1,2,3,4-tetrahydroisoquinolines. Organolithium reagents were added to C-1 to give enamine intermediate 1. Protonation of 1 led to 1-substituted tetrahydroisoquinolines; and addition of alkyl halides to 1 gave 1,4-disubstituted products. Electrophilic addition to C-4 was stereoselective in that the electrophile added selectively cis to the C-1 substituent. 1 was also shown to be an active hydride source. The adduct of sodium triethylborohydride and isoquinoline gave a boron-activated enamine 2 which was also a useful intermediate. Additions of alkyl halides to 2 led to 4,4-disubstituted tetrahydroisoquinolines. In the most useful procedure, aldehydes reacted with 2 in a three-step "one-pot" reaction sequence to give 4-substituted isoquinolines. Other aspects of the chemistry of intermediates 1 and 2 is described as well as the synthetic utility and limitations of this methodology.