A NOVEL SYNTHESIS OF SUBSTITUTED 1-BENZYLOCTAHYDROISOQUINOLINES BY ACID-CATALYZED CYCLIZATION OF N-[2-(CYCLOHEX-1-ENYL)ETHYL]-N-STYRYLFORMAMIDES

The acid-catalyzed cyclization of N-[2-(cyclohex-1-enyl)ethyl]-N-styry lformamides 1-5 gave access to 1-benzyl-2-formyloctahydroisoquinolines 6-10. The reactions were performed in the presence of the Lewis acid 9-borabicyclo[3.3.1]non-9-yl triflate. The cyclization of enamide 1 wa s also studied with Bronsted acid catalysts, such as triflic acid and the heterogeneous catalyst tungstophosphoric acid supported on silica gel. In all cases the 1,2,3,4,5,6,7,8-octahydroisoquinoline formed was accompanied by minor concentrations of one, two, or three isomeric oc tahydroisoquinolines. 1-Benzyl-2-formyloctahydroisoquinoline (6) could also be prepared from N-[2-(cyclohex-1-enyl)ethyl]formamide (11) by r eaction with phenylacetaldehyde in a mixture of acetic acid and triflu oroacetic acid. The octahydroisoquinolines 6, 8, 10 as model compounds , were converted into the corresponding N-formylmorphinans.