Transition-Metal-Free Synthesis of Isoquinolines

Significance

Reported is an efficient synthesis of 1,3,4-trisubstituted isoquinolines involving an iodine-mediated, intramolecular electrophilic heteroannulation of 2-alkynyl benzyl azides. Cyclization with I2 is achieved in excellent yields for substrates bearing bulky R1 = aryl or alkenyl substituents which stabilize the cationic iodonium intermediate formed upon alkyne activation by I2. The reaction failed for R1 = coordinating substituent or H. Formation of side product 3 was minimized by the use of iodine electrophiles having less nucleophilic counterions (i.e. Py2IBF4/HBF4 and NIS) The preliminary investigation of functional group compatibility suggests a dependence of reaction outcome on the iodine reagent and reaction conditions. However, the availability of neutral, basic, or acidic reaction conditions provides options for sensitive substrates, and the method shows promise for the construction of substituted naphthyridines.