Synthesis of (±)‐laudanoeine and a study of 1‐benzyl‐3‐isoquinoione intermediates

A keto acid (2) has been converted to tetrahydropapaverine and (±)‐laudanosine. Some intermediate 3‐isochromanone ( 3 ) and 3‐isoquinolone ( 4 and 5 ) derivatives have been investigated and structures assigned to these compounds from spectroscopic studies. The 3‐isoquinolone tautomer with the o ‐quinonoid structure predominates over the 3‐isoquinolinol form, but the analogous oxygen derivative has preferentially a stilbene structure ( 3 ) with an intact benzene system.