Fritsch's cyclization of N-(α-veratrylveratrylidene)-aminoacetal in sulphuric acid was shown to give 1.1% of papaverine and 23% of an isomer, m.p. 164.5–165.5 °C.; hydrochloride, m.p. 212 °C. decomp., which was supposed to be 4,5-bis(3,4-dimethoxyphenyl)-2H-pyrrolenine, produced by an internal condensation of the acetal or the corresponding aldehyde with the reactive methylene group. A similar structure was proposed for another unidentified isomer prepared by Schlittler and Müller. Hydrogenation of Fritsch's acetal gave N-(α-veratryl-veratryl)-aminoacetal, m.p. 69.5–70 °C., which was cyclized to a base, m.p. 155.5–156 °C.; N-acetyl derivative, m.p. 203.5–204 °C., formulated as 2,3-bis(3,4-dimethoxyphenyl)-3-pyrroline. Substances presumed to be the intermediate aldehyde and aldol were isolated as colorless oils. Condensation of the diketone veratril with aminoacetal, followed by cyclization of the crude product, constituted a new two-step synthesis of papaveraldine in 8% yield, and the reduction of the latter to papaverine was known.Other crystalline compounds prepared incidentally and thought to be new were veratril monoanil, m.p. 172–173 °C.; α,α′-biveratrylideneaminoacetal, m.p. 101–102 °C.; a compound formulated as 2,3-bis(3,4-dimethoxyphenyl)-4-ethylmer-captopyrrolidine hydrochloride, m.p. 184–185 °C; from this an unidentified mercury complex, m.p. 109 °C. decomp.; 4,4′dibenzyloxy-3,3′-dimethoxy-desoxybenzoin, m.p. 141–142 °C; and its oxime, m.p. 137.5 °C.
Synthesis of chiral anti-1,2-diamine derivatives through copper(I)-catalyzed asymmetric α-addition of ketimines to aldimines
