During our studies concerning the syntheses of pyridines and quinolines via an intermolecular condensation reaction^4b we observed a competitive intramolecular cyclization reaction in a single case where a Morgan-Walls⁶ cyclization pathway was possible. N-Phenethylbenzamide (1, Table 1) was used to further investigate this intramolecular condensation reaction. Consistent with our observations on amide activation for the intermolecular addition of σ- or π-nucleophiles,⁴ the use of trifluoromethanesulfonic anhydride (Tf₂O)⁷ and 2-chloropyridine⁸ (2-ClPyr) as the base additive were found to be optimal for a mild cyclodehydration reaction to provide the desired 3,4-dihydroisoquinoline 2 in 95% yield (Table 1, entry 7). The reaction was found to be less sensitive to superstoichiometric quantities of 2-ClPyr as compared to its absence (compare entries 7−9, Table 1), allowing the inclusion of excess base additive for BrØnsted acid sensitive substrates.⁹ Electrophilic amide activation¹⁰ followed by intramolecular π-nucleophilic cyclization and subsequent deprotonation directly provides the desired product 5 (Scheme 1).

A wide range of N-phenethylamide derivatives were found to readily provide the corresponding 3,4-dihydroisoquinoline products (Figure 1, 2, 6−14). Alkoxy and unsubstituted N-phenethylamides provided the desired dihydroisoquinoline products at ambient temperature or with mild heating. The conversion of recalcitrant substrates was found to be optimal via short (5 min) microwave irradiation.¹¹ For example, deactivated halogenated N-phenethylamides did not cyclize at ambient temperature, but provided the desired 3,4-dihydroisoquinolines with microwave irradiation (Figure 1, 9 and 10). The formation of the phenylalaninol–derived dihydroisoquinoline 14 was noted to occur with no loss in optical activity.¹² Significantly, sensitive N-vinyl amides¹³ were used as substrates in this chemistry to directly provide isoquinoline derivatives (Figure 1, 15−18). While (E)-N-styrylcyclohexanecarboxamide did not provide isoquinoline 15, (Z)-N-styrylcyclohexanecarboxamide was converted to the desired isoquinoline 15 in moderate yield (Figure 1). It should be noted that this substrate was sensitive (vide infra) to decomposition/polymerization following electrophilic amide activation. Tri- and tetrasubstituted enamides proved to be excellent substrates for this chemistry and efficiently gave the corresponding azaheterocycles (Figure 1, 16−18). o-Arylaniline derived amides afforded the desired fused tricyclic azaheterocycles, reminiscent of Morgan-Walls cyclization products, in good yield (Figure 1, 19−21). Additionally, the use of tryptamine derived substrates, optimally N-alkyl derivatives, gave the corresponding 3,4-dihydro-β-carbolines (Figure 1, 22−28).

Highly deactivated substrates such as N-(4-nitrophenethyl)cyclohexanecarboxamide or N-(4-(trifluoromethyl)phenethyl)benzamide did not provide the corresponding dihydroisoquinolines.¹⁴ This is likely due to a more rapid rate of elimination/decomposition upon activation as compared to the desired cyclodehydration reaction. Tryptamine derived amides bearing a sulfonyl group on the indolyl nitrogen were not substrates for this chemistry and unsubstituted indole derivatives led to rapid indolyl nitrogen N-sulfonylation of the starting material under the reaction conditions. It should be noted that in some cases minor side products resulting from oxidation (vide infra) of 3,4-dihydro-β-carboline were observed.¹⁵ Additionally, using the phenylalanine derivative 29 as a substrate under the standard reaction conditions competitively gave the oxazole 30 in 84% yield (Equation 1).^16,17 When 2-vinyl-aniline derived amide 31 was exposed to the standard cyclodehydration reaction conditions described above, a highly efficient condensation reaction ensued to afford 2-phenylquinoline (32, Equation 2) in 99% isolated yield.

The direct comparison of the herein described condensation reaction with related protocols further highlights the advantages offered by this chemistry (Table 2).² The synthesis of 3,4-dihydroisoquinoline 2, isoquinoline 15, and phenanthridine 20 is illustrative. Synthesis of 3,4-dihydroisoquinoline 2 was found to be most efficient using the conditions described here as compared to other reported condensation reaction conditions (Table 2). Sensitive substrates, such as the acid sensitive (Z)-N-styrylcyclohexanecarboxamide, were found to be incompatible with the broadly used conditions involving phosphorus oxychloride (POCl₃) in conjunction with heating.^2a Similarly, the use of reaction conditions employing oxalyl chloride and iron trichloride did not provide the desired phenanthridine 20 from the corresponding urea substrate.^2d

While in all three cases (Table 2) the use of superstoichiometric Tf₂O in conjunction with 4-(dimethylamino)pyridine (DMAP) provided the desired product,^2c the competing oxidation reaction in more sensitive substrates is a potential complication. For example, using the herein described conditions, electrophilic activation of amide 33 (Equation 3, 97% ee) afforded the desired optically active 3,4-dihydroisoquinoline 34 in 87% yield and 90% ee without undesired oxidation to the corresponding isoquinoline.¹⁸ However, electrophilic activation of amide 33 using the reported reaction conditions^2c employing excess Tf₂O–DMAP gave the desired product 34 in 31% yield, and with only 63% ee, in addition to 26% yield of the corresponding isoquinoline derivative due to oxidation of 34. Additionally, activation of amide 33 via the typical condensation reaction conditions employing POCl₃ failed to provide the desired product 34 due to competitive decomposition.

As mentioned the 3,4-dihydro-β-carboline condensation products are subject to oxidation with Tf₂O, affording the corresponding β-carbolines.^15,19 In the case of 3,4-dihydroisoquinoline 34 this was a significant complication when excess Tf₂O was used (vide supra). Indeed, exposure of azaheterocycles 2, 6, and 27−28 to Tf₂O and 2-ClPyr resulted in the corresponding oxidation products (Figure 2). Electron rich dihydro-β-carbolines are more sensitive to this oxidation reaction as compared to dihydroisoquinolines (Figure 2). For comparison, while oxidation of 3,4-dihydroisoquinoline 2 to isoquinoline 38 required excess reagents and heating to 140 °C, the oxidation of 3,4-dihydro-β-carboline 27 at 23 °C gave β-carboline 36 in 65% yield within 2 h (Figure 2).²⁰

The chemistry described herein provides an efficient modified Bischler-Napieralski cyclodehydration reaction to access isoquinolines, β-carbolines, and their 3,4-dihydro derivatives. The successful use of unactivated, halogenated N-phenethylamides, sensitive N-vinyl amides, and optically active substrates is noteworthy. The direct comparison of this chemistry with existing methods as shown in Table 2, and the observations discussed regarding epimerization and oxidation challenges in the context of substrate 33, highlight the advantages offered by this methodology.

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