Aromatic substitutions isoquinoline

A variation involves the reaction of benzylamines with glyoxal hemiacetal (168). Cyclization of the intermediate (35) with sulfuric acid produces the same isoquinoline as that obtained from the Schiff base derived from an aromatic aldehyde and aminoacetal. This method has proved especially useful for the synthesis of 1-substituted isoquinolines. 

Heterocyclic amines are compounds that contain one or more nitrogen atoms as part of a ring. Saturated heterocyclic amines usually have the same chemistry as their open-chain analogs, but unsaturated heterocycles such as pyrrole, imidazole, pyridine, and pyrimidine are aromatic. All four are unusually stable, and all undergo aromatic substitution on reaction with electrophiles. Pyrrole is nonbasic because its nitrogen lone-pair electrons are part of the aromatic it system. Fused-ring heterocycles such as quinoline, isoquinoline, indole, and purine are also commonly found in biological molecules. 

The synthesis of 3-aryltetrahydroisoquinolines was accomplished by electrophilic aromatic substitution of polysubstituted phenols and phenyl ethers with Lewis acid-generated tosyliminium ions of 2-tosyl-3-methoxytetrahydroisoquinoline derivatives <00SL801>. In addition isoquinoline was reported to react with N-tosylated (R)- or (S)-amino acid fluorides to afford optically active dihydroimidazoisoquinolinones. The reaction proceeds via acylation followed by attack of the tosylamino group at Cl of the intermediate 2-tosylaminoacylisoquinolinium salt <00TL5479>. 

Furthermore, pyrazole 366 reacts with phthalazine (Scheme 132) to afford pyrazolo[3, 4 4,5]pyrido[6,l-a]phthalazine (367). From a mechanistic viewpoint, no 1,6-dipolar cyclization occurs. Instead, an intramolecular nucleophilic aromatic substitution to the heteroarene is likely. Isoquinoline leads to zwitterionic 368 (94JOC3985). 

Electrophilic aromatic substitutions Quinoline and isoquinoline undergo electrophilic aromatic substitution on the benzene ring, because a benzene ring is more reactive than a pyridine ring towards such reaction. Substitution generally occurs at C-5 and C-8, e.g. bromination of quinoline and isoquinoline. 

As in the Skraup quinoline synthesis, loss of two hydrogen atoms is necessary to reach the fully aromatic system. However, this is usually accomplished in a separate step, utilising palladium catalysis to give generalised isoquinoline 6.14. This is known as the Bischler-Napieralski synthesis. The mechanism probably involves conversion of amide 6.12 to protonated imidoyl chloride 6.15 followed by electrophilic aromatic substitution to give 6.13. (For a similar activation of an amide to an electrophilic species see the Vilsmeier reaction, Chapter 2.)... 

The imine salt is perfectly placed for an intramolecular electrophilic aromatic substitution by the electron-rich dihydroxyphenyl ring. This closes the isoquinoline ring in a Mannich-like process (Chapter 27) with the phenol replacing the enol in the pyrrolidine alkaloid biosynthesis. 

Chloropyrazolo[3,4- ]quinoline, 3-chloropyrazolo[3,4-acid-induced nucleophilic aromatic substitution (SnH) of H-3 in A -hydroxypyrazolo[3,4-first cases of nucleophilic aromatic substitution of a fused pyrazole <2002JOC585>. 

Here, once again, the cyclising step involves electrophilic attack on the aromatic ring so the method works best for activated rings, and meto-substituted-aryl ethanamides give exclusively 6-substituted isoquinolines. 

One of the most powerful methods for the construction of tetrahydroisoquinoline systems is the Pictet-Spengler cyclisation. The reaction consists of the condensation of a b-phenylethylamine derivative with a carbonyl compound, generating an imine (Schiffs base), which undergoes cyclisation via an intramolecular electrophilic aromatic substitution yielding the isoquinoline derivative. The Pictet-Spengler reaction is traditionally carried out in a protic solvent with acid catalysts, usually acetic acid or trifluoroacetic acid. 

Tetrahydroisoquinolines were also prepared by an electrophilic aromatic substitution reaction of 2-amidoacroleins. Exposure of IV-aryl-substituted 3-amido-1,3-dioxins 82 to Lewis acids results in retrocycloadditions to afford 2-amidoacroleins 83 and concomitant electrophilic aromatic substitution to afford tetrahydroisoquinolines 84 <01OL3349>. The synthesis of isoquinoline derivatives via cyclization reactions received attention as well. Some examples include the preparation of isoquinolines by a photocyclization of l-methoxy-2-azabuta-l,3-dienes <01TL3575>. The photochemically induced preparation of 1-methyl-1,2,3,4-tetrahydronaphtho[2,l-/ isoquinolines was also reported <01T1981>. 

Quinoline and isoquinoline show that electrophilic aromatic substitution reactions are the more reactive benzene ring because the pyridine ring is less reactive. Indole undergoes electrophilic aromatic substitution primarily in the pyrrole ring because it is much more reactive than the benzene ring 24, 25, 26, 27, 28, 29, 30, 51, 53. 

A rhodium-catalyzed one-pot synthesis of substituted pyridine derivatives from a,(3-unsaturated ketoximes and alkynes was developed in 2008 by Cheng and coworkers [99], Good yields of the desired pyri-dines can be obtained (Scheme 3.48). The reaction was proposed to proceed via rhodium-catalyzed chelation-assisted activation of the (3—C—H bond of a,(3-unsaturated ketoximes and subsequent reaction with alkynes followed by reductive elimination, intramolecular electro-cyclization, and aromatization to give highly substituted pyridine derivatives finally [100]. Later on, in their further studies, substituted isoquinolines and tetrahydroquinoline derivatives can be prepared by this catalyst system as well [101]. Their reaction mechanism was supported by isolation of the ort/jo-alkenylation products. Here, only asymmetric internal alkynes can be applied. 

Reactions with Electrophiles. The structure of isoquinoline 1 is the result of fusing benzene and pyridine together. Electrophilic aromatic substitution predominately occurs on the benzene ring under acidic conditions and usually addition takes place at the 5-position but can sometimes add to the 8-position. The rate of electrophilic aromatic substitution is slower for isoquinoline compared to naphthalene. The nitrogen in isoquinoline reacts similar to a pyridine nitrogen and will add a variety of electrophilic species such as 0-(2,4-dinitrophenyl)hydroxylamine 2 to aminate the nitrogen (eq 1). Friedel-Crafts acylation and alkylation do not work due to the formation of IV-acyl or IV-alkyl isoquinolinium salts. 

Reactions with Nucleophiles. NucleophiUc aromatic substitution (SnAt) is a common reaction for isoquinoUne. The nucleophile usually adds to the 1- or 3-position on the isoquinoUne ring with addition to the 1-position being about 10 times faster than the 3-position when ethoxide is the nucleophile. SNAr reactions involving A-methyl-isoquinoline are accelerated by as much as 10 times compared to the isoquinoUne. ... 

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