Alkylation of isoquinoline

The reductive alkylation with tertiary halides40 gives no N-alkylation and another product distribution, as discussed below for the alkylation of isoquinoline. 

Imidazolium salts are the most important class of ionic liquids, but preparation of ionic liquids based on different cations has also been reported. Pyridinium salts are prepared in a similar way to imidazolium, by direct alkylation of pyridine. Alkylisoquinolium bis(perfluoroethylsulfonyl)imide salt 13 is similarly prepared by A-alkylation of isoquinoline followed by anion metathesis, and iminium salt 14 is obtained directly associated with the triflate anion (Scheme 6). ... 

This reaction does not take place in the absence of ultrasound. C-Alkylation of isoquinoline derivatives can be effected using sonication under PTC conditions (Scheme 11). 

The N-oxides of isoquinolines have proved to be excellent intermediates for the preparation of many compounds. Trialkylboranes give 1-alkyl derivatives (147). With cyanogen bromide in ethanol, ethyl N-(l- and 4-isoquinolyl)carbamates are formed (148). A compHcated but potentially important reaction is the formation of 1-acetonyLisoquinoline and 1-cyanoisoquinoline [1198-30-7] when isoquinoline N-oxide reacts with metbacrylonitrile in the presence of hydroquinone (149). Isoquinoline N-oxide undergoes direct acylamination with /V-benzoylanilinoisoquinoline salts to form 1-/V-benzoylanilinoisoquinoline [53112-20-4] in 55% yield (150). A similar reaction of AJ-sulfinyl- -toluenesulfonamide leads to l-(tos5larriino)isoquinoline [25770-51-8] which is readily hydrolyzed to 1-aminoisoquinoline (151). 

With pyridine and its alkyl derivatives, and in contrast to chlorination, substantial nuclear fluorination occurred before the side chain was attacked (87TL255). Direct fluorination of isoquinoline was unsuccessful but 2-methylcarbostyril gave the 4-fluoroderivative in 54% yield (82H429). 

Thioxo-2,3-dihydro[l, 2,4]triazino[6,5-c]isoquinolin-6-one 189 was obtained (75ZOR2407) by the cyclization of the 3-thiosemicarbazone 188 with alkali. Compound 188 was prepared by the reaction of 3-anilino-1,4-dihydro- 1,4-isoquinolinedione 187 or the ether 186 with thiosemicarbazide. Compound 187 was obtained by reaction of the ether 186 in benzene with aniline. The ether 186 was obtained from the alkylation of 179 (R = H) via its silver salt 185. 

Vedejs et al. reported catalyst inhibition during a study on the enantioselective transfer hydrogenation of dihydro-isoquinolines using Noyori s catalyst (Scheme 44.2) [27]. Here, the problem is caused by the bidentate nature of the substrate. Whereas the bromo compound 1 a could be rapidly reduced, the tosylamide-sub-stituted compound lb could not be reduced, and although the problem could be alleviated somewhat by alkylation of the sulfmamide to 1 c, hydrogenation of this was still sluggish. Although the authors propose this to be a case of product... 

The Delft synthesis makes use of an acid-catalyzed ring closure - in fact an intramolecular aromatic alkylation - of a l-(3,5-dihydroxy-4-methoxybenzyl) isoquinoline derivative that is prepared starting from (natural) gallic acid. One of the hydroxyl groups is removed via a Pd/ C hydrogenation of the benzyl ether. Other catalytic steps play an important role some steps were improved recently [27]. The crucial step in the Rice synthesis makes use of a l-(2-bromo-5-hydroxy-4-methoxybenzyl)isoquinoline derivative that is also cyclized in an acid-catalyzed ring closure to the morphinan skeleton, followed by catalytic removal of the bromo substituent (Scheme 5.8). 

In another example increased yields of products are obtained under ultrasonic irradiation in the PTC alkylation of the isoquinoline derivatives using 50% aqueous NaOH as base [125]. Efficient mixing is not easy to achieve for this system under normal reaction conditions due to the viscosity of the aqueous base. In the specific case of alkylation with benzyl chloride ultrasound plus [Et3NCH2Ph] Cl achieved 60% yield in 20 min compared with only 50% in 2 h with stirring (Eq. 3.25). 

Scheme 7.2. Centres of attack during reductive alkylation of quinoline and isoquinoline...

Several further publications report on the (—)-sparteine-mediated addition of alkyl-or aryllithium onto imines or the C=N bond of isoquinolines . Usually, the achieved enantiomeric excesses are low and, sometimes, other chiral ligands serve better. As reported by Muller and coworkers, the nucleophilic substitution of arenecarbaldehyde dialkyl acetals by o-substituted aryllithium reagents is an alternative . 

The synthesis of compound 135 (Scheme 12) includes the production of isoquinoline 132 via a Bischler-Napieralski reaction, its reduction and alkylation on the nitrogen atom followed by cyclization of N-alkyl derivatives 134 in the presence of acids (83JHC1477 Scheme 36). 

The formation of a Reissert anion (intermediate of type 16) is usually the introductory step in a great number of synthetic routes leading to isoquinoline as well as indole alkaloids and related compounds. On the one hand, the alkylation of a Reissert anion with alkyl halide followed by alkaline hydrolysis is the most frequently used method for the synthesis of 1-alkyl- or 1-arylalkylisoquinolines (20) (Scheme 4). On the other hand, Reissert anions react with aldehydes to form... 

Skiles and Cava have investigated the alkylation method of isoquinoline Reissert compounds (76), performing a comparative study on the alkylation of... 

Reticuline (38), one of the most important intermediates in the biosynthesis of opium alkaloids, has been synthesized in racemic form (Scheme 7) (78). 6-Methoxy-7-benzyloxyisoquinoline (39), prepared from O-benzylisovanillin via a modified Pomeranz-Fritsch isoquinoline synthesis, was treated with benzoyl chloride and potassium cyanide to obtain Reissert compound 40. Alkylation of the anion generated from 40 with 3-benzyloxy-4-methoxybenzyl chloride gave the corresponding 1-substituted Reissert compound 41 which was hydrolyzed in alkaline medium to 1-benzylisoquinoline derivative 42. Quatemarization of 42 with methyl iodide followed by sodium borohydride reduction and debenzylation led to ( )-reticuline (38) in about 25% overall yield from 39. 

Jackson et al. (22) reported the biomimetic total synthesis of ( )-cularine (67) itself (Scheme 11). Benzyolation of isoquinoline 68 in the presence of potassium cyanide gave Reissert compound 69, the anion of which was alkylated with 3-benzyloxy-4-methoxybenzyl chloride, resulting in intermediate 70. After al-... 

N-Alkylation of 3,3-diethyl-l,3,4,6,7,llb-hexahydro-2//-pyrimido-[2,l-a]isoquinoline-2,4-dione (95, R = R1 = Et, R2 = H) with 3-(dimethylamino)propyl chloride in dioxane in the presence of sodium hydride (53% in oil) gave the l-(3-dimethylamino)propyl derivative (69YZ649. 

C-Alkylation of 1-methyl-1,2,3,4-tetrahydro-6//-pyrimido[l,2-b]isoquino-lin-6-one with benzyl bromide in boiling methylene chloride overnight in the presence of potassium carbonate, and with excess methyl iodide gave 11-substituted derivatives (113 and 114) (88HCA77). The treatment of hydrobromide salt of 11-benzyl derivative 113 with a base yielded 11-benzyl-1 -methyl-1,2,3,4-tetrahydro-6//-pyrimido[l, 2-6]isoquinolin-6-one (115). 

Reissert compounds (l-acyl-l,2-dihydro-2-quinolinecarbonitriles) have been prepared on cross-linked polystyrene and C-alkylated in the presence of strong bases (Entry 8, Table 15.25). Treatment of polystyrene-bound C-alkylated Reissert compounds with KOH leads to the release of isoquinolines into solution (Entry 9, Table 15.25). The reaction of support-bound quinoline- and isoquinoline /Y-oxides with acy-lating agents followed by treatment with electron-rich heteroarenes and enamines has been used to prepare alkylated and arylated derivatives of these heterocycles (Entry 10, Table 15.25 see also Table 15.26). 

This powerful quaternization method enabled the catalytic asymmetric synthesis of quaternary isoquinoline derivatives with 42 (R1 = Me) as a substrate. When 42 (R1 = Me) was treated with a,a -dibromo-o-xylene, CsOHH20 and (S,S)-le (1 mol%) in toluene at 0 °C, the transient monoalkylation product was rapidly produced, and subsequently transformed into the desired 44 (64%, 88% ee) during the work-up procedure. Catalytic asymmetric alkylation of 42 (R1 = Me) with functionalized benzyl bromide 45, followed by the sequential treatment with 1 M HC1 and then excess NaHC03, furnished the corresponding dihydroisoquinoline derivative 46 in 87% with 94% ee (Scheme 5.23) [25]. 

Charge effects may also play an extremely important role in controlling the reactions of co-ordinated amines with electrophilic reagents. This is very clearly seen in the alkylation reactions of nucleophilic sites remote from the metal. On electrostatic grounds we would expect the reaction of positively charged complexes with electrophiles to be less favoured than the reaction of neutral or anionic complexes, and this is indeed the case. Consider the attempted alkylation of the non-co-ordinated isoquinoline rings in the cop-per(n) complexes 5.14 and 5.15. Compound 5.14 is derived from salicylaldehyde and... 

Seebach, D. Lohmann, J.-J. Syfrig, M. A. Yoshifuji, M. Alkylation of the isoquinoline skeleton in the 1-position. Lifhiated 2-pivaloyl and 2-bis(dimefhylamino)phosphinoyl-l,2,3,4-tetrahydroisoquinolines. Tetrahedron 1983, 39, 1963-1974. 

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