Partial and Complete Reduction of Pyridines and their Benzo Analogs

Keay, James G.

doi:10.1016/b978-0-08-052349-1.00239-0

Cited by 31 publications

(15 citation statements)

References 87 publications

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“…It was known [3,4] that the isoquinolinium salts (4) were reduced with sodium borohydride in protic solvents like water, methanol, ethanol, etc., to 1,2,3,4-tetrahydroisoquinolines (5) while in aprotic solvents like anhydrous pyridine or dimethylformamide to 1,2-dihydroisoquinolines (6) (Scheme 2). However, electron withdrawing groups in position 3 [5,6] or 4 [7,8] of the isoquionolinium moiety stopped the reduction at the 1,2-dihydroisoquinoline (6) stage even in protic solvents like methanol or ethanol.…”

Section: Dedicated To Professor Josef Michl On the Occasion Of His 65mentioning

confidence: 99%

Unexpected formation of 1,2‐dihydro‐2‐azolyl‐and azinylisoquinolines by the reduction of the corresponding isoquinolinium salts with sodium borohydride in methanol

Prauda

Toth‐Lauritz

Reiter

2004

Journal of Heterocyclic Chem

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Dedicated to Professor Josef Michl on the occasion of his 65 th birthdayThe reduction of different 2-azolyl-and azinylisoquinolinium salts with sodium borohydride in methanol was studied. Surprisingly, contrary to what is found in the literature 1,2-dihydroisoquinoline derivatives were obtained. Their formation was attributed to the electron withdrawing character of the heterocyclic ring in position 2 of the isoquinolinium moiety. This was corroborated by synthesis and reduction of differently substituted 2-phenyl-and 2-methylisoquinolinium salts. Recently, we have reported on the synthesis of 2-(pyra- [1,2] by the reaction of ortho-acylphenylacetones (1) or the pyrylium salts (1a) formed from them with perchloric acid and the corresponding aminoazoles (2) (Scheme 1).Some of these derivatives moderately but rather selectively inhibited the 5HT 7 receptors showing promising CNS activity. To help them to penetrate through bloodbrain barrier we decided to remove their ionic character by reduction.It was known [3,4] that the isoquinolinium salts (4) were reduced with sodium borohydride in protic solvents like water, methanol, ethanol, etc., to 1,2,3,4-tetrahydroisoquinolines (5) while in aprotic solvents like anhydrous pyridine or dimethylformamide to 1,2-dihydroisoquinolines (6) (Scheme 2). However, electron withdrawing groups in position 3 [5,6] or 4 [7,8] of the isoquionolinium moiety stopped the reduction at the 1,2-dihydroisoquinoline (6) stage even in protic solvents like methanol or ethanol.Surprisingly, when reducing the 6,7-dialkoxy-3-methyl-1-R-2-(5-substituted-1,2,4-triazol-3-yl)isoquinolinium chlorides (3a, A = N, B = C-R", D = NH) with excess of sodium borohydride in methanol only the corresponding 1,2-dihydroisoquinolines 8a (A = N, B = C-R", D = NH) were formed in good yield (see Table I, for their spectral data see Table II) (Scheme 3).To study further this unexpected reaction it was performed also with 2-pyrazolyl-[3b, A = CH, B = C-R"', D = NH], 2-imidazolyl-[3c, A = NH, B = D = CH] and 2-tetrazolyl-[3d, A = B = N, D = NH] isoquinolinium salts prepared mostly for this purpose (see Table III, for their spectral data see Table IV) yielding again only the corresponding 1,2-dihydro-derivatives 8b (A = CH, B = C-R"', D = NH), 8c (A = NH, B = D = CH) and 8d (A = B = N, D = NH), respectively (see Table I, for their spectral data see Table II) (Scheme 4).To extend the scope of this unusual reduction it was performed also with azinyl-, namely the 3-pyridyl-(9a, E = G Nov-Dec 2004915

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Section: Dedicated To Professor Josef Michl On the Occasion Of His 65mentioning

confidence: 99%

Unexpected formation of 1,2‐dihydro‐2‐azolyl‐and azinylisoquinolines by the reduction of the corresponding isoquinolinium salts with sodium borohydride in methanol

Prauda

Toth‐Lauritz

Reiter

2004

Journal of Heterocyclic Chem

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“…Tetrahydroquinolines [1–4] represent a well-known structural motif found in a large number of biologically active natural products. Optically active tetrahydroquinolines are important building blocks for the pharmaceutical and agrochemical industries.…”

Section: Introductionmentioning

confidence: 99%

A combined continuous microflow photochemistry and asymmetric organocatalysis approach for the enantioselective synthesis of tetrahydroquinolines

Sugiono¹,

Rueping²

2013

Beilstein J. Org. Chem.

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SummaryA continuous-flow asymmetric organocatalytic photocyclization–transfer hydrogenation cascade reaction has been developed. The new protocol allows the synthesis of tetrahydroquinolines from readily available 2-aminochalcones using a combination of photochemistry and asymmetric Brønsted acid catalysis. The photocylization and subsequent reduction was performed with catalytic amount of chiral BINOL derived phosphoric acid diester and Hantzsch dihydropyridine as hydrogen source providing the desired products in good yields and with excellent enantioselectivities.

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“…Regarding heteroaromatic compounds, acridines 1s , quinolines 1t and 1v , isoquinolines 1u , and N -alkylated indoles 1w , were readily reduced (Table ). Dimerization and over-reduced byproducts were not observed in our reactions . Reduction of benzofuran 1y afforded a significant amount of the C–O cleavage ring-opening product 2y′ , while furan is not reduced under these conditions, as is often the case in the traditional Birch reduction.…”

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confidence: 99%