Intramolecular reductive coupling

Scheme 8 Intramolecular reductive coupling of a planar chiral diimine...

Scheme 5 Ni-EtjSiH promoted intramolecular reductive coupling of 15...

One of the most convenient ways to form benzocinnolines 183 is the intramolecular reductive coupling of 2,2 -dinitrobiphenyls 182 exemplified by the work of Kaszynski and Sandhu <00JCS(P1)67>. The reaction is general, effected by most typical reducing agents with good to excellent yields . 

For use of conjugated enones as nucleophilic partners in catalytic intramolecular reductive couplings to aldehydes, see ... 

Sml2 also effects an intramolecular reductive coupling of certain unsaturated (3-keto esters or amides to cyclopentanes with good stereochemical control at three centers (equation II).5... 

A modified procedure suitable for intramolecular reductive coupling is achieved using low-valence titanium prepared by reduction of titanium trichloride with a zinc-copper couple followed by the extremely slow addition of ketone to the refluxing reaction mixture (0.0003 mol over a 9-hour period by use of a motor-driven syringe pump) [S60. ... 

Benzene hydrocarbons are known to undergo radical coupling reactions and the intramolecular reductive coupling of carbonyl compounds with a benzene ring has been achieved. Best conditions for this process are at a tin cathode with isopropanol solvent and tetaethylammonium tosylate as supporting electrolyte [102, 103], The reaction is performed at constant current in a divided cell. A single stereoiso-... 

The synthetic plan called for a late-stage intramolecular reductive coupling of the iododiene 3 to establish the macrocyclic ring of 4. The iododiene 3 was to be assembled by condensation of the highly-substituted cyclohexene 1 with the aldehyde 2. 

Cyclic glycols are obtained in moderate yields by intramolecular reductive coupling of the ketones 106 and 107 at a mercury cathode 41... 

Intramolecular reductive coupling of activated olefins = electrohydrocycli-zation, EHC, has been recently reviewed by Baizer and Petrovich 432 The general reaction scheme of EHC is represented by Eq. (199) ... 

J. L. Chiara, J. Marco-Contelles, N. Khira, P. Gallego, C. Destabel, and M. Bemabe, Intramolecular reductive coupling of carbonyl-tethered oxime ethers promoted by samarium diiodide A powerful method for the stereoselective synthesis of aminocyclopentitols, J. Org. Chem., 60 (1995) 6010-6011. 

Intramolecular reductive coupling of diketo sulfides (3-thiapenetane-l,5-diones)... 

Vogel and coworkers have also explored an alternative approach to the synthesis of isocorroles. This more rational approach involves the low-valent titanium-mediated intramolecular reductive coupling of an a,co-formyl-substituted linear tetrapyrrolic species. Specifically, ring closure of the linear species 2.272, carried out under these McMurry-type conditions, afforded the etioisocorrole 2.270 in 10% yield (Scheme 2.2.4). The putative corrologen , thought to occur as an intermediate in this reaction, was not isolated. Rather, oxidation by molecular oxygen or FeCls during workup was presumed to occur spontaneously. 

An intramolecular reductive coupling of ketones with nitriles has been reported for acyclic and monocyclic substrates y9-amino nitriles were isolated from acyclic malononitrile adducts [87]. The Sml2-initiated reductive cyclization of cyclic a-cyanoalkyl-substituted ketones leads to acyloin products in high yields. In this instance further irradiation of the reaction mixtures was performed to afford complete conversion (Scheme 27) [88]. More applications have been collected in several excellent reviews [89-92]. 

Bis-activated olefins undergo electrohydrocyclization (EHC) by intramolecular reductive coupling. It was proposed that the mechanism involves a concerted electron transfer and cyclization which generally gives a mixture of the cis and tram isomers of the cyclic product (7). The latter product is favored at low water concentration whereas at higher ones intermolecular radical-radical coupling competes ... 

The intramolecular reductive coupling of carbonyl groups to a benzene ring has also been achieved. The best conditions for this process are at a tin cathode with isopropanol as... 

The hydrodimerization is also a useful method in the preparation of heterocycles [7,8], 6>-Bis(yS-bicarbethoxyvinylamino)benzene (III) thus yields by intramolecular reductive coupling 2,3-bis(dicarbethoxymethyl)-l,2,3,4-tetrahydroquinoxaline (IV), which on heating gives quinoxaline [7], as in Eq. (2). Compound IV may be anodically oxidized to the substituted quinoxaline [9],... 

Cobaloxime(I) has been shown to catalyze intramolecular reductive coupling of cyclic bromo acetals with double and triple bonds, although fairly large amounts of the catalyst (0.5 eq.) had to be used [244]. Significantly better yields (75-87%) were obtained when MeOH was used as solvent instead of MeCN or DMF [Eq. (56)]. Formation of six-membered rings could not be achieved [244]. 

Intramolecular reductive couplings between alkenes and aromatic halides have been mentioned in sec. IV.B.5 and IV.B.6. Here, discussion centers on reductive coupling either between two aromatic rings or between an aromatic ring and another, nonolefinic molecule. 

Intermolecular coupling of pyridines with ketones under acidic conditions has received attention but like the intramolecular reductive coupling of aromatic rings linked to aliphatic ketones [Eq. (62)], the reaction is essentially a ketone reduction, and the reader is referred to Chapter 10. 

Intramolecular reductive coupling between the isoquinolinium systems (107) and an attached aromatic halide [Eq. (67)] [308], gives products similar to those obtained by the intramolecular coupling of aromatic halides with other aryl groups (Sec. V.B). 

Reduction and reductive coupling. The use of polymer-supported PhjP for debromination of a-bromoketones has been advocated. Intramolecular reductive coupling of bis(acrylic esters) to form carbo- and oxacycles is effected photochemically in the presence of Ph,P and 9,10-dicyanoanthracene. ... 

In an analogous manner, novel 2-(2-hydroxyaryl)quinolines can be prepared via a one-pot synthesis from intramolecular reductive coupling reactions of 2 -hydroxy-2-nitro-chalcones [75]. Accordingly, 2 -hydroxy-2-nitrochalcones 177a-b were treated with ammonium formate and Pd/C at room temperature to give a mixture of 2-(2-hydroxy-aryl)quinolines 178a-b and 2-(2-hydroxyaryl)quinoline-N-oxides 179a-b. 

Palladium(0)-catalyzed intramolecular reductive coupling of the diacetates 1 and 3 with hexa-methyldistannane leads to tricyclic compounds 2 and 4, respectively, in a completely regioselec-tive manner2. Intermediate formation of a monoallyltin species followed by attack of this allyl tin moiety on the rc-allyl complex generated from the remaining allyl acetate is a likely route to the products. Since a substrate similar to 3 has been shown to isomcrizc to a nonstereogenic terminal diacetate under the conditions listed, the stereochemical results obtained for both 1 and 3 reflect the simple diastereoselectivity of these cyclizations. 

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