Furans, Birch reduction

Birch reductions in ammonia-tetrahydro-furan-/-butyl alcohol, 49... 

Donohoe reports a novel and unprecedented reductive aldol process involving the Birch reduction of furans and pyrroles (e.g., 62) which presumably generates a dianion 63 and subsequently (after protonation at C-5 by ammonia) an enolate 64. After quenching excess... 

Furopyridines have a fused structure containing both a 7t-excessive furan ring and a n-deficient pyridine ring. Interesting results were obtained during the Birch reduction of the four different furopyridines, and a mechanistic interpretation for the results was presented. An example is shown in the following scheme <02JHC335>. 

Although furan itself is reduced by Li/MeNH2 only under forcing conditions to give a complex mixture,the Birch reduction of furoic acids proceeds easily, is synthetically useful, and has been scrutinized by several groups, as summarized in equations The reduction of 5-alkyl-2-furoic... 

Phenylmenthol is the auxiliary in an asymmetric Birch reduction of pyrroles by Donohoe37 Lithium in ammonia does the reduction and the enolate is trapped with various alkyl halides. Hydrolysis of the esters 227 releases the enantiomerically enriched (78-90% ee) dihydropyrroles 228 in good yield. Furans give similar products with a C2 symmetric amine as auxiliary. This should become a general route to a variety of heterocycles. 

Birch reductions. Derivatives of pyrrole and furan are reduced to the dihydro level opening up many synthetic possibilities. 

Birch reaction is the dearomatization of aromatic rings to unconjugated cyclohexadienes in the presence of alkali metals (Li, Na, K) in liquid ammonia, using alcohol as proton source. Compared to other heterocyclic compounds, including pyridines, furans, and pyrroles, the Birch reduction of thiophenes and their derivatives has been scarcely investigated. 

Reactions. Birch reduction of 2-furoic acid with lithium in liquid ammonia gives the 2,5-dihydro-compound (131) in 90% yield. Hydrogenation of the vinyl-furans (132 R = CO2H, C02Me, Ac, or COPh) in the presence of nickel bromide occurs selectively at the exocyclic double bond. Anodic methoxylation of 2-(2-thienylmethyl)-furan yields a mixture of geometrically isomeric adducts... 

Birch and Slobbe have found that the intermediate anion from the reduction of 2-furoic acid can be alkylated in situ (equation 17). These dihydrofuroic acids can be oxidatively decarboxylated with LTA to afford the corresponding 2-alkylfurans in what clearly represents an attractive alternative to the standard two-step metallation-alkylation of the furan ring. 

The Birch-type reduction of 1,3-diphenylbenzo[c]furan (33) has been studied in detail by Smith and McCall (equation 110). ... 

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