Furoic acids Birch reduction

A novel synthesis of branched-chain monosaccharides was based on the finding that 3-furoic acid readily undergoes Birch reduction, affording 2,3-dihydro-3-furoic acid. Treatment of its methyl ester 398 with methanol and acid gave,264 in quantitative yield, methyl tetrahy-dro-5-methoxy-3-furoate 399 as a mixture of the isomers. Compounds... 

DL-Dihydrostreptose and its ribo isomer were similarly obtained. Birch reduction of 2-methyl-3-furoic acid, followed by addition of methanol, bromination, and dehydrobromination, gave 402 as a mixture of the isomers. Hydroxylation of 402 with osmium tetraoxide-so-dium chlorate, and subsequent treatment with acetone-sulfuric acid afforded three isomeric acetals (403-405). The structures of these compounds were assigned on the basis of their H-n.m.r. spectra. In addition, the relationship between 403 and 404 was established by hydrolysis and reglycosidation. The methyl esters 403-405 were quantitatively reduced to the corresponding alcohols. The mixture of alcohols obtained from 403 and 404 was converted into crystalline 5-deoxy-3-C-(hydroxymethyl)-l,2-0-isopropylidene-a-DL-ribofuran-ose (406), which was compared directly with a sample prepared from D-xylose. Methyl 5-deox y-3-C-(hydroxy methyl)-2,3-O-isopropy lidene-/3-DL-lyxofuranoside (407), obtained by reduction of 405 with lithium aluminum hydride, was hydrolyzed with dilute hydrochloric acid, to give a,/3-DL-dihydrostreptose.2,ifi... 

Pyrroles are not reduced by sodium in liquid ammonia, but the Birch reduction of 2-furoic acid with lithium in liquid ammonia gives the 2,5-dihydro derivative in 90% yield (780PP94). Sodium-liquid ammonia-methanol reduction of thiophene gives a mixture of A2- and A3- dihydrothiophenes together with butenethiols. Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cis- 1,3-dihydro derivative (Scheme 36) (80JOC3982). 

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... 

Kinoshita and coworkers have found that, in contrast to the result obtained using Pr OH (equation 14), in the presence of nucleophilic alcohols (MeOH, EtOH) the Birch reduction of 3-furoic acid leads to the corresponding acetal (equation 15). ... 

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. 

Asymmetric versions of the Birch reduction are now appearing (chapter 28) and a C2 auxiliary attached to the furoic acid 141 allows Birch reduction and alkylation between the ring oxygen atom and the carbonyl group to give, after hydrolysis, enantiomerically pure acids39 144. 

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... 

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