2- methoxyfuran

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

The Diels-Alder reaction of 3-methoxyfuran with octyl vinyl ketone took place at room temperature in quantitative yield to afford exclusively the endo cycloadduct (27) (81CC221). Reduction of the carbonyl group with lithium tri-r-butoxyaluminum hydride produced a single alcohol (28). Ozonolysis of the double bond followed by Jones oxidation yielded the lactone ester (29). Hydrolysis of the ester and lead tetraacetate oxidation gave the lactone acetate. This was converted by further hydrolysis and Jones oxidation to the bis-lactone (30), a known intermediate in the synthesis of ( )-isoavenaciolide (Scheme 6). 

Diazomethane furnishes the methyl ether which has been degraded to 3-methoxyfuran which, however, is more easily available from 3-iodofuran. 3-Methoxyfuran is cleaved by acid to furan-3(2H)-one. Other 3-furanols with ester, acetyl or benzoyl substituents at the 2-position are also available. They exist in the enolic form but their chemistry has not been investigated (76JCS(P1)1688>. Furan-3(2H)-ones with acetyl or ester substituents at the 4-position are readily available. They exist in the keto form but show some evidence for enolic behaviour and their chemistry is similar to that of enolizable ketones. They enter into cycloaddition with maleic anhydride, are alkylated at the 2-position, condense with aldehydes and ketones and are oxidized by LTA to the 2-acetoxy compounds (74BSF2061). 

The formation of maltol from 2-acetyl-3-methoxyfuran via isomaltol O-methyl ether (428) involves a related method. 

Copper(I) and copper(II) salts have been shown to catalyze the reaction of furan with -acetoxyacrylonitrile (CH2=C (OAc)CN). Alkoxy and silyloxy groups activate the furan nucleus to cycloaddition reactions, e.g., 3-methoxyfuran readily undergoes [4 + 2] cycloaddition with electron-deficient dienophiles with regio- and stereo-control to give endo-adducts (Scheme 97). 

When 15 is reacted with KI/I2 and water, 3-iodofuran forms,178 which is converted into 3-methoxyfuran, 3-cyanofuran, and 3-furyl methyl sulfide by various nucleophiles.180... 

It is expected that 3-methoxyfuran will show an increased diene activity over unsubstituted furan. The only reported reaction is with maleic anhydride, thus ... 

It is somewhat surprising that these furans are not particularly good dienophiles. However, this is not so in the case of 3-methoxyfurans, which are substantially more active dienes than furan. 

Recently,262,352 3-methoxyfuran has been synthesized by the following two methods ... 

All of the above compounds have cis double bonds it is not known whether trans forms, which must be acyclic, exist. The low yields of enol ethers obtained by the action of alkali on some 2-0-methylaldoses may be due to the formation of 3,6-anhydroaldoses. For example, 3,6-anhydro-2-deoxy-D-ami no-hexose is formed reversibly from 2-deoxy-o-arabino-hexose ( 2-deoxy-n-glucose ) by the action of alkali. Another possibility is the formation of 5-(l,2-dihydroxyethyl)-3-methoxyfuran by analogy with the reaction of (53) to give (54) (see Section VII, la p. 202). 

HudzikJM, BozzeUi JW. Stmcture and thermochemical properties of 2-methoxy-furan, 3-methoxyfuran, and their carbon-centered radicals using computational chemistry. J Phys Chem A. 2010 114 7984-7995. 

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