Furans rearrangement into

Dihydro-1,2-dioxines (195) (available from dienes and 02) can be transformed into furans (197) by reduction and rearrangement (into 196) and subsequent elimination of water, usually by treatment with base. In some cases Co(II) and Fe(II) catalysts have been used (76TL4363, 77JOC1900, 77SCH9, 82ACS(B)31, 89JOC3475). 

Alkynyl( aryl) iodine (I II) compounds 89 can also be employed in hetero-Claisen rearrangements [151] after reaction with appropriate thioamides yielding thiazoles of type 90 in reasonable yields as shown in Scheme 38 [152]. Ring enlargement reactions of furan derivatives into pyranones by hypervalent iodine compounds were reported as well [153]. 

A polycyclic system with a l,3-dioxa-2-silacyclohepta-4,6-diene fragment was obtained in low yield from bis(diazocarbonyl)silane 671. In the first stage, a biscarbene or a biscarbenoid, generated in the presence of copper triflate or palladium acetate, forms 1,2-diacylcyclopropene, then rearranging into cyclic cumulene triene 672 by migration of a silicon atom to an oxygen atom. The latter dimerizes to [4]-radialen 673 or reacts with furan to afford 674 (Scheme 32) (89AG175()). 

An analogous Au(I)-catalyzed synthesis of furan 86 was recently described by Toste and coworkers [141]. Thus, sulfoxide 84 undergoes an Au(I)-catalyzed rearrangement into the homopropargylic ketone 85, which subsequently cycloisomerizes into the furan product (Scheme 8.33). 

Functionalized benzofurans have been prepared from ort/io-hydroxyphenones and 1,1-dichloroethylene via a chloromethylene furan which rearranges into a benzofuran carbaldehyde under acidic conditions. A synthesis of 3-(but-2-enylidene)quinolin-4(3//)-ones has been described via a triflic acid-mediated Fries rearrangement of 3-butadienyl-2-azetidinones, a protocol that provides access to C(3)-functionalized quinolin-4(3//)-ones. °... 

Tetrakis(trifluoromethyl)furan 137 was reported to be inert in a reaction with common dienophiles and stable to irradiation, unlike the most other furans [139, 149], However, later it was shown that 137 rearranged into cyclopropenyl ketone 220 under irradiation with high pressure Hg-lamp. Similarly, furan 141 was transformed into acyl fluoride 247 [105],... 

These furan endoperoxides are valuable building blocks for the synthesis of di- and tetrahydrofuran derivatives, e.g., bis-epoxides, epoxy lactones, ene diones, cis-diacyloxiranes, enol esters, butenolides, and others. Furthermore, the primary [4 + 2]-cycloadducts may rearrange into 1,2-dioxetanes or 1,2-dioxolenes. ... 

The development of synthetic methods for the selective introduction of short-chain perfluoroalkyl groups into organic molecules is of interest in drug development [464]. Fluoromodifications often confer unique properties on a molecule, for example in terms of increased metabolic stability and lipophilicity and, as a consequence, the pharmacokinetic profiles are often improved [465]. Burger and coworkers developed a domino process consisting of a SN reaction combined with a Claisen and a Cope rearrangement which allows the transformation of simple fluorinated compounds into more complex molecules with fluoro atoms [466]. Treatment of furan 2-917 with 2-hydroxymethyl thiophene (2-918) in the presence... 

An unusual rearrangement of a 3-hydroxymethyl-4-hydroxybutenolide into a furan-3-carboxylic acid occurs in the natural product, photogedunin, when it is treated with alkali the reaction is really related to some of those in sub-sections B, C, and E.llla... 

N-Protected 2-furanylglycine ester (prepared from methyl methoxygly-cinate) was transformed in several steps into the reduced furan derivative which rearranges in the presence of TFA into an unstable pyridone (Scheme 40) (86TL5085). 

A novel entry into the field of isobenzofurans was reported by Maddaford et al. The Wittig rearrangement of 76 with n-BuLi yields a dihydroisobenzo-furan which, with silica gel, gave 77. Probably because of the TBS substituent, this compound is remarkably stable at room temperature (96JA10766). 

The same intermediate occurs in the Lehmann method463 for converting the methiodides of phenolic Mannich bases into 2,3-dihydrobenzo-furans, through reaction with dimethyloxosulfonium methylide. Thus, o-benzoquinone methide (207) leads to 2,3-dihydrobenzofuran according to Scheme 4. Similarly, 2-naphtholgives l,2-dihydronaphtho[2,l-6]furan through 208 and 209, formed and rearranged in situ.i6i The reaction has been applied to the synthesis of polycyclic benzofurans.483,485... 

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