Furan opening, oxidative

Furan opening. Enediones are produced when furans are oxidized with the urea-HjOj adduct catalyzed by MeReOj. 

Selective transformations Selective styrene ring opening [103] One-pot domino process for regioselective synthesis of a-carbonyl furans [104] Tandem process for synthesis of quinoxalines [105] Atmospheric oxidation of toluene [106] Cyclohexane oxidation [107] Synthesis of imines from alcohols [108] Synthesis of 2-aminodiphenylamine [109] 9H-Fluorene oxidation [110] Dehydrogenation of ethane in the presence of C02 [111] Decomposition of methane [112] Carbon monoxide oxidation [113]... 

The intramolecular cycloaddition reactions of the nitrile oxides 357 (n = 1, 2, 3, 9), obtained in situ from the 2,5-difunctional furan hydroximoyl chlorides or nitro compounds (415) has specific features because of the 2,5-arrangement of two open chains bearing acetylenic and fulminic moieties. Only with 357 (n = 3) is the expected furanoisoxazolophane 358 formed, in acceptable yield. Compound 357 ( =9) gives a complex product mixture whereas 357 ( = 1, 2) gives rise to the exclusive reaction of the dipole with a double bond of the furan system. 

A flexible entry into 2,4-disubstituted furan derivatives through condensation of the sulfur ylide derived from 20 with aldehydes, Pd-catalyzed opening of the vinyl oxirane thus formed, and a final oxidative cyclization of the furan ring was reported. Its utility was exemplified by the first total synthesis of the marine natural product ircinin-4 (21) <99SL29>. 

If 2-camphanyloxyacrylonitrile (15 R = C8H 02C00) is taken for cycloaddition, diastereoisomeric cycloadducts can be separated, and the basic system, 7-oxabicyclo-[2.2.1]hept-5-en-2-one 17, can be obtained in optically pure form [36]. Another way of obtaining enantiomeric ketones is based on crystallization of a brucine complex obtained from the corresponding cyanohydrines (see Sec. III). Ketone 17 can be converted [e.g., by cis-hydroxylation (—>18), protection of the diol system, and Baeyer-Villiger oxidation] to lactone 19, the opening of which leads to furanuronic acid 20. A new development in this field is based in cycloaddition between furan and 2-chloro- or 2-bromoacrolein in the presence of 5 mol% chiral oxazaborolidine 21 as catalyst [37],... 

Ring opening is common in the alkali metal and liquid ammonia reduction of furans unless an anion stabilizing group is present, so most work has been done with derivatives of furancarboxylic acids. Treatment of furan-2-carboxylic acid with lithium and ammonia at -78 °C followed by rapid addition of ammonium chloride affords 2,5-dihydrofuran-2-carboxylic acid (80%). Reductive alkylation similarly gives 2-alkyl-2,5-dihydrofuran-2-carboxylic acids. This method has been used in a synthesis of rosefuran, the intermediate dihydrofuran (66) being converted into the product (67) by oxidative decarboxylation with... 

The formation of furans from pyrylium salts under oxidative conditions has been mentioned in Section 2.23.2.2. Ring opening and recyclization of 4-chloromethyl-2,6-dimethyI-pyrylium perchlorate (83) by aqueous alkali and DMF leads to 2-methylfuran-4-ylacetone (84) (77CHE918). When 2,3,4,5-tetraphenylpyrylium perbromide is hydrolyzed to the dienone and then subjected to bromination-dehydrobromination, 2-benzoyl-3,4,5-triphenylfuran (85) is formed. 

Substituted furans 30 can be obtained in moderate yields by irradiation, in DCM with a high-pressure Hg lamp, of pyridazine-1-oxides (28 R2 = H) opportunely substituted with a group able to stabilize a carbene intermediate. The mechanism involves the open-chain diazo-ketone 29, which undergoes a photoinduced loss of molecular nitrogen leading to a carbene, which is a precursor of the final furans 30 (Scheme 12.9) [26]. 

Ring cleavage in the furan system is usually obtained by catalytic hydrogenolysis. Examples are given in Section IV of addition reactions, and in Section VII of conversion reactions. Oxidative ring opening has already been described in a part of Section IV. 

Furans with suitable substituents in the 2-position can also be converted into phenols. These can be transformed into a 1,4-dicarbonyl system by 2,5 oxidation and ring opening. This system can then condense intramolecularly into a six-membered ring.333 334 The synthesis of m-hydroxybenzoic acid in 26% yield from furfuryl chloride288 is an example of this ... 

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