Cycloadditionen von 3,4-Dimethoxyfuran an Dienophile

On cycloaddition reactions with 3,4-dimethoxyfuran We describe new cycloadditions of 3,4-dimethoxyfuran (3,4-DF, 1 ) with various dienophiles. In contrast to furan, the reaction of 3,4-DF with maleic anhydride gives exo- and endo-adducts at approximately the same rate. The thermodynamically more stable product is again the exo-product. Less active dienophiles lead also to mixtures of exo- and endo-adducts, except for citraconic anhydride (methylmaleic anhydride) which forms only the adduct with an endo-methyl group. Dimethylmaleic anhydride could not be reacted with 3,4-DF. All adducts with 3,4-DF contain a hidden 1,2-dicarbonyl group in the form of an endiolether. Its pronounced nucleophilic character allows a series of further additions. Noteworthy are the stereospecific cis-additions of halogens and the preparation of several acetals. Ozonolysis of the enolether in 4 allows the preparation of the hitherto unknown 2r, 3trans, 4trans, 5cis-tetrahydrofuran tetracarboxylic acid and derivatives thereof ( 5 and 6 ).     

Azokupplungen an 3,4-Dimethoxyfuran

Diazo coupling of 3,4-dimethoxyfuran with aryl diazonium ions 3,4-Dimethoxyfuran ( 1 ) easily reacts with aryl diazonium chloride in aqueous pyridine in an expected 1, 4-addition reaction. From the isolable primary addition product pyridine is displaced by alcohols, phenols or thiols to yield 4-alkoxy- or 4-phenoxy- or 4-thiophenoxy-derivatives of 2,3-dimethoxy-2-buten-4-olide ( 3 ). Attempts to convert them into azo compounds by a base catalysed 1, 6-elimination reaction failed. Oxidation of 3a and 3c with DDQ results in the formation of the mono p-nitrophenylhydrazone of 3, 4-dimethoxymaleic acid anhydride. On the other hand, the thiophenoxy compound 3g is smoothly converted by MnO₂ into the authentic furan-2-azobenzene derivative 5 .     

1,3-Dipolare Cycloadditionen von 3,4-Dihydro-6,7-dimethoxyisochinolinium-N-methoxycarbonylmethylid mit N-substituierten Maleinimiden

Summary 1,3-Dipolar cycloadditions of 3,4-dihydro-6,7-dimethoxyisoquinolinium-N-methoxycarbonylmethylide2 with N-phenyl- and N-methylmaleimide (3 and4) have been investigated. Cycloadducts of theendo- andexo-type were formed, the structure elucidation and conformational analysis of which has been performed by NMR methods.

     

3,4-Dimethoxyfuran: Eigenschaften,Berechnungen und Substitutionsreaktionen

3,4-Dimethoxyfuran: Properties, Calculations and some Substitution Reactions 3,4-Dimethoxyfuran (3,4-DF) is much more reactive than furan itself. We present experimental details for the synthesis of this useful compound in 50–100 g batches. 3-4-Dibenzyloxyfuran has also been prepared. Quantum mechanical calculations by PPP-and CNDO-methods analogous to furan itself gave the following information (compared with furan): enhanced negative partial charge at C(2) and C(5), higher acidity at α-position, a stronger and at the same time inverted dipol moment. In accordance with the calculations, 3,4-DF can easily be substituted by weak electrophilic reagents, e.g. in a typical Mannich reaction. The bis-amino compound 8 is produced in good yields. Lithiation by butyl-Li leads to mono- or di-substituted products. The mono- and di-Li-3,4-DF have been used for the preparation of various acylated and alkylated 3,4-DF derivatives. By acid hydrolysis of 3,4-DF crystalline 4-methoxy-3 (2H)-furanone has been prepared.     

Intramolekulare 1,3-dipolare Cycloadditionen mit alkenyl-substituierten 3,4-Diarylsydnonen

Intramolecular 1,3-Dipolar Cycloaddition Reactions of Alkenyl-substituted 3,4-Diarylsydnones The 3,4-diarylsydnones 1 and 12a–e with an allyl or alkenyloxy substituent on the N(3)-phenyl group have been synthesized by classical methods starting from 2-allylaniline and the corresponding alkenyloxyanilines, respectively (Schemes 2 and 3). Whereas the allyl-substituted sydnone 1 undergoes an intramolecular 1,3-dipolar cycloaddition at room temperature in solution to yield 13 (Scheme 4), the alkenyloxy-substituted sydnones 12a–e are thermally stable. On irradiation of 1 and 12a–d , formation of the fused dihydropyrazoles 2 and 14a–d (Schemes 1 and 5) is observed. In the case of 12d , the yield of 14d is very low, and the symmetric 1,2,3-triazole 15a has been isolated as the main product. A very likely reaction mechanism for the formation of the photoproducts involves decarboxylation of the sydnones to give a nitrile-imine which undergoes an intramolecular 1,3-dipolar cycloaddition onto the C?C bond.     

Cycloaddition of 3,4-Dimethoxyfuran with 1,4-Benzoquinones under High Pressure

High-pressure cycloaddition of 3,4-dimethoxyfuran with 1,4-benzoquinone, toluquinone and 2,3-dimethoxy-1,4-benzoquinone show, at 7 kbar and room temperature, prevailing formation of endo-isomers. Raising pressure (or temperature) results in a surprising increase of the exo-isomer. By catalytic hydrogenation of a mixture 3a/4a , followed by chromatography, the pure diastereomers 5 and 6 were obtained.     

Favorskii-Umlagerung in Gegenwart von 3,4-Dimethoxyfuran: 11endo-oxa-tricyclo [4.3.1.12,5]undec-3-en-10-on und Derivate

Favorskii -rearrangement in the presence of 3,4-dimethoxyfuran: preparation of 3,4-dimethoxy 11 endo -oxo-tricyclo [4.3.1.1^2,5]undec-3-en-10-one and any derivatives On treatment with sodiumhydride of 2-chloro-cyclohexanone in the presence of 3,4-dimethoxyfuran, a possible intermediate of the Favorskii-rearrangement has been trapped as 3,4-dimethoxy-11endo-oxa-tricyclo [4.3.1.1^2,5]undec-3-en-10-one ( 3 ). This new compound contains a highly nucleophilic double bond. It can be cleaved in high yield by ozonolysis to 2exo, 4exo-bis (methoxycarbonyl)-3-oxabicyclo [3.3.1]nonan-9-one ( 4 ). Addition of chlorine to 3 occurs in stereoselective exo-cis-manner to the crystalline 3exo, 4exo-dichloro-3endo,4endo-dimethoxy 11endo-oxa-tricyclo [4.3.1.1^2,5]undecan-10-one ( 5 ). Silver ion assisted hydrolysis of 5 , followed by thermal treatment of the intermediate hydrates, leads to the red 11endo-oxa-tricyclo [4.3.1.1^2,5]undecan-3,4, 10-trione ( 6 ), and methanolysis to 3,3,4,4-tetramethoxy-11endo-oxa-tricyclo [4.3.1.1^2,5]undecan-10-one ( 8 ). By photolytic decarbonylation, 8 is converted into 3,3,4,4-tetramethoxy-10-oxa-tricyclo-[4.3.1^2,5.0]decan ( 9 ).