Furan formation reactions

Development of the furan formation reaction by mercury-catalyzed 5-endo-dig cyclization dates back to the middle of the twentieth century [112]. For example, a trisubstituted furan 239 was prepared by the HgCb-catalyzed cyclization of an alkynylated diol 238 (Scheme 19.63). The pioneering discovery of 5-exo-dig cyclization by Heilborn et al. in 1947 is described later (Scheme 19.70). 

Friedrichsen and co-workers (135), along with Padwa, has utilized the carbonyl ylide cycloaddition to generate reactive furan moieties that can be further used in inter- or intramolecular Diels-Alder reactions to prepare aza- and carbocyclic compounds. Friedrichsen conducted a number of synthetic and theoretical studies on the reactivity, regioselectivity, and stereoselectivity of substituted furan formation and subsequent Diels-Alder reaction (Scheme 4.69). 

H-NMR for (t-Adduct Formation from Some Furan Derivatives (Reaction with MeO )... 

Scheme 4.69 Friedrichsen tandem furan formation/Diels-Alder reaction.

Thieno[3,4-d][l,3]dithiol-2-one, 1-nitro-synthesis, 6, 1013 Thieno[3,4-c]furan, 4,6-dichloro-synthesis, 6, 1013 Thieno[3,4-c]furan, tetraphenyl-formation, 4, 1060 synthesis, 4, 1060 UV spectra, 4, 1044 Thieno[3,2-6]furans structure, 4, 1039 synthesis, 6, 1020 Thieno[3,4-c]furans addition reactions, 4, 1060 dipole moments, 4, 1040 synthesis, 4, 1073 Thieno[3,4- c]furazan synthesis, 6, 414... 

The mechanism of furan formation by this route is determined by the structure of the sulfonium salt the course, hence the end product, is governed by whether an a-substituent is present. This must be considered when syntheses based on this procedure are being planned. Plausible mechanisms for the reaction have been suggested.3... 

In order to study the mechanism of the furan formation, the following reactions were carried out. The reaction of isomeric 2-butynyl methyl carbonate (160) and 1-m ethyl propynyl methyl carbonate (161) with methyl acetoacetate (162) gives the same product 163. This shows that the furan formation from 160 and 161 proceeds via a common intermediate. 

In the following scheme, the benzo[Z>]furan core of ( )-frondosin B was built up by the palladium-catalyzed quinone reduction, followed by Lewis acid-mediated benzo[Z>]furan formation <07OL3837>. In the total synthesis of bisabosquals, the core structure of benzo[ ]furan was constructed by an epoxide-ring opening reaction <07T10018>. 

Scheme 11.58 Furan formation of intramolecular reaction of propargylic carbonate with alcohol...

Scheme 11.49 Mechanism of furan formation by the reaction of acetoacetate.

Furan Formation by Reaction with Acetoacetate (Scheme 11-49) [44]... 

Reaction of carbonylcarbenes with a triple bond results in either 1,1-cycloaddition product 326 or 1,3-cycloaddition product 327. Substituted furans 327 are also accessible through photolysis, thermolysis, or catalytic rearrangement of carbonylcyclopropenes 326. However, furan formation can be imagined as a cyclization of 6ir-electron system 328, incorporating a singlet carbene or carbenoid center and a conjugated heteroatomic diene... 

The formation of furans from 3-acylcyclopropenes is catalyzed by copper(I) but not copper(II) salts, and can also be brought about photochemically. With an optically active cycloprop-2-enecarboxylate, racemization in a photochemical reaction occurred 2.5 times faster than furan formation, supporting the intermediacy of a singlet vinylcarbene in equilibrium with the cyclo-propene. ... 

The reactions are highly substituent dependent and in other cases gave dihydropyrans, or ring-opened allenes or ketones. The mechanism of the furan formation has been examined by deuterium labeling, and some stereoselectivity was observed in the derived products. ... 

The formation of furans as byproducts during photolysis of dimethyl benzocyclopropene-1,1 -di-carboxylates is mentioned in Section 4.1. These furans 1 were also obtained when the ben-zocyclopropenes were heated above their melting point. The reaction proceeds by thermolysis of the cyclopropane bond to a biradical, which is intercepted by one of the carboxylate groups. Furan formation also occurred when methyl 1-methylbenzocyclopropene-l-carboxylate was thermolyzed. By analogy, heating of 2,3,4,5-tetraphenylbenzocyclopropene-1,1-dicarboxylate in quinoline afforded the decarboxylated product, 4,5,6,7-tetraphenylben-zofuran-2(3//)-one (2). ... 

The procedure described here provides a direct synthesis of highly substituted furans (see Table). Reaction of keto carbenoids with acetylenes is normally an efficient method to prepare cyclopropenes. In numerous systems, however, the formation of furans was observed as a competing side reaction. Furan formation is particuiariy favored when the carbenoid is a pyruvate or contains two electron-withdrawing groups,and when eiectron-donating groups are present on the acetyiene. >8... 

In addition, another important product was observed, furan (C4H4O, m/e 68 and 39) at 550 K with comparable yield to acetaldehyde (42 % carbon selectivity). Thus, furan formation indicated that U surfaces are also active for C-C bond formation in their oxidised form, in addition of being an active C-0 bond formation catalyst. The key route to this reaction is the formation of C4 olefin (most likely butadiene) which in its turn reacts with the surface oxygen to give furan as follow... 

A further example of this type of reaction was shown by Tsutsumi and Yoshisato in furane formation. a-Bromoketones react with Ni(CO)4 the products of the reaction are different depending on the solvent used in the reaction. Thus, in tetrahydrofurane, diketone is obtained. On the other hand, furane derivatives are obtained in dimethylformamide. As an intermediate of the furane formation, an epoxy compound was isolated. 

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