Acid-catalyzed furan ring-opening

Tri- and tetrasubstituted furans are obtained from benzofuranyl carbinols and 1,3-dicarbonyls via a Bronsted acid-catalyzed benzofuran ring opening and furan ring closure sequence (13JOC10427). 

Diels-Alder cycloaddition of 3,4-bis(trifluoromethyl)furan with ethyl propynoate involved addition of two a,/3-unsaturated esters followed by acid-catalyzed ring opening, rearrangement, and elimination of ethanol to give a 6,7-bis(trifluoromethyl)isocoumarin-3-carboxylate [92JFC(56)359]. 

Acid-catalyzed ring opening of the furans 659 forms the intermediate 660, which recyclize to afford 4-aroyl-2H-pyran-2-ones in good yield (Scheme 156) <2004TL8583>. 

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>. 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

A somewhat similar problem is revealed by the acid-catalyzed ring opening of oxetans (104). These are formed by photochemical [2 + 2] additions, and with boron fluoride or aluminium chloride they supply 3-furylcarbinols (105) mixed, however, with the isomeric 2-furylcarbinols (106). With toluene-sulfonic acid only the expected 3-furylcarbinol results, but this is changed into the 2-isomer when treated with one of the Lewis acids. The rearrangement is believed to be effected by dissociation into furan and a carbenium ion... 

The majority of the bicyclic cyclopropanes so obtained undergo facile acid-catalyzed ring open-ing/isomerization to the corresponding (Z,Z)-6-oxohexa-2,4-dienoates this transformation already occurs when the workup by column chromatography is performed on silica gel rather than on alumina. Treatment of the bicyclic systems (as well as the E,Z- and Z,Z-dienes) with iodine in dichloromethane results in isomerization to (is,is)-l,4-diacylbuta-l,3-dienes in high yield.The ring-opened products in carbenoid reactions of furans are not likely to result from isomerization of the alkyl-2-oxabicyclo[3.1.0]hex-3-ene-e c/o-6-carboxylates. For mechanistic proposals, see refs 300 and 301. 

The acid-catalyzed ring opening-ring closure reaction of an azetidinone yields a dihydrofuranone (29), which can converted to a furan using either a reduction-dehydration step or reaction with phenyllithium and subsequent dehydration (Scheme 29) <82ICS(P1)257>. 

The first mention of polythiophenes as potential conducting polymers can be found in the literature in 1967. A. G. Davies and coauthors investigated the acid catalyzed polymerization of furan, pyrrole, and thiophene hetero-cycles. Although this polymerization reaction was known before, several new and interesting facts could be elucidated. Besides the confirmation of the cyclic structure of the polymer units (instead of ring opened), the electric conductivity was investigated. In contrast to the already known types of electronically conducting polymers (polyaniline and polypyrrole, see Chapter 1), the conductivity of all polyheterocycles in Armour s paper, isolated as trichloro- or trifluoroacetates (triflate), is ionic. In the case of the polythiophene, ion pairs formed by the reaction of polythiophene with trifluoro acetic acid dissociated in methylene chloride solution to the protonated polymer and triflate anions. The polythiophene triflate decomposed at relatively low temperatures (60°C-80°C). ... 

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