Diels-Alder additions furans

Furan also undergoes cycloadditions with allenes and even with simpler dienophiles, like acrylonitrile and acrylate (specifically enhanced by the presence of zinc iodide),and with maleate and fumarate esters, if the addition is conducted under high pressure. This device can also be used to increase markedly the reactivity of 2-methoxy- and 2-acetoxyfuran towards dienophiles. Lewis acid catalysis can also been used to accelerate furan Diels-Alder additions. At higher reaction temperatures alkynes and even electron-rich alkenes will add to furan. 

Potential 2,5-dihydroxy compounds (185) exist in the dicarbonyl forms (186). Succinic anhydride (186 Z = O) on silylation is converted into 2,5-bis(trimethylsilyloxy)furan (187) the latter compound readily participates in Diels-Alder addition reactions (80TL3423). Reaction of thiosuccinic anhydride (186 Z = S) with the triphenylphosphorane Et02CH=PPh3 gives a product which exists in the aromatic form (188) (75LA1967). 

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

Table 9. Diels-Alder additions of furans 87a-e to methyl 2-chloro-2-cyclopropylideneacetate (4) [16] R1 R1 R1 Rl...

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

The intramolecular Diels-Alder addition of a benzyne to a furan has been exploited in a synthesis of mansonone E (81TL4877). The benzyne (211), generated from the anthranilic acid (210), yields the adduct (212 86%) which is easily converted into the naphthalene (213 Scheme 79). A similar addition was achieved by generating the benzyne by treating an o -dibromobenzene with butyllithium. 

Phenyl-A2-oxazolin-4-one forms the furan (282) on treatment with dimethyl acetyl-enedicarboxylate. The reaction may be interpreted as proceeding through a Diels-Alder addition to the tautomeric oxazole (281) (Scheme 74) (72CC1000). 

Tetraphenyltellurophene fails to undergo Diels-Alder addition with maleic anhydride even when heated at 220 °C for 21 h. Tetraphenyltellurophene 1,1-dibromide, however, on heating with maleic anhydride undergoes elimination of tellurium to produce tetraphenyl-furan and/or the dibenzopentalene derivatives (76) and (77) (61JA4406). 

In a recent report, Padwa [64] disclosed a general method of wide applicability for constructing a variety of heterocyclic systems. It involved an amino furan ester as a more reactive four carbon component vis avis a pyrone in intramolecular Diels-Alder addition to an unactivated olefin. This method as applied to the synthesis of anhydrolycorinone consisted of the preparation of the tertiary amide 240 (Scheme 40) and its subsequent thermolysis to the N-o-bromoaroylindoline 241. It is believed that the initially formed cycloadduct 242 opened to the acyl iminium oxyanion 243, which by prototropy and dehydration generated 241. The latter, on photolysis in the presence of bis (tri-n-butyltin), furnished the tetracyclic ester 244, which was hydrolysed and decafboxylated to anhydrolycorinone as in the previous synthesis. 

Benzo-phosphabarrelenes have been obtained from the addition of cyclooctyne (14) to 2-phenyl-l-phosphanaphthalene 226>. Cyclooctyne (14) has recently been used to synthesize a [6]-paracyclophane (68) the first step being a Diels-Alder addition of (14) to a substituted furan 220 222). 

There are essentially two mechanisms to be considered for these orthofurylation reactions. First, a direct nucleophilic addition of furan, to the acylquinone at the 2-position is postulated, which is probably catalyzed by traces of acid (route A). The other leads through a Diels-Alder addition, and the adduct yields a stable hydroquinone by a dienone-phenol rearrangement (route B). 

A multi-gram enantioselective synthesis of (+)-37, a key precursor to the tetracycline antibiotics, has been performed from simple heterocyclic compounds. Key steps in the route involve enantioselective addition of divinylzinc to 3-benzyloxy-5-isoxazolecarboxaldehyde (with 93% ee) for the preparation of 36 and, after its nucleophilic addition to 35, an intramolecular furan Diels-Alder cycloaddition <07OL3523>. 

Little is known about the reactivity of the group 14 cations in solution. Clearly, one would expect these strong electrophiles to react with common nucleophiles, but such trivial reactions were not documented except for the reaction of Bu3Sn+CBnMei2 with PhLi which produced Bu3SnPh. [(c-C6Hn)3Sn(NCMe)2] SbF6 and compounds of this series have been shown to be effective promoters of Diels-Alder additions to furan. ... 

Structurally related is biflorin (351), isolated from Capraria biflora, and the first naphtho[l,8-hc]pyranquinone found in nature. It has antibiotic properties. Its structure was determined by Prelog and co-workers (58HCA1386 63HCA409, 63HCA415) and it was synthesized by an intramolecular Diels-Alder addition between a benzyne and furan (86AJC647). 

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