1,4-Benzoquinone diene synthesis with

A double diene synthesis occurs with benzoquinone. With 2 molecules of cyclohexadiene, for instance, this affords the 1,4,5,8-diethanoanthracene ring system the resulting l,4,4a,5,8,8a,9a,10a-octahydro-l,4,5,8-diethanoanthra-quinone readily loses four hydrogen atoms, to give the 1,4,5,8-tetrahydro compound and, when that is heated, it spits off ethylene to afford anthraquinone ... 

The Lewis acid-catalysed orientation reversal in the reaction between substituted cyclohexa-1,3-dienes and 2,6-dimethyl-l,4-benzoquinone ° has been employed in an interesting synthesis of quassin (218). ° Thus, reaction at room temperature of the diene (215) with the above quinone in the presence of an equivalent quantity of Bp3,OEt2 gave the adduct (216) which was converted by several subsequent steps into (218). In the absence of the catalyst the alternative adduct (217) was obtained. Periodic acid oxidation of substituted o-cresols ° and of 2-methoxyphenols in methanol solution affords intermediate o-quinol methyl ethers or o-quinone dimethyl ketals which dimerize to give dienediones with structures related to those of (216) and (217). Another report concerns the formation of a Diels-Alder dimer upon hypochlorite oxidation of 2,2 -methylenebis(4-methyl-6-t-butyl)phenol. ... 

Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

Wefero-Dlels-Alder reactions of 3,5-di-fe/-f-butyl-o-benzoquinone with acyclic dienes novel synthesis of 1,4-benzodioxanes [135]... 

The Diels Alder reaction, involving the [4 + 2]-cycloaddition of an unsaturated group (di-enophile) to a 1,3-diene, has been studied for the synthesis of ladder polymers, such as the reaction of 2-vinyl-l,3-butadiene with benzoquinone [Bailey, 1972] (Eq. 2-247). Related polymerizations are those utilizing the [2 + 2]-cycloaddition reaction [Dilling, 1983]. While [4 + 2]-cycloaddition reactions are thermally induced, [2 + 2]-cycloaddition reactions are... 

The key step in the convergent synthesis is a Diels-Alder reaction. Diene 9 is accessible in three steps from 2-methylcyclohexa-none 6. The dienophile is an acylated p-benzoquinone. Surprisingly, this Diels-Alder reaction is completely rxo selective and introduces the sesquiterpene skeleton of the target compound with the correct stereochemistry in a single step... 

After Buchi confirmed this finding in the 19S0s, numerous other investigators immediately recognized the rapidity with which complex molecular architectures could be constructed by this reaction. Cookson irradiated the Diels-Alder adduct (3) of cyclopentadiene and benzoquinone to produce the caged diketone (4) (equation 2) while Eaton utilized a photochemical cycloaddition of diene (5) in a synthesis of the platonic solid cubane (6) (equation 3). ... 

Cyclobttlttdiene. Cyclobutadicne itself is unknown but the complex cyclobuta-dicnciron tricarbonyl (I, 2,140 3, 101) is readily available. The diene can be liberated from the complex as a transient intermediate for use in synthesis by oxidative degradation with ceric ammonium nitrate. If the reactants or products arc sensitive to the acidic solutions of CAN, lead tetraacetate in pyridine can be used as oxidant. Thus the reaction of cyclobutadienciron tricarbonyl with p-benzoquinone in the presence of CAN leads to formation of e [Pg.72]

A general synthetic route to several polyhydroxylated agarofurans was developed by J.D. White and co-workers and the total synthesis of (+)-euonyminol was achieved. The key intermediate was prepared via a Diels-Alder reaction between a diene and a substituted benzoquinone. The resulting bicyclic homoannular diene was reduced under the Luche conditions with excellent regio- and stereoselectivity at C6. The substrate was mainly in the boat conformation and the (3-face of the ketone was more exposed to hydride attack. The C6 ketone was also more sterically accessible and more basic than the C9 ketone functionality. 

The adducts obtained from acyclic dienes and cyclic dienophiles are frequently formed in accordance with the endo rule. A classic example is found in the Woodward synthesis of reserpine, which started with the Diels-Alder reaction of i -pentadienoic acid and benzoquinone (3.67). In this cycloaddition reaction, the endo adduct 78, in which the carboxylic acid and the benzoquinone carbonyl groups become cLs to one another, is obtained as the exclusive product. 

Another approach by Naruta and co-workers to the synthesis of 9,9a-dihydro-3H-pyrrolo[l,2-a]indoloquinones was based on photocyclization of a diene and an azidoquinone [23] (Scheme 5). Thus, irradiation of a mixture of two equivalents of 2-azido-5-methoxy-6-methyl-l,4-benzoquinone (40) and one equivalent of doubly protected cis, cfs-2,4-hexadien-l,6-diol (41) gave an in-doloquinone 42 that had the 2,3-trans and 8,9-cis configurations. Selective desilylation followed by mesylation and treatment with DBU at reflux temperature then gave the desired tricyclic compound 43 in 35% overall yield. 

A particularly attractive application of hydrotropy in organic synthesis arises when the product is bulkier than the reactant, with the result that it has lower solubility than the reactant in the hydrotrope solution. Consequently, it selectively precipitates out of the reaction mixture and can be easily filtered out. Then the hydrotropic solution can be recycled, thus minimizing the environmental hazards associated with waste disposal. An important example is the synthesis of Diels-Alder adducts that act as flame retardants for polymer blends and formulations. One of these is also used in the manufacture of the pesticide Endosulfan. The reaction involves a diene such as hexachloro-pentadiene or anthracene and a dienophile such as p-benzoquinone or maleic anhydride. The following typical reaction carried out by Sadvilkar (1995) gave excellent results ... 

In 1985, O Malley et al. published the total syntheses of rac-averufin (103) and rac-nidurufin (104) (65). These are both early precursors of the aflatoxins in their biosynthesis. Nidurufin (104) is the direct successor of averufin (103) and the direct precursor of versiconal hemiacetal acetate (12, see Scheme 2.1). Nidurufin (104) and averufin (103) are accessible by the same synthesis route only the two last steps differ firom each other (see Scheme 2.17). The first reaction was a double Diels-Alder reaction with dichloro-p-benzoquinone (97) and two equivalents of diene 98. Then, three of the four alcohol functions were selectively MOM-protected (—> 99). The remaining alcohol was converted into the allyl ether and then subjected to a reductive Claisen rearrangement, followed by MOM-protection of the redundant alcohol ( 100). By addition/elimination of PhSeCl, 101 was formed. Deprotonation of t-butyl 3-oxobutanoate, followed by reaction with 101 yielded the pivotal intermediate 102. This could be converted into rac-averufin (103) by deprotection of the alcohols and decarboxylation at the side chain. The last step was a p-TsOH-catalyzed cyclization to give 103. By treating 102 with /m-CPBA, the double bond is epoxidized. rac-Nidurufin (104) was then formed by cyclization of this epoxide under acidic conditions. 

Various approaches to the assembly of the 1,3-dienes that engage in the Diels-Alder cycloaddition reactions leading to substrates for DPM and ODPM processes have been reported. Thus, for example. Yen and Liao demonstrated, during the course of a total synthesis of the Lycopodium alkaloid magellanine, that oxidation of acetovanillone (109) rScheme 9.151 with diacetoxyiodobenzene (DAIB) in the presence of methanol afforded the o-benzoquinone monoketal 110. The latter conpound engaged in an in situ Diels-Alder reaction with added cyclopentadiene (111) and the resulting adduct 112 proved to be an excellent substrate for the ODPM rearrangement reaction. Thus, photolysis of 112 as a solution in acetone afforded the pivotal tetracyclic diketone 113 in 92% yield. 

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