O-quinodimethanes via

An efficient method for the generation of o-quinodimethanes via desilylation reaction involves treatment of a 2-[(trimethylsilylmefi3yl)benzyl]trimethylammonium iodide with fluoride ion (equation 31). This reaction was applied by Saegusa and coworkers for the synthesis of estrone in which an intramolecular Diels-Alder reaction of an o-quinodimethane, generated in situ, served as the key reaction (equation 32). ... 

Nicolau and co-workers have independently reported parallel studies on the generation of o-quinodimethanes via thermal elimination of sulfur dioxide from... 

Utley et al. were able to perform Diels-Alder reactions in aqueous solution via electrogenerated or//zo-quinodimcthancs.34 They cathod-ically generated the or// o-quinodimethanes in aqueous electrolyte in the presence of /V-mcthylmaleimide, which is both the redox mediator and the dienophile. Competition from the electrohydrodimerization of /V-mcthy Irrialci midc is suppressed, allowing for the efficient formation of the endo-adduct (Scheme 12.1). 

The forerunner in the Co-catalyzed [2+2+2] cycloaddition domino processes was that identified by Vollhardt and colleagues [273], with their excellent synthesis of steroids. Reaction of 6/4-1 with [CpCo(CO)2] gave compound 6/4-3 with an aromatic ring B via the intermediate 6/4-2. In this process, trimerization of the three alkyne moieties first takes place, and this is followed by an electrocyclic ring opening of the formed cyclobutene to give o-quinodimethane. This then undergoes a Diels-Alder reaction to provide the steroid 6/4-3 (Scheme 6/4.1). 

Isochromones lose carbon dioxide on heating via retro-Diels-Alder pathway to result in o-quinodimethanes (equation 81)1241,129. An isochromone route to podophyllotoxin derivative has been described (equation 82)130. Diels-Alder adducts of a-pyrone readily extrude carbon dioxide on thermal activation to furnish cyclohexadienes, which are useful substrates in tandem Diels-Alder reactions (equation 83)131. 

Retro-Diels-Alder reactions can be used to regenerate dienes or alkenes from Diels-Alder protected cyclohexene derivatives under pyrolytic conditions144. Most of the synthetic utility of this reaction comes from releasing the alkene by diene-deprotection. However, tetralin undergoes cycloreversion via the retro-Diels-Alder pathway to generate o-quinodimethane under laser photolysis (equation 89)145. A precursor of lysergic acid has been obtained by deprotection of the conjugated double bond and intramolecular Diels Alder reaction (equation 90)146. 

The thiophene ring system can be utilized as a synthetic scaffold for the preparation of nonthiophene materials as the sulfur moiety can be removed by reduction (desulfurization) or extrusion (loss of SO2). The extrusion of sulfur dioxide from 3-sulfolenes (2,5-dihydrothiophene 1,1-dioxides) give dienes (butadienes or o-quinodimethanes) that can be utilized to prepare six-membered rings by cycloaddition chemistry. For example, thermolysis of 3-sulfolene 120 provided tricyclic pyrazole 122 via an intramolecular cycloaddition of the o-quinodimethane 121 that results by extrusion of sulfur dioxide <00JOC5760>. Syntheses of 3-sulfolenes 123 and 124 <00S507> have recently been reported. 

Very interesting Diels Alder comonomers for BMI are the bis(benzocyclobutenes). Under appropriate thermal conditions, the strained four-membered ring of benzocyclobutene undergoes electrocylic ring opening to generate, in situ, o-quinodimethane, which, in the presence of BMI, reacts via a Diels-Alder reaction (83). The chemical structure of a bis(benzocyclobutene-imide) is provided in Fig. 28. The synthesis and properties of BCB and BMI/BCB blend systems is described in detail in chapter I of this book. 

The meso-diepoxide 76 produces via valence tautomerism the unstable o-quinodimethane diether 318, which also can be trapped by using NMM to give the 1 1 adduct 319. Trapping of either 76 or 318 by dienophiles depends on the molar excess of the added dienophile. Depending on the dilution, the reaction... 

When A.A-dimethylisoindolinium bromide is treated with phenyl-lithium, it gives V-methylisoindole via the ylid (54).2 59,60 An attempt to prepare benzo[c]thiophene via the analogous ylid (55) failed. Thus, when l,3-dihydrobenzo[c]thiophene methylsulfonium iodide was treated with phenyllithium, it gave a mixture of methyl phenyl sulfide, spiro[5.5]-l-methylthio-2,3-benzo-6-methylthio-methyleneundeca-7,9-diene (56), and 3,4-bis(methylthio)-l,2 5,6-dibenzo-l,5-cyclooctadiene (57).59,60 The formation of methyl phenyl sulfide may be explained by the formation and ring cleavage of compound 58, and compounds 56 and 57 arise by Diels-Alder dimerization of the o-quinodimethane (59) formed by ring cleavage of the ylid (55). 

A simple procedure to prepare 5-aryl- and 5-pyridyl-2-furaldehydes from inexpensive, commercially available 2-furaldehyde diethyl acetal was reported. The reaction proceeded in a four-step, one-pot procedure and the yield of coupling step was usually between 58-91% <02OL375>. A facile route to 3,4-furandicarboxylic acids was developed. DDQ-oxidation of 2,5-dihydrofuran derivatives, which were produced from dimethyl maleic anhydride, furnished the desired esters of furan-3,4-dicarboxylic acid <02S1010>. The furan-fused tetracyclic core of halenaquinol and halenaquinone possessing antibiotic, cardiotonic, and protein tyrosine kinase inhibitory activities was synthesized. Intramolecular cycloaddition of an o-quinodimethane with furan gave the adduct as a single isomer via an enrfo-transition state, which was converted to trisubstituted furan by oxidation-elimination reactions <02T6097>. 

Several stereochemical facets show up on Diels-Alder reactions of 7,8-disubstituted o-quinodimeth-anes. A seminal study of addition reactions of 7,S-diphenylbenzocyclobutenes provided strong evidence for a conrotatory opening of the four-membered ring, followed by a supra-supra-facial endo-selective Diels-Alder reaction with TV-phenylmaleimide, as exemplified by the transformation (607) - (609) (Scheme 137). Based on this result, it is now generally agreed that the trans relation of substituents at C-7 and C-8 in benzocyclobutenes translates, via the transient ( , )-o-quinodimethanes, e.g. (608), into the cis disposition in the corresponding cycloadducts. 

This approach was dramatically shortened (from six to two operations) by providing the required oxidation level via the o-quinodimethane partner (Scheme 143). Benzocyclobutenedione (643), readily available through flash vacuum pyrolysis of phthalide (642), furnished divinylketene (644) on photo-... 

Fukumoto et al have taken advantage of the propensity for inward rotation of an acyl group to prepare the isochroman-3-one (36), an intermediate in their synthesis of geneserine, a constituent of the calabar bean. The conversion of the benzocyclobutene (37) to (36) proceeds via ring opening to the o-quinodimethane (38) followed by cyclization and a 3,3-sigmatropic rearrangement. 

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