Reactions via o-Quinodimethanes

2- [(trimethylsilyl)methyl]benzyl carbonates 69 with or without a fluoride ion (method 4) have frequently been utilized for the transition-metal-catalyzed reactions of o-QDMs [36], 

Yoshida et al. also applied the catalytic system above to the insertion reaction into a Si—Si CT-bond [40]. Thus, naphthalene-fused disilacyclopentane 77 readily underwent the insertion of variously substitued o-QDMs, furnishing nine-membered disilacar-bocycles, benzodisilonines 78 straightforwardly (Table 28.3). The disilylation was also applicable to simple disilacyclopentane 79, and furthermore, a 10-membered disilacarbocycle, benzodisilecine 82, could be synthesized by treatment of biphenyl-fused disilacyclohexane 81 with o-QDMs. 

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Scheme 2 Diels-Alder reaction via o-quinodimethane intermediate...

Octahydrophenanthrenes have also been synthesized via IMDA reactions of o-quinodimethane species generated by cheletropic elimination of sulfur dioxide from 1,3-dihy-drobenzo[c]thiophene 2,2-dioxides28,19. 

Photolysis of o-allgrlphenacyl benzoates (205) afforded indanones (206) and benzocyclobutanols (207) via o-quinodimethane, in a way depending on the reaction temperature (Scheme 40). ... 

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). 

An efficient method for the generation of oquinodimethanes via desilylation reaction involves treatment of a 2-[(trimethylsilylmethyl)benzyl]trimethylarmnonium iodide with fluoride ion (equation 31)64. 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)65. 

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. 

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... 

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. 

Azabutadiene systems, as more or less formal carbonyl derivatives, have been used for [4 + 2] cycloaddition reactions. The fluoride-induced stereoselective transformation of (144) to the polycyclic steroid-like system (145), obviously proceeding via the corresponding reactive aza-o-quinodimethane (Scheme 67), shows the synthetic utility of such heterodienes. 

As mentioned earlier, direct irradiation of 1,2-dihydronaphthalenes leads to a variety of photoisomerization reactions which can be attributed to the initial formation of the )-vinyl-orf/ o-quinodimethane isomer by electrocyclic ring opening. For example, irradiation of the parent compound 115 with an intense, broad-band light source yields the isomeric benzobicyclo[3.1.0]hexene derivative (158) as the main photoproduct, via secondary photolysis of the initially-produced cu-vinyl-orf/zo-quinodimethane isomer 116 (equation Such compounds are short-lived due to rapid thermal ring closure to... 

In a reinvestigation of the reaction of Schlenk and Bergmann, Miller and Boyer 64) found that reaction of 1,1,3-triphenylindene with sodium or sodium-potassium alloy in THF gave, after addition of water, 1,2,3-triphenylindane (rather than the indene). These workers suggest that the reaction likely proceeds via the radical anion 95 which by [1,2] migration of phenyl gives a more stable o-quinodimethane anion radical % which upon reduction leads to the dianion 97. Evidently in THF hydride loss from 97 is slower than in diethyl ether, where 99 is formed prior to... 

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