Quinodimethane intermediate

CATEGORY llegf CYCLIZATIONS - CYCLOADDITIONS INVOLVING PYRROLE-2,3-QUINODIMETHANE INTERMEDIATES AND EQUIVALENTS... 

This category corresponds to the construction of the carbocyclic ring by 2 + 4 cycloaddition with pyrrole-2,3-quinodimethane intermediates. Such reactions can be particularly useful in the synthesis of 5,6-disubstituted indoles. Although there are a few cases where a pyrrolequinodimethane intermediate is generated, the most useful procedures involve more stable surrogates. Both 1,5-di-hydropyrano[3,4-b]pyrrol-5(lf/)-ones[l] and l,6-dihyropyrano[4,3-b]pyrrol-6-(in)-ones[2] can serve as pyrrole-2,3-quinodimethane equivalents. The adducts undergo elimination of CO2. 

Synthesis of carbazoles from indoles by quinodimethane intermediates... 

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

Several procedures have been developed for obtaining quinodimethane intermediates from w-substi Luted benzylstannanes. The reactions occur by generating an electrophilic center at the adjacent benzylic position, which triggers a 1,4-elimination. 

Several ingenious syntheses of natural products have been developed by exploiting benzcyclobutene ring opening to o-quinodimethane. Particularly, the intramolecular Diels-Alder strategy employing o-quinodimethane intermediates has been very effective for the construction of polycyclic structures. Selected examples are gathered in Table 12. 

Photochemical hydrogen abstraction reaction for the silylimine 161 give an o-quinodimethane intermediate 162 which could be trapped with dimethyl fumarate, dimethyl maleate, trans-methyl cinnamate, methyl acrylate, acrylonitrile (equation 94)... 

Nitric oxide can be scavenged as a stable, ESR-detectable free radical by a quinodimethane intermediate, l,2-disopropylidene-3,5-cyclohexadiene, which can be generated by photolysis of l,l,3,3-tetramethyl-2-indanone. This approach offers the possibility of detecting and possibly quantitating NO produced by biological systems [24]. Free NO in solution is not detectable by ESR. 

An alternative polymerization mechanism and polymer architecture has been proposed by Kirchhoff [1, 2, 3], Tan and Arnold [77], By this mechanism, polybenzocyclobutenes which do not contain reactive sites of unsaturation are proposed to polymerize by the 1,4 addition of the o-quinodimethane intermediates to give a substantially linear poly(o-xylylene) structure. Since the monomers all contain at least two benzocyclobutene units the net result of this reaction will to a first approximation be a ladder type polymer as shown in Fig. 17. The formation of a true ladder polymer however would require that all... 

Takahashi and co-authors (1996) described another case of cation radical cycloreversion. Benzocyclobutenols undergo ring opening induced by electron transfer to generate quinodimethane intermediates, which then tautomerize to benzenoids. The reaction proceeds upon irradiation in the presence of tetracyanoanthracene (X > 350 nm). Yields (based on PMR analyses) are quantitative (Scheme 6-42). 

Formation of ring c ( disconnection b) was readily accomplished by addition of o-quinodimethane intermediates, e.g. (562) or 7,8-dibromo-o-quinodimefliane, to tetrahydronqihthoquinones such as racemic or enantiomerically pure (632) (Scheme 141). 

The stoichiometric reaction of proparene 113 and the carbene complex 114 at 25 °C gave rise to trace amounts of styrene, dibenzocyclooctadiene 117, and other polymeric products (Scheme 4.43) [89]. Dibenzocyclooctadiene 117 was considered to be formed from the mthenacyde 115 via a quinodimethane intermediate, while styrene was formed by the decomposition of the isomeric ruthenacycle 116. The qui-... 

Imidazole o-quinodimethane intermediate 8, synthesized from 2-bromo-... 

There are four main routes to the synthesis of PAVs [9] polymerizations via quinodimethane intermediates, polycondensations, transition-metal-mediated polycouplings, and metathesis polymerizations. Other methods such as chemical vapor deposition and electropolymerization have also been used on occasion but generally give poorer quality polymers. 

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