Cycloadditions 2,3-dichloro-5,6-dicyano- 1,4-benzoquinone

Acyl-substituted quinolizinium ylide 63 was obtained by treatment of its 1,2-dihydro analogue with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). Its 1,3-dipolar cycloaddition with an acetylenic ester in excess was regioselective and was accelerated in polar solvents yielding the intermediate adduct 64 and finally the corresponding cyclazine 65, as shown in Scheme 2 <2001JOC1638>. 

For the first time, application of sequential Diels-Alder reactions to an in situ-generated 2,3-dimethylenepyrrole was shown with various dienophiles 548 to afford 2,3,6,7-tetrasubstituted carbazoles (549). This novel tandem Diels-Alder reaction leads to carbazole derivatives in two steps, starting from pyrrole 547 and 2 equivalents of a dienophile, and is followed by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) oxidation of the intermediate octahydrocarbazole. Mechanistically, the formation of the intermediate octahydrocarbazole appears to involve two sequential [4+2] cycloadditions between the exocyclic diene generated by the thermal elimination of acetic acid and a dienophile (529) (Scheme 5.17). 

Cycloadditions of alkenes and alkynes onto imine cation radicals have been reported, with the cation radicals generated by either PIET mediated by DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)180, or by TIET mediated by FeCb1066. The reaction is shown in Scheme 77. 

N-Substituted 1,2-diazocine 34, containing a conjugated diene moiety, was reacted with dimethyl acetylenedicar-boxylate (DMAD) to give the cycloaddition adduct 35 in excellent yield. The latter was oxidized with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) to give the benzodiazocine 36 (Scheme 7) <2004TL3757>. 

The high-pressure cycloaddition of 2-vinylbenzothiophene 848 with 3-nitro-2-cyclohexen-l-one gives adduct 849, which is treated with l,5-diazabicyclo[4.3.0]non-5-ene (DBN) to afford benzothiophene 850. The reaction with indenone affords the thiophene 852 via oxidation of 851 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) (Scheme 130) <2001T4959>. 

It has been demonstrated that N-hydroxytryptophan can be converted to /3-carbolines in two ways (Fig. 41). Pictet-Spengler reaction of 1 with acetals provided the N -hydroxytetrahydro-/8-carbolines (2) (287). A modified Bischler-Napieralski reaction of 1 with trimethylorthoformate gave N -0X0-3,4-dihydro-/3-carbolines (3), the nitrone function of which can undergo 1,3-dipolar cycloaddition with alkenes (288) and nitriles (289), providing isoxazolidine (4) and dehydro-1,2,4-oxadiazoline (5), annulated TBCs, respectively. Nitrone 3 also was obtained by oxidation of the N-hydroxy-j8-carboline 2 with 2,3-dichloro-5,6-dicyano-l, 4-benzoquinone (DDQ). N-Oxygenated TBCs showed no affinity for the benzodiazepine and tryptamine receptors (290). Unfortunately, no toxicity data were recorded for these substituted hydroxylamines. 

Several isoxazolidines such as 188 were prepared by Yb(OTf)3-catalyzed cycloaddition of polymer-supported nitrones and 3-(2-butenoyl)-l,3-oxazolidin-2-one. Oxidative cleavage from the resin with 2,3 dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) afforded 2-isoxazoline derivatives in good overall yields (Equation 21) <1998TL9211>. 

Fowler, F. W. Cycloadditions of N-methoxycarbonyl-2,3-homopyrrole . Angew. Chem. 1971, 83, 148-149. Angew. Chem. Int. Ed. Engl. 1971,10, 135-136. Baldwin, J., Pinschmidt, R. K. jr. The Cycloaddition of Bicyclo2.1.0pent-2-ene with Tetracyanoethylene . Tetrahedron Lett. 1971, 935-938. Christl, M., Brunn und E., Lanzendoerfer, F. Reactions of Benzvalene with Tetracyanoethylene,2,3-Dichloro-5,6-dicyano-p-benzoquinone, Chlorosulphonyl Isocyanate, and Sulphur Dioxide. Evidence for Concerted 1,4-Cycloadditions to a Vinylcyclopropane System . J. Am. Chem. Soc. 1984,106, 373-382. 

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