Quinodimethanes precursors

A traceless linker for solid-phase homo- and hetero-Diels-Alder reactions was based upon resin bound quinodimethane precursors... 

Entry Quinodimethane precursor Dienophile Product Yield (%) Ref. 

Several reviews have dealt with Diels-Alder reactions of o-quinodimethanes, 2" emphasizing the more powerful, selective and thus extensively used intramolecular versions (cf. Scheme 135) introduced in 1971. 2 The most recent survey also tackled the issue of asymmetric induction by means of a chiral auxiliary group attached to the o-quinodimethane precursor.This section aims at a complementary coverage of intermolecular Diels-Alder reactions of o-quinodimethanes, underlining synthetically relevant aspects. 

As to the underlying reaction mechanism, it appears that vacuum thermolysis of (545), (554) and (561) generates in each case, initially, a transient o-quinodimethane which cyclizes in the highly dilute gas phase. o-Quinoid vinylketene intermediate (562) could be regenerated in solution from the isolated benzocyclobutenone (563) on heating or irradiation and trapped by electron-poor dienophiles, e.g. (632) (Scheme 141). = Alternatively, benzocyclobutenones can be easily functionalized at the carbonyl group or at Ca, affording further o-quinodimethane precursors. 

DiffCTent methods for the generation of indolo-2,3-quinodimethanes and subsequent cycloaddition have been reported. Indole-2,3-quinodimethane precursors include 3-cyanomethyl-2-vinylindoles <05BMC2263> and furoindolones <05SL994>. 

The quinodimethane can also be trapped by SO2 which gives the sulfone (84), which can itself serve as a quinodimethane precursor (Equation (149)). 

Another indole-2,3-quinodimethane precursor is the Boc-protected 2-(TMSmethyl)gramine... 

The Miller group also reported the synthesis of nonacene derivatives 75 and 76 using their thioaryl approach discussed above in the context of substituted heptacenes [62]. The synthesis is reminiscent of that of heptacene derivatives [58], with the key step being the Diels-Alder reaction between anthracenequinone 72 and the bis-ortho-quinodimethane precursor 73 (Fig. 35). The persistence of the less substituted derivative 76 is insufficient for characterization by solution phase methods. Derivative 75, however, could be investigated by NMR, UV/vis-NIR, and fluorescence spectroscopies as well as by LDI-TOF-MS [62]. 

Beyond RCM and CM strategies, Craig has reported cleavage using Diels-Alder reactions (Scheme 1.16). ° [4 + 2] Cycloaddition (with concomitant aromatization) of the o-quinodimethane precursor (52) with dimethylacetylene dicarboxylate (DMAD), trichlor-oacetonitrile, and benzoquinone provided dimethyl naphthalene-2,3-dicarboxylate (53), 3-(trichloromethyl)isoquinoline (54), and 2,3-naphthoquinone (55), respectively. The diverse products from a single polymer-supported intermediate, such as the bismuth linkers discussed previously, make Craig s multifunctional linker unit attractive for approaches toward diversity-oriented synthesis. 

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