Quinomethide intermediate

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

Biosynthesis of the spirobenzylisoquinoline alkaloid ochotensimine (282) via the quinomethide intermediate (Scheme 49) was proposed by Shamma and Jones (7J0). On the basis of this hypothesis, several biomimetic transformations of phenolic protoberberines to spirobenzylisoquinolines have been realized by the base-induced rearrangement via the quinomethide. 

An attractive route to dialkoxy PTV derivatives has recently been reported by Elsenbaumer and co-workers [131, 132], The method employs the thermal elimination of a sulfmyl group from a soluble sulfoxide precursor polymer (Scheme 1-31). In addition, the authors claim the first isolation of the quinomethide intermediate and characterised it by NMR and mass spectrometry. Dibutoxy-PTV produced by this methodology exhibits a significantly smaller band gap (1.2 eV) and higher molecular weight (M =8700) than the same material obtained by other methods. Poly(3,4-dimethoxy-2,5-thienylene vinylene) PDMoxTV was deep blue in color... 

Upon treatment of a triol with HCl in a mixture of EtOH and CH2CI2, biomimetic cyclization occurred efficiently through the quinomethide intermediate to afford exo-exo furofuran as a single diastereomer in 72% yield. This process was applied in the synthesis of (+)-hedyotol A (140L1976). 

Salicylaldehydes cannot be used in this process, as they form dihydropyrans, most likely via an intermediate o-quinomethide. 

A fascinating contrapolarization-driven reaction is the conversion of a protoberine skeleton to an ochotensine-type alkaloid [211], The quinomethide carbon in the seco intermediate assumes a -character which is made possible by electron supply from the oxy substituent at the meta position. Note the C-N bond cleavage step entrusts this carbon an acceptor role. 

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