Ortho-quinodimethanes

Craig D, Robson MJ, Shaw SJ. Traceless linkers for solid-phase synthesis. Homo- and hetero Diels-Alder reactions of ortho-quinodimethanes. Synlett 1998 12 1381-1383. 

Scheme 6.2S9 Diels-Alder cycloadditions of pyrazine ortho-quinodimethane with dienophiles.

The generation and Diels Alder reaction of 4,5-bis(bromomethylene)-4,5-dihydrothiazole 39 has been investigated. 39 is a heterocyclic analogue of ortho-quinodimethanes and is generated by treating 4,5-bis(dibromomethyl)thiazole 38 with sodium iodide in dimethylformamide. 39 can be trapped in situ with symmetrical dienophiles to give substituted benzothiazoles such as 40 <98EJOC2047>. 

Intramolecular Cycloaddition Reactions of ortho-Quinodimethanes in Organic Synthesis W. Oppolzer, Synthesis, 1978, 793-802. 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyciobutene derivatives tend to open to give extremely reactive dienes, namely ortho-quinodimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G. Aalbersberg, 1975 R.P. Thummel, 1980). 

Heterocyclic analogs of ortho-quinodimethane have become increasingly popular for Diels-Alder functionalization of C60. Several methods for generating these highly reactive species have been developed however, most attention has been paid to their generation via chelotropic elimination of S02 from het-... 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

Aldol reaction and alkylation of ketones at the a-position via enol silyl ethers (11) under neutral conditions are good examples [8]. Ortho-quinodimethane (15) is afforded at room temperature and undergoes [4 + 2] cycloaddition [10]. In contrast, the thermal ring opening of benzocyclobutane requires a much higher temperature (180-190°C) [11]. The fluorine-mediated Si—C activation protocol was the preferred method for the formation of tetrafluoro-para-quinodimethane (19), which then dimerizes to octafluoro-para-cyclophane (20) [12]. Azomethine ylide (22) can be generated from 21 byAgF catalysis and is used for heterocycles syntheses [13]. 

Heterocyclic ortho-quinodimethanes . Collier, S. J. and Storr, R. C., Prog. Heterocycl Chem., 1998, 10, 25. 

The intermolecular dimerization of ortho-quinodimethanes to [2.2]paracyclo-phanes is an established synthetic procedure [39]. However, with extended exo-cyclic double bond systems,para- or orf/zo-quinodimethanes yield cydophanes with alkyne groups in their bridges, such as 69, as products of an intermolecular cyclization [44]. Such para-quinodimethanes with cumulated double bonds are accessible via l,x elimination of bis-propargyl or benzyl bromides with Bu3SnSi-Me3/CsF [45]. Carbo- and heterocyclic unsaturated cydophanes have been pre-... 

Three main strategies have been employed for the production of heterocyclic ortho-quinodimethanes a 1,4-elimination, the chelotropic loss of sulfur dioxide from a 2,5-dihydrothiophene 5,5-dioxide, and the electrocyclic ring opening of a cyclobuteno-heterocycle each of these is illustrated diagramatically below. 

This type of [60]fullerene derivative (13a-e), which contains a thiophene ring, was, nevertheless, synthesized by Chung and coworkers [23] using thienosultines (12a-e) as precursors for the corresponding ortho-quinodimethanes (Scheme 21.7). The reaction was markedly accelerated by microwave irradiation and gave yields... 

Collier SJ, Storr RC (1998) Heterocyclic ortho-quinodimethanes. Prog Heterocycl Chem 10 25-48... 

Terzidis M, Tsoleridis CA et al (2005) Chromone-3-carboxaldehydes in Diels-Alder reactions with indole-ortho-quinodimethane. Synthesis of tetrahydrochromeno[2, 3-b]carba-zoles. Tetrahedron Lett 46 7239-7242... 

---