Hetarynes, cycloaddition

Hetarynes (dehydroaromatic heterocycles) can also act as dienophiles in Diels-Alder reactions. 2 This capacity and the inherent regiochemical problems are illustrated by two syntheses of the alkaloid ellip-ticine, which both feature a [4 + 2] cycloaddition of 3,4-pyridyne (533) (Scheme 126). 2 The first approach used the triazenylcarboxylic acid (534) as a precursor for hetaryne (533), which then underwent an addition to pyranoindolone (535) spontaneous CCh extrusion of the initial cycloadducts (536 X = N, Y = CH) and (536 X = CH, Y = N) gave directly a 50 50 mixture of ellipticine (537) and its undesired regioisomer (538). 2 ... 

They are highly strained and have a singlet ground state.73,74 Because such intermediates have two reactive carbon centers they are attractive substrates for cycloaddition reactions. However, little consideration will be given to this important use of arynes and hetarynes because it is outside the scope of this article. Information dealing with the generation of new intermediates is included, although attempts have not yet been made to exploit them in syntheses. 

The long-standing doubt over the existence of this hetaryne continues. The suggestion2 that it may be involved in some of the nucleophilic displacement reactions of 2- and 3-bromobenzo[b]thiophene has now been refuted.214 216 Heating bis(3-bromo-2-benzo[b]thienyl)mercury with tetra-phenylcyclopentadienone at 275°C produces 1,2,3,4-tetraphenyldibenzo-thiophene (54%).217 It is tempting to conclude that the reaction proceeds via the aryne. However, 3-bromobenzo[b]thiophene undergoes the same reaction, which might therefore proceed via a cycloaddition reaction of the... 

Despite its obvious attractiveness as a potential route to isoindoles and benz-fused isoindoles, no new examples of the dipolar cycloaddition of munchnones and arynes have been reported since the first report by Kato and co-workers. This area is ripe for further exploration, particularly because many new mild methods for generating arynes and hetarynes have been developed since 1976. 

Diaz MT, Cobas A, et al (1998) Polar eontrol of the regioselectivity of hetaryne cycloadditions. Synthesis of elliptieine. Synlett. 157. 

Diaz M, Cobas A et al (2001) Synthesis of elliptieine by hetaryne cycloadditions - control of regioselectivity. Eur J Org Chem 4543 549... 

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