Triene-conjugated nitrile ylides

This method has been used extensively for the generation of diene- and triene-conjugated nitrile ylides (see Section 7.4.1.2) using strong bases. Potasium tert-butoxide (58) was used for the most part but in recent work (59,60) it was reported that lithium bisftrimethylsilyl)amide is both more effective and more convenient. It has also been shown that thermally unstable imidoyl chlorides for use in these reactions can be prepared by reaction of the corresponding amides with chlorodi-methylformiminium chloride at 0 °C, a reaction that is more effective than using either thionyl chloride or phosphoms pentachloride at higher temperatures (61). 

When the conjugation is further extended, as in the triene-conjugated nitrile ylides (347), it is interesting that 1,1-cycloaddition still appears to be the favored primary process. In the case of the cis isomers 347 [R R =Me, Ph or (CH2)3 R = Ph, Me, C02Me] the proximate products (348) were not isolated but rearranged spontaneously via an aza-Cope process to give the bridged isoquinolines (349) in moderate yield (20-65%) (60,206). 

It is rare that a year passes without a report on the cyclisation of triene-conjugated nitrile ylides from Sharp and co-workers, and this year is no exception. In this example. 

The work of Sharp and his many co-workers at Edinburgh cannot be underestimated. In a more recent communication they have extended the scope of his cyclisation of diene-conjugated nitrile ylides to triene homologues <96CC2739>. Thus cyclisation of the triene 21 afforded the cyclopropa[c]isoquinoline 22, which on heating gave a mixture of 23 and 24. The isomeric triene 25 also gave 24 as the sole product (Scheme 4). In this instance the intermediate cyclopropane could not be isolated. 

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