Ketenes, vinyl cycloaddition intramolecular cycloadditions

Ketene reactivity in intramolecular cycloadditions parallel those in intermolecular reactions in which chloro-, vinyl-, aryl- and alkoxyketenes are more reactive than the alkylketenes. In most instances the ketene is generated by amine dehydrohalogenation of an acid chloride. There are, however, a few examples of ketenes prepared along less conventional routes as by the examples for the formation of 11.150 12,151 and 13.151... 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

The reaction course is shown in Scheme 4. Enyne 12 reacts with 2 to give vinyl carbene complex 17, which is in a state of equilibrium with vinyl ketene complex 21. [2+2] Cycloaddition of the ketene moiety and alkene part in 21 gives cyclob-utanone 22. On the other hand, the vinyl carbene complex 17 reacts with the alkene intramolecularly to produce metalacyclobutane 18. From metalacyclob-utane 18, reductive elimination occurs to give cyclopropane derivative 23. Ret-... 

The intramolecular photocycloaddition of the vinyl ketenes (6) has been reported. This system affords good yields of the (2t2)-adducts shown in Scheme 1. Primarily cycloaddition... 

Alkyne-tethered vinyl ketenes 441, generated in the thermolysis of propargyloxo-cyclobutenones 440, undergo an intramolecular [4+2] cycloaddition reaction, followed... 

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