Cobalt carbonyl Pauson Khand annulation

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

The intermolecular Pauson-Khand annulation using dicobalt hexacarbonyl-alkyne complexes met only with limited success, possibly because of the higher energy of the cobalt-carbonyl bond. In contrast, chromium carbonyls dissociate more easily, and the reactions of metallocarbenes of chromium, e.g., the Dotz annulation of alkynes, are improved by sonication. 

KHAND PAUSON Cyclopentenone Annulation Cyclopemenone synthesis from carbon monoxide acetylene and olefins, cobalt carbonyl catalyzed. 

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See also in sourсe #XX -- [ ]

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