Tandem Pauson-Khand/4 + 2-cycloaddition

The highly chemoselective and stereoselective tandem Pauson-Khand/4 + 2-cycloaddition of the diene-diyne substrate (3) produced the spirotricyclic core analogue (4) of the sesquiterpene polyol, mangicol (Scheme 2)J... 

It is also worthy of note that the construction of tetracyclic compounds 44 by cobalt-catalyzed tandem Pauson-Khand/[2 + 2 + 2] cycloaddition reactions of 1,6-diynes 43 was also reported by Chung et al. [45,46]. It was experimentally shown that the [2 + 2 + 2] cycloaddition reaction occurs after the [2 + 2 + 1] cycloaddition between CO and the substrate (Scheme 22). 

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. 

Few other qfdoaddition reactions with microwave irradiation have appeared in the literature since the first applications were published. In particular, the [2+2+1] reaction, known as the Pauson-Khand reaction, [6+4], and tandem [6+4]-[4+2] cycloadditions have been reported as being performed successfully under micro-wave conditions. A range of examples is described below. 

Shi and co-workers reported a Pauson-Khand type [3+2+1] cycloaddition reaction of ene-vinylidenecyclopropanes and CO in the presence of rhodium(I) catalyst [122]. Both cyclic and noncyclic substituent could be tolerated (see (48)). Shi also reported a tandem process that craiverted 1,4-enynes tethered by cyclopropyl group... 

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