THE TANDEM PAUSON-KHAND REACTION

The conditions for the novel tandem Pauson-Khand reaction have been ex plored. The highly functionalized tetracyclic compounds 11c, 11d, and 16 were prepared from the ene-diynes 4c, 4d, and 10 by treatment with 2 equiv. of [Co-2(CO)(8)] and 4-methylmorpholine N-oxide (NMO) or Me(3 )NO in yields of 24, 22, and 53%, respectively (Table). In the presenc e of 1-3 equiv. of H2O added to the NMO used for induction of the Paus on-Khand reaction of 6d, a mixture of cyclopentanones 17/18 and cyclop entenones 12/13 was obtained (Scheme 5). The first example of a [Co-2( CO)(6)]-induced highly stereoselective ene reaction is described. To a ccount for these results, the formation of intermediates are proposed (Schemes 6 and 7) which hitherto have not been considered in the mecha nistic description of the Pauson-Khand reaction.