THE SEQUENCE OF A STEPWISE AD(E) REACTION AND INTRAMOLECULAR PAUSON-KHAND CYCLOADDITION AS AN ENTRY INTO THE SYNTHESIS OF POLYCYCLIC COMPOUNDS

A stepwise Ad(E) acylmethoxylation across the double bond of dicobalt hexacarbonyl complexes (DCHCC) of conjugated enynes was elaborated as an efficient and general route for the synthesis of DCHCC of 1,6-enynes containing a combination of five- and six-membered-ring fragments. Depending on the structure of these adducts, the latter either were subjected to 1,2-carbonyl reduction followed by an intramolecular Pauson-Khand (IMPK) cyclization or were directly utilized as substrates for this process. A list of model polycyclic systems which were assembled using this approach includes [5.5.5] angularly fused compounds, [6.5.5] and [5.5.5] linearly fused tricyclics, and linearly and angularly fused [6.5.5.5] and [5.5.5.5] tetracyclic products. A novel convergent and general method for the synthesis of various cyclic compounds is suggested on the basis of the Ad(E)-IMPK tandem sequence as the key steps for the assemblage of polycyclic frameworks. This option seems to be especially promising for the tetracyclic derivatives mentioned above, as in these cases only two operationally simple steps are required to convert readily available starting blocks into the target structures related to natural polyquinanes.