Utility of the Tandem Pauson−Khand Reaction in the Construction of Tetracycles

• Published in: Journal of organic chemistry 2000-04, Vol.65 (7), p.1957-1971
• Main Authors: Van Ornum, Scott G, Bruendl, Michelle M, Cao, Hui, Reddy, Mundala, Grubisha, Desiree S, Bennett, Dennis W, Cook, James M
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9914014

The scope of the tandem Pauson−Khand reaction has been explored for the regiospecific construction of [5.5.5.5]- and [5.6.6.5]tetracyclic systems via the photolytic method of Livinghouse. The rapid regiospecific entry into the two dicyclopentapentanoid systems 17 and 29 was accomplished from the key diene-diynes 11 and 19b. A photochemically mediated catalytic tandem Pauson−Khand cyclization was employed to prepare the parent ring systems of dicyclopenta[a,e]pentalene (from 19b) and dicyclopenta[a,f]pentalene (from 11) in regiospecific fashion in a one-pot process. Under these conditions, conversion of acyclic diene-diyne 16 into tetracyclic system 17 was achieved in 74% yield, while a similar process was employed to convert 28 into tetracycle 29 in 90% yield. This is much improved over the previous conditions that employed NMO. Six carbon−carbon bonds were generated in this process constituting up to 98% yield for each carbon−carbon bond so formed. Furthermore, tetracyclic [5.6.6.5] systems such as dicyclopenta[b,g]decalins 37, 38, and 40 were prepared from similar diene-diyne precursors via the tandem Pauson−Khand cyclization. Importantly, acetal 36 provided the desired cis-fused [5.6.6.5] system 38a (via 40a/b) in stereospecific fashion. This reaction is unique in that it provides a cis-decalin ring system; moreover, the yield of each of the six carbon−carbon bonds formed in this process was at least 89%. The structure of cis diol 38a was confirmed by X-ray crystallography.