Since its discovery in the early seventies the intermolecular Pauson-Khand reaction has made considerable progress towards a powerful synthetic method. This account describes the major accomplishments with respect to reactivity, stereoselectivity and catalytic versions, which have been achieved over the last decade and summarizes mechanistic information being obtained by theoretical and experimental studies.

A concise synthetic route to highly substituted pentalenones 4 and 5, respectively, is reported. The key step is a Pauson–Khand reaction of norbornadiene (7) with the functionalized acetylenes 8 to give the methanoindenone derivatives 6. 1,4‐Addition of organocuprates resulted in clean formation of 2,3‐trans‐disubstituted methanoindenones 9. After ozonolysis and reductive workup, the bis(hydroxymethyl)pentalenones 10 were obtained in moderate to good yields. A differentiation of the hydroxy groups became possible by lipase‐mediated acetylation with vinyl acetate, giving highly selectively monoacetates 4 with the acetyl group at C‐6 of the pentalene system. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The utility of cyclopentadiene (4) in intermolecular Pauson-Khand reactions was investigated. Subsequently, a 1,4-addition of lithium organocuprates was carried out followed by desilylation. This synthetic concept allows the preparation of β -functionalized bicyclo[3.3.0]octane derivatives 1a -c in only three steps in total yields up to 53%.

Cyclopentane derivativesCyclopentane derivatives Q 0030 Synthesis of Pentalene Systems Employing a Sequence of Pauson-Khand Reaction, Michael Reaction, and Desilylation. -The reaction sequence is applied to norbornene, norbornadiene and pentadiene. The resulting products are further desilylated [cf. (V)] or alkylated followed by desilylation. -(BECHEANU, A.; BELL, T.; LASCHAT, S.; BARO, A.; FREY, W.; STEINKE, N.; FISCHER, P.; Z. Naturforsch., B: Chem. Sci. 61 (2006) 5, 589-596; Inst. Org. Chem., Univ. Stuttgart, D-70569 Stuttgart, Germany; Eng.) -Y. Steudel 40-058

Cyclopentane derivatives Q 0030Synthesis of Highly Functionalized Pentalenes via Intermolecular Pauson-Khand Reaction. -On treatment with diorganocuprates, Pauson-Khand cyclization products (I) give the corresponding 2,3-trans-disubstituted products which are transformed to diols via ozonolysis and reduction of the intermediate dialdehydes.A differentiation of the hydroxy groups is accomplished by lipase-mediated acetylation. For products (IV) and (IX) the enzymatic reaction proceeds with >99% regioselectivity. Surprisingly, the acylations proceed without any enantioselectivity. -(BECHEANU, A.; BARO, A.; LASCHAT*, S.; FREY, W.; Eur.