There are still some synthetic chemists who hesitate to use metal-mediated or -catalysed reactions. The Pauson-Khand reaction (PKR) is a powerful transformation that has now been sufficiently well developed to be routinely considered when planning a synthesis, especially of polycyclic complex molecules. This tutorial review aims to encourage the use of this process explaining the best ways of performing a PKR both in the stoichiometric and the catalytic version, showing the scope of the process and its limitations. Additionally, asymmetry can be introduced in the reaction using several strategies, which will be discussed. The most recent examples of the synthetic applications of the PKR in natural product synthesis will give the reader an idea of the great usefulness of this reaction.

Activation of alkynes with platinum salts allows their reaction with alkenes, giving rise to a myriad of interesting processes like cyclizations, formation of cyclopropanes, or metathesis. Weak nucleophiles can also participate in the processes leading to alkoxycyclizations. There are intriguing mechanistic aspects of these processes in which subtle variations of the reaction conditions and/or substrate structures can lead to completely different products.

Pauson-Khand reactions of functionalized allenes with different alkynes give cyclopentenones with generally high regio-and stereoselectivities. The allenes react through their external double bonds, giving cyclopentenones with exocyclic double bond at their β positions and predominantly with E stereochemistry. Some intramolecular reactions with al-

Abstract-Intermolecular Pauson-Khand reactions are still quite limited in scope and yields. There are also few reports using allenes as the olefinic part in this version. We report herein new regio and stereoselective reactions of allenamides with several alkynes. These reactions give functionalized cyclopentenones bearing an exocyclic enamide, which can be useful synthetic intermediates. Ó 2004 Elsevier Ltd. All rights reserved.Allenamines and allenamides are an emerging group of substrates that are becoming useful in organic synthesis. 1 Although there are still very few literature examples using allenamines, this electron enriched allenes have found use in some reactions with electrophiles and in certain cyclizations. The parent allenamides are more stable and thus are increasing their use in synthetic methodologies. 2 On the other hand, the Pauson-Khand reaction (PKR) is a powerful tool in the synthesis of cyclopentenones, a structural feature present in a large variety of natural products. 3 The intermolecular version of this reaction is still quite limited, as unstrained olefins scarcely react. 4 Some vinylethers/esters 5 and vinyl sulfoxides 6 are also useful in the intermolecular PKR. The use of traceless tethers 7 or directing groups like pyridisilyl 8 are strategies that circumvent partially this problem, also avoiding the formation of regioisomeric mixtures. In the majority of cases, the intermolecular PKR is regioselective from the alkyne being the bulkier substituent situated adjacent to the carbonyl in the final product. It has been demonstrated recently that both steric and electronic effects may be responsible for this regioselectivity. 9 Unsymmetrical olefins usually give mixtures of regioisomers (Scheme 1).There are a wide variety of catalytic protocols available to effect this reaction, some of them under mild conditions. Nevertheless the catalytic PKR is not a completely solved problem specially in the intermolecular processes where there are still few results. 3bAllenes are one of the most promising substrates for the PKR introduced in the past years. Several groups have used the intramolecular allenic PKR for the synthesis of medium sized rings. However there is only one precedent in the use of simple allenes in an intermolecular PKR due to Cazes and co-workers. 10 There is also a precedent of an intramolecular PKR with an allenamide equivalent. 11 We herein report new intermolecular PKR of allenamides. These reactions show many possibilities in selectivity, and as we will see have resulted to be very regio and stereoselective.To explore this chemistry we carried out the synthesis of a series of allenamides bearing aromatic rings. This synthesis was effected using a classical strategy depicted Tetrahedron Letters in Scheme 2. Starting from tosylanilides and acetanilides 1, propargylation under sonication using solid KOH as the base gave the corresponding propargylamides 2 in good yields. These compounds were converted totally into the corresponding allenamides 3 by treatment with t BuOK. In...