Pauson-Khand reactions allenynes

Wilkinson s catalyst also allows the intramolecular cycloisomerization of allenynes 243 to interesting cross-conjugated trienes 244 (Scheme 15.76) [146], Similar compounds are observed as side-products in Pauson-Khand reactions of allenynes [147]. 

The key step employed a rhodium(l)-catalysed allenic Pauson-Khand reaction to generate the tricyclic ring system 276 from the allenyne 275. However, all attempts to introduced the missing methyl group by a 1,4-addition protocol failed to provide the completed neodolastane framework. Allenyne 275 was synthesized from the enone 274 by a multistep procedure. The quaternary atom was constructed by sequential enolate alkylations. 

Parasites, antimicrobials for, 12, 458 Pauson-Khand reaction allenic substrates, 11, 352 and allenynes, 10, 356-357 with aminocarbonylation, 11, 531 asymmetric catalysts, in desymmetrizations, 11, 357 catalytic... 

The Pauson-Khand reaction, promoted by (CO)3Mo(DMF)3, has been found to take place under very mild conditions in the absence of any promoter. High yields of the adducts were obtained in the cyclization of a wide variety of functionalized 1,6- and 1,7-enynes. Enynes bearing electron-withdrawing groups at the alkene terminus proved to be particularly good substrates.121 The exclusive formation of cyclopentenones was observed in the molybdenum hexacarbonyl (10 mol%)-catalysed Pauson-Khand reactions of 1,6-allenynes under 1 atm of CO (balloon) in excellent yields.122... 

Rhodium(I)-catalysed Pauson-Khand reaction of allenynes has been shown to be applicable for constructing azabicyclo[5.3.0]decadienone and oxabicyclo[5.3.0] decadienone frameworks. The 10-monosubstituted-bicyclo[5.3.0] deca-l,7-dien-9-one ring system has been prepared by the rhodium(I)-catalysed Pauson-Khand reaction of allenynes under 10 atm of CO (Scheme 86).129... 

Most Pauson-Khand reactions have been conducted with an alkene, an alk5me, and CO. However, PKRs have been developed with aUenynes (Scheme 17.36). Narasaka reported the first example of an intramolecular allenyne PKR catalyzed by an iron complex, and Brum-mond has extensively studied the allenic Pauson-Khand reaction. These reactions have been catalyzed by the combination of Mo(CO) and DMSO or by [Rh(CO)2Cl]2, and several examples are shown in Equations 17.77 and 17.78. - The reactions in Equations 17.77 and 17.78 illustrate the different regiochemistry of the products from reactions catalyzed by molybde num and rhodium. Computational studies indicate that different geometries of octahedral Mo(0), and square-planar Rh(I) species account for the different regioselectivities. ... 

Currently, these reactions are typically conducted with Rh(l) or Ir catalysts. The Pauson-Khand-type reaction of allenynes has also witnessed important developments, especially in its applications to natural products synthesis.388 Brummond s group has been very productive in both areas. Duality in the reaction of allenynes is shown below. In the context of diversity-oriented synthesis, simply changing the reaction conditions gives versatile heterocycles in high yields (Scheme 116).389... 

Allenyne represents an interesting substrate for the intramolecular Pauson-Khand(-type) reaction, where an allene moiety acts as an ene component. Here, there are two possible reaction pathways (Scheme 11.21) (i) the reaction of an external tr-bond of allene moiety gives a bicyclic dienone (type A) or (ii) the reaction of an internal 7i-bond gives a bicyclic cyclopentenone with an alkylidene substituent (type B). 

Cyclopentenones. Allenynes undergo a Pauson-Khand-type reaction in the presence of Mo(CO)g in DMSO. The substitution pattern of the allenyl moiety has profound... 

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