1.3- Dienes multi-component reactions

Dienes such as 90 can be accessed by a multi-component reaction under ruthenium catalysis involving an allene 88 and an enone (methyl vinyl ketone in this case), with cerium(m) chloride as an additive in DMF (Scheme 26).95,96 With an allene concentration of 0.25 M, yields are moderate to good. Different ruthenium catalysts and additives were tested in order to optimize this reaction. CpRu(COD)Cl 89 and CpRu(MeCN)3PF6 appeared to be more versatile ones. The mono-, di-, tri-, and tetrasubstituted allenes have been investigated with methyl vinyl... 

Multi-component reactions open a direct access to poly-functional derivatives/ For instance, reactions of unsaturated substrates and aldehydes in the presence of EtaSiH opened a rapid access to allyllic, homo-allylic or co-unsatu-rated alcohols, and NHC-Ni complexes were recently reported to be efficient catalysts for such oxidative coupling. Mori and co-workers described the first example of such condensation in 2001. They showed that an in situ generated IPr/Ni 1 1 complex led to the stereoselective formation of (Z)-homoal-lylic silyl alcohols [eqn (10.33)], while classical Ni -PPhj catalyst produced only (Ej-coupling products. Of note, IPr/Ni 1 2 and IMes/Ni 1 2 were less efficient in this reaction. These first results were extended to silylated dienes. Interestingly, it was shown that a mixed complex [(IPr)Ni(PPh3)] stereoselectively produced (Z)-allylsilanes while [Ni(PPh3)4] led only to (Ej-isomeric products. 

The aza-Diels-Alder reaction of Danishefsky s diene with imines provides a convenient method for the synthesis of 2-substituted 2,3-dihydro-4-pyridones, a compound class that has important synthetic applications. Kobayashi and co-workers have studied the reaction in detail using ytterbium (III) triflate as the Lewis acid30. Although the reaction is often run at low temperature (—78°C to 0°C) for a number of hours, we have found that the reaction also worked well at elevated temperatures (150°C) in the microwave for a few minutes (J. Westman and A. Hurynowics, unpublished results) (see Scheme 5.15). The imines could either be preformed prior to the addition of the Danishefsky s diene or the reaction could be performed as a multi-component protocol, where all components were added at once. 

Ni-catalyzed multi-component coupling reactions are an efficient protocol for carbon-carbon bond formation [97]. The Ni-catalyzed asymmetric intermolecular reductive coupling of 1,3-dienes and aldehydes with Et Zn as a reducing reagent was realized... 

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