Polyunsaturated substrates

Polyolefin cyclization.1 This Pd(0) catalyst in combination with a phosphine ligand and acetic acid effects cyclization of polyunsaturated substrates to polycyclic products in one step (zipper reaction). However, a triple bond is required for initiation. This polyolefin cyclization has been used to prepare tetra- and pentaspirocycles, as a mixture of only two stereoisomers. 

EpoxUatioa. Orison and co-workers have compared (he relative rate of epoxi-dation of peroxybenzimidic acid with that of peracetic acid and note little correlation. They conclude that this pcracid is relatively indiscriminate and is not the reagent of choice for selective epoxidation of polyunsaturated substrates. 

One method by which double bonds can be distinguished in polyunsaturated substrates is to position one or more of them in conjunction with an ester (or ketone) group which renders them open to f 1.4]-reduction [(122)- - (123)] amongst other possibilities. Some new reagents for performing this transformation are magnesium in... 

Another synthetically useful isomerization that can be combined with a metathesis event in a domino transformation is a metallotropic [l,3]-shift of a transient ruthenium aUdnyl carbene complex [33]. Treatment of the polyunsaturated substrate 162 with Grubbs catalyst 2 afforded the oligoenyne 163 in 86% yield... 

Metal-catalyzed domino reactions are characterized by organometallic specimens that are regenerated in multiple insertion events and yield after an elimination step the complex, often polycyclic organic product. Predominantly in palladium chemistry, unimolecular reactions proceeding in an intramolecular fashion, for example, by cyclic carbopalladation [5], furnish complex polycyclic structures starting from linear polyunsaturated substrates. 

It is only in recent years, following what is commonly referred to as the Gold Rush in the scientific literature, that NHC-Au complexes have gained popularity and have seen their potential unfold. The first application in catalysis of a NHC-gold species appeared in 2003, but the real surge in publication only started in 2006. Since then, there is hardly a week without a report on the activity of NHC-Au catalysts in organic synthesis is published. The field is dominated by two main topics of crucial importance in modern catalysis, namely cycloisomerization of polyunsaturated substrates and hydrofunctionalization of 7i-bonds. NHC-Au species have been the subject of several reviews... 

Cycloisomerization is described, the intramolecular reaction of a polyunsaturated substrate for the formation of... 

Permanganate was the first reagent that was demonstrated to have the ability to carry out the oxidative cyclization of 1,5-dienes [47]. Brown and coworkers have continued the development of this area by discovering a diastereoselective KMn04-mediated 1,5-diene cyclization that leads to 2,5-cis-tetrahydrofuran derivatives. They have used a camphorsultam chiral auxiliary to ccaitrol the facial selectivity and have found that polyunsaturated substrates can be chemoselectively manipulated. As an... 

The iron(0) complex [CpFe(cod)][Li(dme)] is able to catalyze the intramolecular Diels-Alder reaction of unactivated diene-ynes (Scheme 4-307). Moreover, also [5+2] and [2+2+2] cycloaddition reactions of polyunsaturated substrates are achieved with the iron(O) catalysts [CpFe(cod)] [Li(dme)] and [CpFe(C2H4)2] [Li(tmeda)]. ... 

The enantioselective transition-metal-promoted skeletal rearrangements of polyunsaturated substrates possessing olefin, alkyne, or allene functions have been reviewed (Scheme 97). ... 

Van Tamelen showcased the ability of homoallylic alcohols to bias the stereochemical course of epoxidations in investigations of the biosynthesis of sterol precursors (Equation 11) [68]. The polyunsaturated substrate 39 readily participated in a vanadium-catalyzed epoxidation of the 1,1-disubsti-tuted olefin with complete regio- and stereoselectivity. Monoepoxide 40 was isolated as the only detectable product in quantitative yield. 

An impressive sequence involving concomitant inter- and intramolecular Diels-Alder reactions was reported by Roush during the total synthesis of chlorothricolide (63, Scheme 17.11) [48]. When polyunsaturated substrate 58 was allowed to react with chiral enone 59 in the presence of BHT (60)... 

Beside Alder-ene cyclizations, only the cycloisomerizations of polyunsaturated substrates (Sects. 10.3.2 and 10.3.3) as well as reactions involving electrophilic activation of the alkyne, notably the conversion of 1,6-enynes into bicycHc scaffolds (Sects. 10.4, 10.5 and 10.6), have been investigated in some depth. 

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