Sigmatropic rearrangements 3.3 , catalyzed

Sigmatropic rearrangements catalyzed by silver salts have been used in total synthesis. The synthesis of three monoterpene compounds that comprise the pheromone of male boll weevil Anthonomus grandis has been achieved using the silver-carbonate catalyzed rearrangement of an acetoxycyclohexyl acetylene as the key step (Scheme 3.35).56... 

These results clearly demonstrated that, depending on substrates, apparent [3,3]-sigmatropic shifts actually resulting from 1,2 shifts could occur. Further studies are thus clearly needed to better understand the mechanism of apparent sigmatropic rearrangements catalyzed by silver and other metals. 

A slereoselectrve base catalyzed [2,3] sigmatropic rearrangement of allyl ethers to honwallylic alcohols (stereoselectrve)... 

A more direct access to the unstable and non isolated sulfonium ylides 58a- c is the reaction of diisopropyl diazomethylphosphonate 57 with allylic sulfides, catalyzed by Cu(II), Rh(II) [39], or ruthenium porphyrins.[40] For example, the a-phosphorylated y,d-unsaturated sulfides 59-61 are obtained through the [2,3] -sigmatropic rearrangement of 58a-c. This method allows the use of a greater variety of starting allylic sulfide substrates, such as 2-vinyl tetrahydrothiophene, or propargylic sulfides (Scheme 15). 

A nice application of this reaction for the synthesis of cyclic a-sulfanylphos-phonates 63 has been reported [42]. It involves a Rh(II)-catalyzed [2,3]-sigmatropic rearrangement and a ring-closing metathesis of the resulting a-(S-allyl) y,d-unsaturated phosphonates 62 (Scheme 16). However, the last step occurs with a low yield (19%) when R = H. 

Entry 7 illustrates reaction conditions that were applicable to formation and rearrangement of an isopropenyl allylic ether. The tri-isopropylaluminum is thought to both catalyze the sigmatropic rearrangement and reduce the product ketone. 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocydic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

Finally, Katsuki and coworkers [271] described an enantioselective Ru-catalyzed domino reaction, which includes a sulfamidation of an aryl allyl sulfide 6/3-111 using the chiral Ru(salen)-complex 6/3-115, followed by a 2,3-sigmatropic rearrangement of the formed 6/3-112 to give N-allyl-N-arylthiotoluenesulfonamides 6/3-113. On hydrolysis, 6/3-113 yielded N-allyltoluenesulfonamides 6/3-114 (Scheme 6/3.33). The enantioselectivity ranged from 78 to 83% ee. 

Olefins analogous to 158 and 159 were also isolated from the CuS04-catalyzed decomposition of ethyl diazoacetate in the presence of 2-isopropenyl-2-methyl-1,3-dithiane (total yield 56%, E Z — 4 1) a butadiene was absent from the reaction mixture 161). With dimethyl diazomalonate instead of ethyl diazoacetate, only the Z-olefin resulting from a [2,3]-sigmatropic rearrangement of the corresponding sulfur ylide was obtained in 36 % yield 161). When the same procedure was applied to... 

In a clever application of the hetero-Cope rearrangement, Martin used a Pd-catalyzed coupling of W-arylhydroxamates 362 with vinyl acetate to set up the [3,3] sigmatropic rearrangement 363 to 364 and final cyclization 364 to 365 [432-435]. Applications of this novel indole ring... 

Keyword Carbocycles a Cascade Reactions a Cycloadditions a Combinatorial Chemistry a Domino Reactions a Enantioselective Transformations a Ene Reactions a Eieterocydes a Natural products a Preservation of Resources and Environment a Sigmatropic Rearrangements a Tandem Reactions a Transition Metal-Catalyzed Transformations... 

Uemura and co-workers (91) demonstrated that copper catalysts effectively transfer nitrenoid groups to sulfides generating chiral sulfimides. A complex obtained from CuOTf and 55d catalyzes nitrenoid transfer to prochiral sulfides to afford products such as 139 in moderate to poor enantioselectivities (<71% ee, Eq. 78). Nitrenoid transfer occurs selectively to the sulfur atom of allylic sulfides generating allylic sulfenamide (140) in moderate selectivity, after [2,3] sigmatropic rearrangement of the initial sulfimide 141, Eq. 79. 

The combination of a TiCl4-catalyzed formation of an iminium salt from 193, a subsequent [3,3]-sigmatropic rearrangement followed by a return of the cyanaide provided 194 (Scheme 1.87) [611]. 

An intriguing approach to nitrogen-substituted allenes such as 253 was reported by van Vranken and co-workers (Scheme 8.67) [148]. Iron(II)-catalyzed sulfimidation of propargyl sulfides 252 and subsequent [2,3]-sigmatropic rearrangement furnish 253 in moderate to good yields. 

Thermolysis of 164 bearing a pendent olefin at 150 °C also allows the aryl radical in 167 to be captured in a 5-exo fashion, leading to 165 as a 1 1 mixture of diastereo-mers (Scheme 20.33). The corresponding Lewis acid-catalyzed [3,3]-sigmatropic rearrangement promoted by AgBF4 was facile at room temperature, making it possible for 166 to be isolated. Thermolysis of 166 at 75 °C afforded 165 in 80% yield. 

Phenyl- and 2-(2-thienyl)-3,3-dimethyl-3//-pyrroles (58) were obtained by the reaction of the corresponding ketoximes 56 with acetylene catalyzed by MOH (M = Na, K) in DMSO. The reaction intermediate observed is the corresponding O-vinyl oxime 57 which undergoes [3,3] sigmatropic rearrangement and cyclization to products 58 (equation 24). The yield of the products obtained strongly depends on the structure of the ketoxime . 

One potential approach for the preparation of benzofuran derivatives involves the acid-catalyzed cyclization of O-aryl oximes 1193. 39 (equation 36). The transformation is analogous to well-known Fisher indole synthesis. The reaction proceeds through a [3,3]-sigmatropic rearrangement of enehydroxylamine 120 to compound 121 followed... 

It is known that 5-acyloxyoxazoles 132 rearrange to 4-acyl-5(4/l/)-oxazolones 133 in the presence of 4-(dimethylamino)pyridme or 4-(pyrrohdino)pyridine. Recently, an asymmetric variant of this nucleophUe-catalyzed rearrangement that employs a chiral derivative of 4-(pyrrolidino)pyridine has been described. This procedure allows the construction of quaternary stereocenters with high levels of enantioselectivity (Scheme 7.38). Representative examples of saturated 5(4//)-oxazolones prepared via sigmatropic rearrangements are shown in Table 7.16 (Fig. 7.18). 

The mixture of 5 and 6 can be converted to 9 by reduction, separation and then epimerization/reduction of one isomer. Alcohol 9 is then further subjected to similar procedure as for 1 to give tricyclic ether 12, through the same Cu(tfacac)2-catalyzed ylide formation/[2,3]-sigmatropic rearrangement of diazo compound 10 (Scheme 2). 

Uemura and co-workers developed an interesting and unique catalytic system, in which the Rh(ii) intermediate is generated by Rh2(OAc)4-catalyzed reaction of conjugated ene-yne-carbonyl compounds (Equation (16)). The Rh(ii)-carbene is trapped by allyl sulfide to give [2,3]-sigmatropic rearrangement product 121 in good yields." ... 

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