Selenium methodology

As can be observed in Scheme 70, these transformations have parallels with well-established selenium processes.195 Contrary to the cyclofunctionalization using selenium reagents, the tellurium version does not tolerate the presence of amines to capture the HC1 formed during the process, since amines react with organotellurium trichlorides.5,196 In this way, although several cyclizations can be successfully performed using tellurium electrophiles, the selenium methodology continues to be the method of choice for this purpose in view of the milder cyclofunctionalization conditions and the easier removal of the heteroatom at the end of the process. 

Selenium has now been added to the growing list of transition metals shown capable of catalyzing the reductive A/-heterocyclization of 2-nitrostyrenes to indoles with carbon monoxide <99TL5717>. Utilizing the palladium-phosphine catalyzed methodology, Soderberg achieved the synthesis of several 4-substituted 2-methylindole natural products isolated from TricMoma... 

In this section, ring synthesis of 1,2,5-selenadiazoles and their fused systems such as 2,1,3-benzoselenadiazoles are described. Most synthetic methods involve the reaction of 1,2-diamines with selenium dioxide or selenium oxychloride. The methodology has been described <2004HOU777, 1996CHEC-II(4)743, 1984CHEC(6)513>. A related investigation toward 2,1,3-benzotelluradiazoles is also described here. 

J. Ruiz Encinar, M. Sliwka-Kaszynska, A. Polatajko, V. Vacchina, J. Szpunar, Methodological advances for selenium speciation analysis in yeast, Anal. Chim. Acta, 500 (2003), 171D183. 

Krief and Barbeaux have also applied the selenium-lithium exchange methodology to some cascade carbolithiation reactions as shown in Scheme 3667. The l,l-(bis-methylseleno) derivative 128 undergoes a selenium-lithium exchange at —78 °C giving... 

The synthesis and application of selenium intermediates and seleno sugars, and the subsequent study of their transformation into various classes of useful precursors and target derivatives, are witnessing rapid new development. This new field of synthetic carbohydrate chemistry offers new methodologies for general organic and natural-product syntheses, and new... 

The reaction between benzyne derivatives and selenium analogues of Barton s thiopyridone esters provided a convenient entry into complex, fused benzo[3]selenophenes <2004JHC13, 2004ARK51>. For example, the generation of the benzyne 86 in the presence of selenoester 85 provided benzo[. ]seleno[2,3-. ]pyridine 87, presumably via a single electron transfer (SET) pathway (Equation 11). This methodology was examined utilizing a number of benzyne precursors (anthranilic acids, iodium triflates, and trimethysilyl triflates) and provided access to an impressive number of fused benzo[. ]selenophenes. 

Sulfur ylides are most frequently employed in this methodology. The cyclopropanations of Michael acceptors with arsenic, selenium and tellurium ylides " have also been... 

Nicolaou [47] has applied this methodology in synthetic studies towards the antibiotic everinomicin 13,384-1. Glycosylation of immobilized fluoride donors such as 258 (Scheme 65) with sugar acceptors allowed the synthesis of disaccharides such as 259, attached to solid support through a selenium fink-... 

The same methodology was employed by Tiecco to effect the cyclizations of 0-allyl oximes 164 indicated in Scheme 25. Treatment of the intermediate iminium salt 165 with sodium borohydride affords M-alkyl isoxazolidines 166 [95]. Alternatively, 165 can be treated with water to produce M-unsubstituted isoxazolidines 167 [96]. This synthesis of M-alkyl isoxazolidines represents a valid alternative to that described in Scheme 23. Moreover, the present cyclization reaction is much more stereoselective than the cyclization of 0-allyl hydro-xylamines. In fact, when the methyl derivative 168 was employed, the two isomeric isoxazolidines 153 and 154 were obtained in a ratio of 95 5. The same two products were, in contrast, formed in a 1 1 ratio when the reaction was carried out starting from the corresponding 0-allyl hydroxylamines. Clearly, the steric requirements for the selenium-induced cyclization of the 0-allyl oximes are much greater than those for the corresponding 0-allyl hydroxylamines. 

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