Selenium stereochemistry

Part B of Scheme 4.5 gives some examples of cyclizations induced by selenium electrophiles. Entries 9 to 13 are various selenyletherifications. All exhibit anti stereochemistry. Entries 14 and 15 are selenyllactonizations. Entries 17 and 18 involve amido groups as the internal nucleophile. Entry 17 is an 5-exo cyclization in which the amido oxygen is the more reactive nucleophilic site, leading to an iminolactone. Geometric factors favor N-cyclization in the latter case. 

In this chapter, recent development on the synthesis and stereochemistry of optically active chalcogen compounds over the last 10 years will be described, focusing mainly on chiral selenium and tellurium compounds. 

Optically active selenoxides are known to be unstable toward racemization. An optically active selenoxide having a steroidal frame was obtained for the first time by Jones and co-workers in 1970.7 Enantiomeric selenoxides were prepared by Davis et al. in 1983,8 and an enantiomerically pure selenoxide was isolated for the first time by us in 1989.9 Many optically active selenoxides, which are kinetically stabilized by bulky substituents, were synthesized over the last two decades, and their stereochemistry and stability toward racemization were studied.3,5,10 Recently, some optically active selenoxides, which were thermodynamically stabilized by the intramolecular coordination of a Lewis base to the selenium atom, have been isolated. Optically active selenoxides 1 and 2 were obtained by optical resolution on chiral columns, and their stereochemistry and stability toward racemization under various conditions were clarified (Scheme 1).11,12... 

Michalski, J. The chemistry and stereochemistry of organic selenium-phosphorus acids and phosphine selenides. Ann. N.Y. Acad. Sci. 192, 90 (1972). 

Sulfur, Selenium, Tellurium and Polonium Table 1 Compounds of Group VI Elements and their Stereochemistries... 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Azocanes with nitrogen at a bridgehead such as fused azocane 289 were prepared starting from /V-protected amino aldehydes 286. Those amino aldehydes were converted into allylic alcohols by the classical Morita-Baylis-Hillman reaction or by condensation with selenium-stabilized carbanions, followed by oxidation <2007JOC5608>. Fused azocane 289 was prepared in good yield as described in Scheme 120. Formation of [ z, ,0]-bicyclic structures via these reactions is general and the stereochemistry of the starting amino-aldehyde is preserved. 

Mono- and 1,2-di-substituted alkenes react with PhSeQ/Hg(SCN)2 in benzene (0.5-96 h, at 20 C), giving -tra/if-phenylselenoalkyl isothiocyanates in 70-94% yields.2 Terminal alkenes generally give the product with the selenium terminal (an exception is the prc uct from Bu CH H2) internal alkenes show the expected stereochemistry (cis to threo, trans to erythro). Oxidation to selenoxides could be achieved cleanly only with ozone, and the products cis eliminate in the usual manner to give predominantly the vinylic isothiocyanates (Scheme 73). 

Two characteristic structural and bonding features of the chlorine and bromine compounds of selenium(IV), in which the role of the inert pair determines much of stereochemistry and reactive properties of the entire class of compounds, are already evident in the simple binary halides. 

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