Selenides cyclic

More recently, cyclizations have been performed via cyclic selenides (265,324-327) and by ring opening of epoxides (328,329). Similar cyclizations have been achieved using alkenes complexed to Hg(II) (265) or Pd(II) salts (267,330) which. 

The introduction of selenium in the (4 +1) sense was achieved by the use of aqueous sodium hydrogen selenide in several instances where the appropriate di(halomethyl) precursors were readily available. Thus, the dichlorides (254c) and (287) and the dibromides (288) and (289) were converted to the five-membered cyclic selenides (290), (291), (292) and (293) by reaction with aqueous sodium hydrogen selenide high dilution conditions were used to minimize the formation of the dimeric diselenides (294)-(297) (Scheme 99) (77H(6)1349, 77CC163). 

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

The 13-membered cyclic selenide 65 and 15-membered 66 were obtained by treatment of a CH2Cl2/MeOH solution of the corresponding dipropargyl dibromide 179 with an aqueous solution of sodium hydroselenide under high-dilution conditions (Scheme 23) <2002EJ03198>. 

The oxidation of linalyl acetate with selenium dioxide in methanol affords, in addition to the usual allylic oxidation products, two cyclic selenides 1 and 284. 

The reaction of aryl selenoketones [72], selenothioacetic acid S-esters [73], and iV -alkylidene selenoamides [74] with TONE and acetylenes having electron-withdrawing groups have been carried out to afford cyclic selenides 40-42 and acyclic selenides 43. 

Oxidation of the cyclic selenides 61 and 119 by sulfuric acid or nitronium hexafluorophosphate has provided selenurane salts 2 and 62 (Equation 26) <1996T10375>. 

S. Uemura, and M. Okano,/. Chem, Soc., Perkin Trans. 1. 1978, 1206 [diene + KSeCN and CuClj saturated cyclic selenides rather than selenocyanatesl S. Rajappa, T. G. Rajagopalan,... 

The iodo-formates obtained as described above are useful intermediates, which can be further transformed into cyclic amines, cyclic sulfides, cyclic tellurides, and cyclic selenides by the sequences outlined in Scheme 49. A variety of heterosteroids have been synthesized from the corresponding steroidal... 

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