Carbon-selenium rings

Treatment of sodium selenide with cw-dichloroethene in the presence of the phase-transfer agent, 15-crown-5, gave 1,4-diselenin along with a family of cw-unsaturated selenacrown ethers, e.g., (Z,Z,Z,Z,Z)-l,4,7,10,13-pentaselenacyclopentadeca-2,5,8,ll,14-pentene (simply 15-US-5) as well as 18-US-6,21-US-7, 24-US-8, and 27-US-9 05JOC5036 . 

Three new azamacrocyclic-cyclophane, hybrid receptors have been created based on the incorporation of either 1,4,7,10-tetraazacyclododecane or 1,4,7-triazacyclononane units tethered by a short amidic spacer to an electron donor and a //-bonding crown ether. These direceptors are designed to act as hosts for biologically relevant guests 05JOC115 . 

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Allenes have also been used as substrates in free radical cyclizations. Dener and Hart demonstrated that such entries are valuable in constructing pyrrolizidine and indolizidine ring systems [71]. In a total synthesis of pyrrolizidine base (+)-heliotri-dine (340), compound 338 possessing an allene functionality was used as a key intermediate (Scheme 19.62). Tri-n-butyltin radical-mediated carbon-selenium bond homolysis of 338 followed by the addition of the free radical to the allene moiety... 

It has been shown recently that at 20 °C the unstable iodosulfanes (n = 1,2,...) decompose to give homocyclic sulfur molecules and iodine In a similar fashion SCjIj prepared from Se Clj and solid KI in carbon disulfide solution produces Seg and polymeric selenium Mixed sulfur-selenium ring molecules can be prepared by the reaction of mixtures of Se2Cl2 and chlorosulfanes (S Cl2 n = 1, 2, 4, 6-8) with potassium iodide in carbon disulfide solution. The subsequent decomposition of the unstable iodides results in the formation of mixed SeoS8 molecules. The general reaction scheme can be presented as follows ... 

The mass spectra of a number of heterocyclic compounds have been reported. The fragmentation patterns of thiophosphoryl derivatives of phosphorinanes are sensitive to stereochemistry, thus for the series (95) facile loss of the HS radical is indicative of an axial PS bond. The seven-membered heterocycles (96 Ch = S, Se) undergo a remarkable migration of sulphur or selenium from phosphorus to carbon with ring cleavage. While exocyclic P-C bonds of five-and six-membered heterocycles in the phosphonic class may be readily cleaved with retention of the phosphorus ring system, the seven-membered ring (97) exhibits facile expulsion of a phosphorus radical. ... 

It is interesting to note that the latter result is exceptional since l,l-bis(phenyl-seleno)cyclopropane is the only selenoacetal derived from ketones to be at least partially cleaved under these conditions and even the homologous cyclobutyl derivative is inert under these conditions. This may be due to the extra stabilization introduced by the cyclopropyl ring. The case of 2-decy 1-1,1-bis(methylseleno)cyclo-propane merits further comment. It is difficult to assess whether the cleavage of the carbon-selenium bond occurs on the methylseleno moiety cis or tram to the alkyl group, since this organometallic leads to a mixture of the two possible stereoisomers on further reaction with electrophiles (Scheme 16). 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

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