Selenium-mediated ring

C-labeled carbamoyl compounds can be prepared by using a selenium-mediated reaction with [ carbon monoxide. The urea formation works excellent in the case of cyclizations but in other cases satisfactorily only with primary amines (Kihlberg et al. 2002). Since selenium is practically insoluble in most solvents the use of primary alkyl amines or tetrabutylammonium fluoride is necessary for the formation of soluble and reactive complexes with selenium. Except for ring closures, rhodium-promoted carbonylations are probably more useful than the corresponding selenium reactions (Ohad et al. 2009). 

A highly efficient synthetic route to neurotoxic spirocyclic alkaloids such as perhydrohistrionicotoxin has been described in which the key steps are a selenium-mediated [2,3]-sigmatropic rearrangement and a ruthenium-catalysed ring-closing metathesis. 

Scheme 23 Selenium-mediated alkene cyclization to construct the B ring of leucascandrolide A [22]...

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... 

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. 

---