Selenyl groups

In a fascinating development of the methodology described in Section 10.12.4.3.1, Hanessian and co-workers122 have shown that it is also possible to take a tri-selenylated peptide and to substitute the three phenylselenyl groups with three allyl groups in a one-pot procedure (Scheme 13). In each case the selenyl groups are replaced with almost total retention of configuration. 

A valuable preparative method for the generation of cyclobutenyl anion has been developed by reductive removal of the selenyl group with lithium naphthalenide.24... 

Many reports of selenium relate to its use as a source of double bonds. Continuing this theme, phenols are formed from cyclohexenones by phenyl-selenation (PhSeCl) and oxidative removal of the selenyl group (MCPBA). In their turn cyclohexenones have been prepared by a similar sequence. The interesting point here is that the double bond can be inserted at the less substituted conjugated position by base isomerization of the phenylselenocyclo-hexanone. For example, the phenylseleno-group in the cyclohexanone (283)... 

Selenyl groups can be abstracted from acyl selenides to generate radicals on reaction with stannyl radicals. The resulting acyl radicals can be reduced to aldehydes or undergo decarbonylation prior to reduction. 

The modification of [T-amino acids by a-selenylation and then free radical allylation leads to the formation of (A-amino acids bearing an a-allyl group (Scheme 12). Hanessian and co-workers 22 initially used this approach to produce p2 3-amino acids which were then subjected to suitable coupling procedures. Interestingly, the coupling method chosen made use of PyBOP and hence demonstrates that other peptide coupling procedures can be used in the synthesis of (3-peptides. 

The reaction of primary amines with arenesulfenyl halides leads to the formation of sulfonamides (12). ° These compounds are most stable when the aryl group has electron-withdrawing substituents at Ae 2- and 4-positions. Selenamides are formed in an analogous manner but are somewhat less stable aliphatic amines give isolable compounds, but most anilines react with selenyl halides to give products of ring substitution. An example of an isolable selenamide is compound (13), which was prepared (79%) from the amine and 2-nitrobenzeneselenyl chloride. This compound was used as an intermediate in the preparation of 7a-methoxycephalosporins. 

In connection with the synthesis of cytochalasans, electrophilic selenylation of cycloalkenyl silanes provided exclusively the selenides with the endocyclic double bond. Treatment of the selenides with oxidizing agents (periodate, 3-chloroperbenzoic acid) resulted in the formation of allylic alcohols with the CO bonding at the less hindered /l-alkene face33 - 35. The methylthio group in the molecule is not oxidized under the conditions used for the oxidation of the selenide. 

Particularly interesting results have been observed from -hydroxy-a-trimethylsilylalkyl selenides since vinylsilanes are exclusively produced under conditions where both the selenyl and the silyl moieties are removed with the hydroxy moiety. Apparently the selenyl moiety seems to stabilize a p-car-benium ion to a better extent than a silyl group. 

The presence of two potential leaving groups p to each other makes p-hydroxyalkyl selenides valuable candidates for epoxide synthesis, as well as for pinacol-type rearrangement These concurrent transformations, which usually require the selective activation of the selenyl moiety to a better leaving... 

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