Selenophenols reduction

The third method makes use of the one-flask procedure, which is advantageous from the preparative point of view. However, opening of certain stereoisomeric epoxides (263) with selenophenol suffers from low regioselectivity, resulting in a low yield of the final product. The other disadvantage is the basic reaction-medium occasioned by the method used for the generation of selenophenol, namely reduction of diphenyl diselenide with sodium borohydride in solution in anhydrous alcohol (see Ref. 356) some epoxides are sensitive to basic media. However, David (see Ref. 356) did not observe side reactions in his syntheses of 256. 

Reductive alkylations of amines with carbonyl compounds can be also accomplished quantitatively by means of selenophenol (PhSeH) at room temperature in chloroform198 and by sodium hydrogen telluride (NaHTe) in ethanol at room temperature199. 

The synthesis of 3,4-diaminothiophene has been standardized and optimum conditions established for its condensation with a-diones to produce thieno[3,4- ]pyrazines <2002JOC9073>. Selective monocarbamoylation of 3,4-diaminothiophene has given access to several 2-alkyl-3,4-diaminothiophenes <1997T10331>. Acid-catalyzed reductive alkylation of the monocarbamate by treatment with an aldehyde and selenophenol in the presence of -toluenesulfonic acid gave the 2-alkylthiophene the carbamate could then be cleaved by means of HBr to give the diamine (Scheme 51). 

Michael reaction of selenophenols. Selenophenols (and other selenides) undergo enantioselective 1,4-addition to cyclohexenone in the presence of catalytic amounts of cinchona alkaloids. Chemical yields are high optical yields are 10-43%. Usually the optical yield can be enhanced by crystallization. In one case the addition product was converted into an optically active allylic alcohol by hydride reduction followed by selenoxide fragmentation. ... 

Epoxide cleavage, Sodium selenophenolate reacts with the furanose 1, prepared from D-glucose, to form the diselenide 3 in quantitative yield. This reaction involves formation and cleavage of the epoxide 2, which can be also prepared by treatment of 1 with NaH. The epoxide is cleaved regiospecifically by attack from the less hindered side at C4. The phenylseleno group can be removed reductively by Raney nickel. The product was used to prepare isoepiallomuscarine (5). 

If selenophenol is used in place of thiophenol, ethyl (7 )-2-(phenylseleno)propionate (177) is formed also in high yield. Partial reduction of the ester to aldehyde affords (/ )-2-(phe-nylseleno)propanal (178) with 90-98% ee. This intermediate has been used for the synthesis of a variety of Cbz-protected D-amino acids [66] (Scheme 25). 

Elaboration of the thiophene ring can be achieved by standard electrophilic substitution reactions, which are normally favored at the a-carbons for thiophene and at the 3-position for benzo[fc]thiophene. Reductive alkylation of aminocarbamate 72 with butyraldehyde in the presence of selenophenol gave thiophene 73 . Treatment of 3-methylthiophene with chlorosulfonic acid followed by tcrt-butylamine led to selective sulfonamidation producing 74 . Similarly, selective 2-iodination of 3-alkylthiophenes was ob.served upon treatment with iodine in the presence of mercuric oxide . Exhaustive bromination of thieno[3,2-7i]thiophene 27 gave 2,3,5,6-tetrabromothieno[3,2-ii]thiophene which was... 

Generation of a phenyl selenoglycoside from 2,3,4,6-tetra-O-acetyl-a-D-galacto-pyranosyl bromide via displacement of the anomeric halide with a selenophenol anion generated by reduction of diphenyldiselenide [44]. 

The reduction of Schiff s bases is a well used and convenient method for the preparation of secondary amines. Selenophenol has now been found to be an excellent and advantageous reagent to effect this, and the reductive alkylation of amines with carbonyl compounds. The preparation of tertiary amines bearing three different substituents can also be achieved by the successive addition of carbonyl compounds to a primary amine in a one-pot reaction (Scheme 15). The... 

---