With selenourea

As in the case of the thiazoles, a variation on the Hantzsch s method has been used. This consists of using a nonhalogenated carbonyl derivative directly in the presence of iodine in the reaction with selenourea (Scheme 7) (20). However, in this case the reaction with selenourea is slower than with thiourea, and normally an excess of carbonyl compound is used. 

Reaction of a-chlorocyclohexanone (93) with selenourea has been reported to yield 2-amino-4,5,6,7-tetrahydrobenzoselenazole (99) (43RTC580, 5UA1864). 

A variation of the general method for the synthesis of 2-amino-selenazoles is to avoid the use of the free a-halogenocarbonyl compound and in its place react the corresponding ketone and iodine with selenourea.This procedure is also taken from thiazole chemistry. By contrast with thiourea, the reaction with selenourea needs a longer reaction time and the work up of the reaction mixture is somewhat more difficult. Usually an excess of the ketone is used. In the preparation of 2-amino-4-( n-nitrophenyl)selenazole, a very high yield, calculated on the amount of iodine used, was obtained. To explain this peculiar result, the oxidative action of the nitro group was invoked. This liberates free iodine from some of the hydrogen iodide eliminated in the condensation reaction, and the free iodine then re-enters into the reaction. 

The preparation of 2-amino-5-arylselenazoles 99 and 3,5-diaryl-1,2,4-selenadiazoles 101 has been described. Starting from readily available a-arylsulfonyl-a-bromoacetophenones 98 reaction with selenourea gave 2-amino-5-arylselenazoles 99 in good yield. Reaction of 98 with selenobenzamide 100 did not give the expected selenazole the 3,5-diaryl-1,2,4-selenadiazole 101 was obtained in moderate yield. Compound 101 is a known oxidation product of selenobenzamide and a mechanism for its formation is proposed <99JHC901>. 

General methods for preparation of imidazolidin-4-one-2-thiones include the reaction of thioureas with a-dicarbonyl compounds,37,38 with ethyl phenylpropiolate,39 and with phenyl chlorothiolacetate,40 and cyclization in acid of l-(carbethoxymethyl)-2-thioureas.41 The analogous reaction of a-diearbonyl compounds with selenoureas produce imidazolidin-4-one-2-selones.42 Berlin and Levi43 utilized the acid-catalyzed cyclization of l-aryl-3-carbalkoxymethyl-2-thioureas to produce 3-arylimidazolidin-4-one-2-thiones (23). 

Dunn (156) showed that colloidal selenium likewise is a sensitizer and produces similar effects to those obtained with selenoureas. Spencer, Atwell, and Levy (94) confirmed this and showed that silver selenide can be formed by reaction of silver bromide with selenium. They further demonstrated the similarity between sulfur and selenium sensitization. In the absence of evidence to the contrary, the presumption is that the mechanism or mechanisms of sensitization by the two agents is the same. 

N,N -diarylglyoxaldihydrazonoyl dihalides (91), versatile intermediates for the synthesis of heterocyclic systems, reacted with KSeCN in ethanol at reflux to yield 2,2 -bi(4-aryl-4,5-dihydro-5-imino-l,3,4-selenadiazoles) (92) directly in good yield. Similarly, the reaction of the dihalides with selenourea and acylselenourea gave the biselenadiazoles in one step by elimination of ammonia or aniline. [94PS129]... 

Method B was evaluated with the diglucosyl diselenide 216, prepared in two steps by reaction of the 6-deoxy-6-iodo derivative 214 with selenourea and transformation of the pseudourea resulting 215 and 217. The selenoate was generated by reduction of 216 and allowed to react with 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide in DMF, affording protected disaccharide 217. The free (1 - 6)-/Minked 6-selenodisaccharide 218 was obtained in 70% yield after deacetylation. This methodology could find... 

Caution. Selenourea is toxic and should be handled with care. Experiments with selenourea should be conducted in an efficient fume hood or in a glove box vented to a fume-extraction system. The explosive hazard of perchlorate salts of metal complexes should also be noted. 

Complexes of thiourea with mercury(II) halides have been known for almost a century, and compounds containing one, two, three, or four molecules of coordinated thiourea can be isolated from aqueous solution. With selenourea in acetone, it is possible to prepare 1 2 complexes Hg[(NH2)2CSe]2X2 (X = C, Br, I) and a dimeric 1 1 complex [Hg (NH2)2CSe Cl2]2 in excellent yields. 

Treating the quinoline 230 with selenourea gave l,3-thiaselenolo[5,4-. Similarly, 232 was converted into compound 233 (Equation 50) <1996PJC54>. 

The modified Hantzsch synthesis first described by Hofmann <1889LA(250)294> has remained the general method for preparing 1,3-selenazoles. Depending on the substitution required in the selenazole the requisite selenocarboxamide and a-halocarbonyl derivatives are coupled (Scheme 7) <79HC(34)217>. Since a chlorooxirane is isomeric with an a-chlorocarbonyl, 2-aminoselenazoles are also formed with selenourea (Equation (6)) <81T2607>. The synthesis of a 5-quinonyl-l,3-selenazole... 

The reaction of polymer-bound 5-(2-bromoacetyl)pyrroles with selenourea in DMF led to selenazolylpyrroles (Fig. 6.16c, d) with HPLC-purities ranging from 0% to 85%. No product was obtained when 3-amino-l-propanol was used as the amino component purities in all other cases were > 70 %. 

Most selenium-substituted benzo[6]thiophenes are obtained by metalla-tion reactions (Section V). 2-Bromobenzo[6]thiophene-3-carboxaIdehyde (and the 3-Br, 2-CHO isomer) reacts with selenourea, to give the correspond-... 

---