Thiourea oxidative cleavage

The first enantioselective organocatalytic oxo-Michael reaction to enones was reported by Falck [109]. Falck developed the intramolecular addition of boronic acids hemiesters to enones catalyzed by thiourea catalysts. Alkyl, aryl, and heteroaryl y-hydroxy enones react with phenylboronic acid to furnish the corresponding boronic hemiesters 118, which after oxidative cleavage of the resulting dioxaboro-lane renders the chiral 1,2 diols 119 in excellent yields and enantioselectivities (Scheme 33.34). 

Polystyrene-bound isothiuronium chloride, which is readily prepared from chloro-methyl polystyrene and thiourea, has been used as a starting material for the preparation of various substituted pyrimidines (Entries 9-11, Table 15.28). After oxidation to the corresponding sulfones, nucleophilic cleavage with amines proceeds smoothly to yield substituted 2-aminopyrimidine derivatives (see Section 3.8). 

Selective bromine-mediated addition of BOC-protected-guanidine 81 to dihydropyridine 56 occurs across the electron-rich 5,6-alkene to give, after acid deprotection, r-2-amino-l,3a,5,7a-dihydroimidazo[4,5-b]pyridine 82 (Scheme 23). Aminal bond cleavage under basic conditions affords substituted 2-aminoimidazole 83 <2004OL3933>. Replacement of guanidine 81 with urea or thiourea leads, similarly, to 2-aminooxazoles or 2-ami-nothiazoles, respectively however, the yields are considerably lower than that of 82 due to the sensitivity of the ureas to bromine oxidation <2005JOC8208>. 

The formation of 8-0x0- or 8-thio-purines requires one-carbon components at a higher oxidation level urea and thiourea are appropriate. The products of chloroformate-initiated tive-membered ring cleavage of purines (27.1.1.3) can be recyclised to produce 8-oxo-purines." ... 

Resin-bound benzonitriles 189 were reacted with trimethylsilyl azide in the presence of dibutyltin oxide followed by acidic cleavage to give 5-biphenyl-2-yl-l//-tetrazoles 190 <04OL1143>. Resin-bound thioureas 191 reacted with sodium azide in the presence of mercury(II) chloride followed by acidic cleavage to give 5-aminotetrazoles 192 <04TL7787>. 

The rates of the thermal cleavage of 7V-alkyl groups from 1,3-dialkyl-imidazolium halides (484 R = R = alkyl, R =H, R = H, Ph, or NO2) have been measured. Deprotonation of 1,2,3-trimethylimidazolium salts (484 R —R = Me R = H) at the 2-methyl group has been investigated in a model study of thiamine catalysis. Photolysis of the 4-diazo-imidazole (485) in the presence of ethanol leads to a mixture of the carbene-insertion products (486) and (487). Treatment of 2-diazo-4,5-dicyanoimidazole (488) with tetramethyl-thiourea gives the stable ylide (489). The photochemical rearrangement of imidazole oxides (490) to 2-imidazolinones (491) has been described. 

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