Thiouracils hydrogenation

Among the dihydro analogues of thiouracil, 2-mercapto-6-methyl-5,6-dihydro-uracil (LXXX) retains some of the antithyroid activity exhibited by 2-thiouracils [642] (see Part 1 of this review. Volume 6). This compound can be obtained either by the condensation of thiourea with ethyl crotonate or by the hydrogenation of 2-mercapto-6-methyluracil. 

Botta and coworkers recently developed an Oxone cleavage methodology for the solid-phase synthesis of substituted uracils (equation 57)165. Whereas some common methods of thioether cleavage, such as reduction with Na/NH3, acid catalyzed hydrolysis, heavy metal ions followed by treatment with hydrogen sulfide and iodotrimethylsilane proved to be unsuccessful for this reduction, Oxone was shown to be an efficient and selective reagent for cleavage of polymer bound thiouracils. 

Hemideprotonation of a metalated nucleobase and the resulting selfcomplementarity is also the key in rationalizing the triple hydrogen-bonding pattern seen in an associate of a neutral and an anionic purine-6-thione Ru(III) complex (130), which is analogous to that seen with 2-thiouracil [(84), see Section II. D]. 

A/-AIkyIpyrimidinethiones cannot be converted into the corresponding pyrimidinones under regular hydrolytic conditions but they can be so converted oxidatively, although examples are few. Thus, l,3,6-triphenyl-4-thiouracil (865 X = S) is converted by hydrogen peroxide in acetic acid into the corresponding uracil (865 X = 0) (64G606) and the pyrimidinethione (866 X = S) into the pyrimidinone (866 X = 0) by mercuric acetate... 

The present work discusses some properties of thiouracil nucleobases calculated at the B3LYP/6-31+G(d,p) computational level, namely, their structures, most stable tautomers, proton affinities and deprotonation enthalpies, hydrogenation, interaction with water, and base pairing, in order to shed a certain light on the problem of why their functioning in RNA and modified DNA base pairs is so unusual. 

Table 7. Geometrical parameters and hydrogen bond lengths of the adenine-uracil and adenine-thiouracil Watson-Crick base pairs. Distances in A, angles in degrees. In the distance notations, the former base belongs to adenine while the latter one to uracil or thiouracils.

Reduction of [IrCls] by a variety of agents has been cited. The reduction of [IrXs] " (Cl, Br) by 2-thiouracil and 2-thiopyrimidine follows second-order rate laws, first-order with respect to both the concentration of the oxidant and the reductant. A pH-rate profile as well as activation parameters have been presented, and a free radical mechanism proposed." Reduction in aqueous alcoholic solution by aquated electrons, hydrogen and alkyl radicals was investigated by pulse radiolysis, and found to result in the production of [IrClg] " A kinetic examination of reaction (123) in aqueous solution shows that this outer-sphere electron transfer reaction proceeds by parallel paths first and second order in substrate (SCN ). The kinetics of the oxidation of hydrazine by [Ir(Cl)4(H20)2], [Ir(Cl)5(H20)], [IrClef , as well as [IrBrsf , in aqueous acidic perchlorate solution have been investigated." ... 

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