Thiohydroxamate systems

Free radicals can also be generated by the photolysis of thiohydroxamate systems derived from thiazolones and thiopyridones. Photolysis of the thiazolone derivative (236) yields the pyrrolo[l,2-ajindole (238), together with a trace of the related indoline and some dimeric product (239). Reaction of the pyridone thiohydroxamate (237) gives a dimer as the major product (Scheme 72) <93JOC2768>. 

Structure and Reactivity PTOC Esters Related Thiohydroxamate Systems Preparation of Barton Esters 67-1... 

For a general review on the chemistry of thiohydroxamate containing compounds, see Chimiak, A., Przychodzen, W., and Rachon, J., The thiohydroxamate system, Heteroatom Chem., 13,169, 2002. 

The following discussion of hydroxamic acids includes saturated systems, e.g., 2, compounds such as 3, derived from aromatic systems, 7V-hydroxyimides such as 7V-hydroxyglutarimide (78), and certain of their derivatives including thiohydroxamic acids. Naturally occurring cyclic hydroxamic acids are discussed to show the range of structural types that has been found, hut macrocyclic polyhydroxamic acids are mentioned very briefly, because several comprehensive reviews of these compounds are already available. The main purpose of this review is to summarize the methods available for the synthesis of cyclic hydroxamic acids, to outline their characteristic reactions, and to present some useful physical data. Their synthesis and some biological properties have previously been reviewed by Coutts. ... 

Kinetic and equilibrium data are available for complex formation between iron(III) and 4-Me0C6H4C(S)N(0 )H, a system studied in relation to the possibility that some natural siderophores may bind iron through a thiohydroxamate moiety. The Fe " " complex of this... 

TABLE 3. Periodic system showing the number of reagents from the group of hydroxamic acids with which a certain element can be determined. Superscripts denote the number of reagents considered as hydroxamic acids and subscripts denote the number of elements considered as thiohydroxamic acids... 

The reactions based on A-hydroxy-2-thiopyridone derivatives are clearly radical chain reactions. In a recent paper,91 we have reported quantum yield measurements for a number of reactions based on iV-hydroxy-2-thiopyridone. Most of the reactions had quantum yields of 10-30. Synthesis of the N-hydroxyquinazolin-4-thione 139 (Ar = Ph, An, 1-Naph) by an improved route gave thiohydroxamic acids which were more sensitive to light than iV-hydroxy-2-thiopyridone. The quantum yield for bromination was in the range 30-60. More important, while the JV-hydroxy-2-thiopyridone system at — 30 °C, makes21 radicals only in a non-chain fashion, the derivatives of 139 continue radical chain reactions even at — 60 °C. 

O-Acyl derivatives of thiohydroxamic esters (Barton esters) react with benzynes to afford a tricyclic thiophene ring system thus the pyridine derivative 30 gives benzo[4,5]thieno[2,3-A pyridines 31 (Scheme 14) <2002JOC3409, CHEC-III(3.11.3.3)900>. 

There have been further reports on pyridine C-nucleosides. 4-Carbam-oyl-2-p-D-ribofuranosylpyridine 54 and 3-carbamoyl-5-P-D-xylofuranosyl-pyridinel55 have been prepared by Alderweireldt s approach (see Vol. 21, p. 211), and some 2 -deoxy-r,2 -didehydro-pyridine C-nucleosides have been prepared by base-catalyzed elimination. Addition of lithiopyridines to 2,3-C-isopropylidene-5-C-TBDMS-D-ribonolactone gives adducts such as (133), which can be reduced to p-pyridyl C-nucleosides with EtaSiH.l T Coupling of the anomeric free radical, produced by photolysis of an iV-acyl-thiohydroxamate, with pyridinium and quinolinium salts gave 2-pyridyl C-nucleosides and 2-quinolinyl systems such as (135), where P-selectivity was... 

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