Replacement of sulfur by oxygen

Desulfuration. Replacement of sulfur by oxygen is known to occur in a number of cases, and the oxygenation of the insecticide parathion to give the more toxic paraoxon is a good example of this (Fig. 4.25). This reaction is also important for other phosphorothionate insecticides. 

A replacement of sulfur by oxygen takes place on treatment of the l,3-dithiole-2-thione... 

Trialky loxonium fluoroborate Replacement of sulfur by oxygen Carboxylic acid amides from thioamides... 

Without additional reagents Replacement of sulfur by amino groups and oxygen... 

Replacement of a hydroxy oxygen atom of an aldose or ketose, or of the oxygen atom of the carbonyl group of the acyclic form of an aldose or ketose, by sulfur is indicated by placing the prefix thio , preceded by the appropriate locant, before the systematic or trivial name of the aldose or ketose. 

Replacement of the ring oxygen atom of the cyclic form of an aldose or ketose by sulfur is indicated in the same way, the number of the non-anomeric adjacent carbon atom of the ring being used as locant. 

As with five-membered heterocycles, phosphorus pentasulfide may replace a ring oxygen atom by sulfur, or it may effect replacement in an acyclic precursor and then bring about cyclization. Thus isochroman-l-one (99) is converted into thioisochroman-l-thione (100) under the same conditions a 3-arylisocoumarin( 101) produces a 3-ary 1-1-thioisocoumarin (102) only, with no replacement of the ring oxygen atom.122 A typical example of thiation, followed by cyclization, is the formation of 2-alkyl- or 2-aryl-3,l-benzo-thiazine-4-thiones (103) from /V-acylanthranilic acid esters (104) by treatment with phosphorus pentasulfide in boiling xylene.122... 

With respect to OZTs anchored on carbohydrate scaffolds, the direct replacement of the sulfur by oxygen showed to be tricky (Scheme 51). 

Rj and Rj can be varied. Replacement of the lactam oxygen by sulfur decreases activity. 

However, in contrast to the cyclic flow of electrons in purple bacteria, some electrons flow from the reaction center to an iron-sulfur protein, ferredoxin, which then passes electrons via ferredoxin NAD reductase to NAD+, producing NADH. The electrons taken from the reaction center to reduce NAD+ are replaced by the oxidation of H2S to elemental S, then to SOf, in the reaction that defines the green sulfur bacteria. This oxidation of H2S by bacteria is chemically analogous to the oxidation of H20 by oxygenic plants. 

Important reactions of this type include the replacement of a ring oxygen by a ring sulfur or nitrogen. Table 8 gives some examples and references to the section where they are discussed. 

Replacement of the carbonyl oxygen atom by sulfur may be effected by heating with phosphorus pentasulfide (49JCS2142), boron sulfide or silicon disulfide which gives high yields under mild conditions, as, for example, in the synthesis of 7-methoxy-2-methylchromene-4-thione (438) (69JCS(C)2192). Such compounds are more easily converted into their oximes or hydrazones than the oxygen compounds. 

Dioxane absorbs at 180 nm and is thus transparent in that part of the UV region accessible to basic spectrometer systems. Replacement of the ring oxygen atoms by sulfur... 

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