Rearrangement carbon oxidation states

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

There are many [2,3] -sigmatropic rearrangements involving a variety of heteroatoms as well as carbon. We shall describe just one more because it involves no ions at all. The key is an element that is prepared to change its oxidation state by two so that we can start and finish an arrow on that element. The element is sulfur, which can form stable compounds at three oxidation states S(II), S(IV), or S(VI). 

This ester is one carbon atom short of the full side chain of grandisol, so an Arndt-Eistert reaction was used to lengthen the chain by one atom. First, the ester was converted into the diazoketone with diazomethane and, then, the Wolff rearrangement was initiated by formation of the carbene with a Amdt-Eistert chain extension of ester silver compound at the Ag(II) oxidation state. 

A variety of interesting chemical rearrangements occur in the catabolic pathways of amino acids. It is useful to begin our study of these pathways by noting the classes of reactions that recur and introducing their enzyme cofactors. We have already considered one important class transamination reactions requiring pyridoxal phosphate. Another common type of reaction in amino acid catabolism is one-carbon transfers, which usually involve one of three cofactors biotin, tetrahydrofolate, or A-adenosylmethionine (Fig. 18-16). These cofactors transfer one-carbon groups in different oxidation states biotin transfers carbon in its most oxidized state, CO2... 

In several biological reactions the oxidation state is not changed but rearrangements occur. Vitamin B, an alkyl-cobalt(III) complex of a substituted corrin, is a cofactor of enzymes catalyzing 1,2-carbon rearrangements. Another important reaction is the conversion of citrate to isocitrate in the citric acid cycle catalyzed by the enzyme acotinase containing a FeS-cluster. 

While such a process had initially been observed as an undesired side-reaction in transformations where copper salts were employed as re-oxidants [13], Chemler demonstrated that various aminohalogenation reactions proceed in THF or acetonitrile in the presence of potassium carbonate as base [14]. These reactions employ palladium trifluoroacetate or palladium dibromide as catalyst source and require a moderate excess of the copper oxidant (3-4 equiv) giving moderate to excellent yields. However, they usually suffer from rather low selectivity, either in the initial aminopalladation or via subsequent rearrangement pathways to provide mixtures of pyrrolidines and piperazines (Scheme 4.2, Eq. (4.3)). A stoichiometric control reaction in the presence of palladium bromide led only to the Wacker cydization together with an alkene isomerization product, suggesting that the presence of copper(II) salts is crucial for the overall process. The exact role of the copper(II) salts has not yet been darified and palladium intermediates of different oxidation states may be involved in the final stage of carbon-halogen bond formation. 

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