Cleavage semicarbazones

Mattox and Kendall have suggested that loss of bromide from the initially formed hydrazone of the a-halo ketone is facilitated by participation of the electron pair on the amine nitrogen atom. The same group has shown that semicarbazones are often superior to 2,4-dinitro-phenylhydra-zones since they can be obtained in higher yields and undergo cleavage more... 

Efficient cleavage of A.A-dimethylhydrazones to yield the parent ketones (>90%) is effected under neutral conditions with tetra-n-butylammonium persulphate [40], The procedure is particularly useful for compounds with acid-sensitive substituents, e.g. vinyl or ketal groups. Similarly, reaction times for the oxidative cleavage of semicarbazones with sodium nitrite or nitrate/trimethylsilyl chloride are reduced by the addition of benzyltriethylammonium chloride [41]. 

Oxidation of p-toluenesulfonylhydrazones.1 Carbonyl compounds can be regenerated in good yield from the tosylhydrazones by treatment with 1 equivalent of Tl(OAc), in HOAc. Room temperature suffices in the case of aldehydes, whereas reflux temperatures are used for ketones. The other product is T1(0S02C6H4CH3). This method is less useful for oxidative cleavage of semicarbazones long reflux periods are necessary, and acetoxylation is a side reaction. 

Different solid-phase techniques for the synthesis of C-terminal peptide aldehydes have gained much attention and allowed greater accessibility to such compounds. Solid-phase techniques have been used to synthesize peptide aldehydes from semicarbazones, Weinreb amides, phenyl esters, acetals, and a, 3-unsaturated y-amino acids)47-50,60 63 The examples presented below use unique linkers to enhance the automated efficiency of C-terminal peptide aldehyde synthesis)47 For instance, the reduction of phenyl esters led to the aldehyde as the major product, but also a small amount of alcohol)50 The cleavage of u,p-unsaturated y-amino acids via ozonolysis yielded enantiomeric pure C-terminal peptides)49,61 The semicarbazone from reduction of peptide esters technique laid the initial foundation for solid-phase synthesis. Overall, Weinreb reduction is an ideal choice due to its high yields, optical purity, and its adaptability to a solid-phase platform)47 ... 

Cleavage of hydrazones, oximes, and semicarbazones. Amberlyst 15, an ion-exchange resin with sulfonic acid groups, is a superior reagent for exchange of nitrogeneous derivatives of carbonyl compounds with acetone. Yields are 85-95% with reactions that proceed at room temperature.1... 

A reduction route similar to that of phenylhydrazones [229] seems to be rather general for azomethine derivatives of hydrazine [229] as it s followed by cyclic and acylic phenylhydrazones, semicarbazones, azines, cyclic hydrazones, and acylated cyclic and acyclic hydrazones [231] under pro tic conditions in DMF, acylated hydrazones of aromatic aldehydes are reduced with saturation of the C=N bond and cleavage of the N-N bond at a more negative potential [232]. The suggestion that the cleavage of the N-N bond precedes the saturation of the azomethine bond is also an essential part of the interpretation of many of the ring contractions of heterocyclic compounds (Chapter 18). 

Cleavage of—N—derivatives. This reagent is more effective than pyruvic acid for the cleavage of phenylhydrazones and semicarbazones. Reaction with a phenyl-hydrazone produces 3,5-dimethyl-N-phenylpyrazoIe and liberates the free carbonyl compound. 

The Cj p3-O bond of 2-tert-butoxy-furan and -thiophene is cleaved catalytically by BiCl3 or Bi(OTf)3 to produce 2(5H)-furanone and -thiophenone, respectively [254]. The selective hydrolysis of aryl esters is catalyzed by bismufh(III) mandelate in DMSO [255]. In the presence of 50 mol% Bi(NO3)3-5H2O, 10 mol% Cu(OAc)2, and Montmorillonite KIO ketoximes undergo facile deoximation in acetone-H2O [256]. Under microwave irradiation the BiCls-promoted hydrolytic cleavage of the C=N bond of dimethylhydrazines, tosylhydrazines, semicarbazones, and oximes proceeds in wet THF (Scheme 14.120) [257-259]. 

A. R. Hajipour, H. Adibi, and A. E. Ruoho, Wet silica-supported permanganate for cleavage of semicarbazones and phenylhydrazones under solvent free conditions, J. Org. Chem., 68 (2003) 4553-4555. 

---