Hydrazones cleavage, oxidative

The asymmetric hydrogenation of imine derivatives that contain a second ligating functionality to create two-point binding of the substrate have been investigated. Reactions of two types of such substrates are shown in Equations 15.85 and 15.86. In the first example, an aroyl hydrazone undergoes selective hydrogenation to form the enantioenriched N-alkyl hydrazone. Oxidative cleavage of the N-N bond then leads to the primary amine. 

It has been reported that the cleavage of SAMP hydrazones can proceed smoothly with a saturated aqueous oxalic acid, and this allows the efficient recovery of the expensive and acid-sensitive chiral auxiliaries SAMP and RAMP. No racemization of the chiral ketones occurs during the weak acid oxalic acid treatment, so this method is essential for compounds sensitive to oxidative cleavage.393... 

A general oxidative cleavage reaction of ketone-derived SAMP-hydrazones can be realized without racemization by reaction with magnesium monoperoxyphthalate (MMPP) hexahydrate in methanol or in methanol/pH 7 phosphate buffer within 2 hours26. 

A further method for estimation of sialic acid using periodic acid has been reported by Massamiri and coworkers.121 Fonnaldehyde produced by mild, oxidative cleavage of the D-en/f/iro-glycerol-l-yl side-chain of Neu can be detected by 3-methyl-2-benzothiazolinone hydrazone, which yields a green-blue color (absorbance maximum 625 nm). This test, which is slightly more sensitive than the thiobarbi-... 

Alkyl-s-caprolactones are formed (Scheme 10) in good overall yields (up to 75%) and with high enantiomeric purity (ee = 90%, 94%) by alkylation of SAMP-hydrazones 25 (SAMP = (A)-l-amino-2-methoxymethylpyrrolidine) and subsequent oxidative cleavage of the hydrazone 26 with ozone <1995S947>. 

Besides the oxidative cleavage by ozonolysis, the optically active carbonyl compounds can be alternatively obtained by acidic hydrolysis of the corresponding SAMP-/RAMP-hydrazone methiodides in a two-phase system. > a... 

Oxidative cleavage of N,N-dialkylhydrazones These hydrazones are cleaved ay the parent ketone by treatment with this oxidant in aqueous methanol at 0° in yields if 76-91%. No racemization occurs in the case of SAMP or RAMP hydrazones. 

Oxidative cleavage of N,N-dimethylhydrazones. m-Chloroperbenzoic acid is as efficient and more economical than NalOi (7, 126) for cleavage of these hydrazones. The cleavage of the hydrazone 1 was effected without epimerization of the axial methyl group. 

Oxidative cleavage of hydrazones. Carbonyl compounds can be regenerated from 2,4-dinitrophenylhydrazones, tosylhydrazones, or N-methyl-N-tosylhydra-zones in 80-95% yield by oxidation with sodium nitrite in TFA or HOAc (0-5°, 1-2 hours). In the case of tosylhydrazones, the other product of cleavage has been identified as tosyl azide, the product of reaction of tosylhydrazine with nitrous acid. 

Enders, D., Plant, A. Enantioselective synthesis of a-substituted nitriles by oxidative cleavage of aldehyde SAMP-hydrazones with magnesium monoperoxyphthalate. Syn/eff 1994,1054-1056. 

The asymmetric Michael addition of metaUated SAMP hydrazones to a variety of diethyl 1-alkenylphosphonates followed by oxidative cleavage of the 1,4-adducts produces 2,3-disubstituted 4-oxoalkylphosphonates with good yields (58-80%), but low to moderate de s (6-74%) and excellent ee s (> 93%). ... 

Oxidative cleavage of pyrazol-4,5-dione-4-[Af-(pyrazol-4-yl)]hydrazone 512... 

A new method for cleavage was recently reported by Enders. Treatment of the hydrazone with three equivalents of sodium perborate regenerate the carbonyl compound in 70-95% yield according to equation (36). This oxidative cleavage was chemoselective and left carbon-carbon double bonds intact. This chemoselectivity plus the neutral hydrolytic conditions and the low cost of the oxidant make this method quite attractive, liie main disadvantage of this method is the relatively long reaction time required, ranging from 4-24 h for aliphatic ketone hydrazones to 2-3 d for aromatic ketone hydrazones. 

Oxidative cleavage of pyrazol-4,5-dione-4-[7V-(pyrazol-4-yl)]hydrazone 658 occurred when this compound was oxidized with 4-chloroperbenzoic acid (73G179) (Scheme 193) to give pyrazole diazonium salt 659 in 72% yield, oxadiazol-5-one 660 in 0,5% yield, hydrazone derivative 661 in 10% yield and the bispyrazole 662 in 17% yield. Oxidation of hydrazone 663 under similar conditions... 

Altamura, A. Curci, R. Edwards, J.O. Facile regeneration of carbonyl compounds by oxidative cleavage of hydrazones using dioxiranes. J. Org. Chem. 1993, 58, 7289. 

Oxidative cleavage of hydrazones. Dimethylhydrazones and tosylhydrazones are reductively cleaved to carbonyl compounds by either M0OCI3 or MoFo. Yields are generally in the range of 70-90%. Cf. Tungsten hexafluoride, this volume. 

Oxidative cleavage of hydrazones. This salt converts dimethylhydrazones or tosylhydrazones into the carbonyl compound under mild condilions (CHCla or Freon 113, 75-95% yield).i... 

Oxidative cleavage of hydrazones and oximes to give the parent ketones occurs on refluxing in chloroform containing cobalt(ili) fluoride, followed by hydrolysis. 

By contrast, very high diastereoselectivities have been observed by the use of chiral hydrazones prepared from the C2 symmetric hydrazine 2,5-dimethyl-pyrrolidin-l-ylamine. The asymmetric induction comes at a price, however The hydrazine is not commercially available, and it cannot be recovered as such following oxidative cleavage from the P-lactam. More readily available, and therefore more expendable, hydrazines are SAMP analogs derived from L-proline. ... 

The new hydrazones were first tested in simple a-alkylation reactions. As is shown in scheme 14, metalation with Lochmann-Schlosser base in THF at low temperature yielded highly reactive azaenolates, which were alkylated by a number of alkyl halides at -100°C in good yields and with high diastereomeric excess (de = 85 - > 95%). It is noteworthy that, besides the usual oxidative cleavage with ozone, the SAMP-hydrazones could be converted without racemization to the final 3-substituted 2-oxoesters under very mild conditions with boron trifluoride-ether in acetone/water with the addition of paraformaldehyde. Thus, this method permitted for the first time the highly enantioselective transfer of a homologous pyruvate unit to electrophiles (scheme 14) [41]. 

Regeneration of carbonyl compounds from their hydrazones and oximes is most commonly carried out by acid hydrolysis and the expulsion of the analyzed carbonyl compound by a more reactive carbonyl compound in the case of oximes and in special circumstances oxidative cleavage can also be applied. 

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