A number of aldehyde arylhydrazones have been oxidised with lead tetra-acetate. In many cases diacylhydrazines, RCO-NHaNAcAr, can be isolated in good yield. With benzaldehyde phenylhydrazone, provided that precautions are taken to avoid autoxidation, a-phenylazobenzyl acetate can be isolated in up to 27% yield. Evidence has been obtained that this compound is not the main precursor of the diacyl derivative PhCO*NH*NAcPh or of further oxidation products, but that these arise via the nitrilimine PhCrN-NPh ; this I ,3-dipolar compound has been trapped with acrylonitrile, to form 1.3-diphenylpyrazole-5-carbonitrile together with a small quantity of the isomeric 4-carbonitrile. Analogous nitrilimines have been similarly trapped during the oxidation of the p-nitrophenylhydrazones of benzaldehyde, propionaldehyde, benzoin, and benzil. Oxidations with lead tetrabenzoate, and with lead tetra-acetate in methanol, have also been carried out; they lead to azo-dibenzoates and azo-dimethyl ethers, respectively, and benzaldehyde phenylhydrazone also forms an azo-benzoate.

Four 2-pyrazolines have been oxidised to the corresponding pyrazoles in high yield by lead tetra-acetate a t room temperature ; this reagent has advantages over others which have been used previously. Lead tetra-acetate converts chalcone phenylhydrazone directly into 1.3.5-triphenylpyrazole, in contrast to its reactions with the arylhydrazones of aromatic ketones which give azoacetates. Two 2-pyrazolines which cannot undergo direct aromatisation have been converted into pyrazoles by oxidation with lead tetra-acetate to 4-acetoxy-2-pyrazolines followed by acid-catalysed rearrangement and elimination.

Coupling of 3β-acetoxy-5α-etiojerv-12(13)-en-17-one (1) with the lithio derivative of 2-ethyl-5-methylpyridine provides the crucial intermediate (3) for the subsequent elaboration to verarine. Aromatization of 3 to 4 and reduction of the latter provides a mixture from which N-acetyl-5α,6-dihydroverarine (11) was isolated. Subsequent introduction of the 5,6-double bond in the latter and removal of the N-acetate function completed the synthesis of verarine (31). In a similar sequence of reactions employing 1 and the lithio derivative of 2-ethyl-3-methoxy-5-methyl-pyridine, the resultant intermediate (54), was elaborated to 5α,6-dihydroveratramine (56). Due to known conversions of the latter to veratramine (14), jervine (32), veratrobasine (33), and 11-deoxojervine (34), the formal total synthesis of these natural products is complete.

No abstract

Benzophenone phenylhydrazone and p-nitrophenylhydrazone are oxidised by lead tetra-acetate in the presence of alcohols to mixtures of the azoacetate and an azoether; the latter product does not arise by alcoholysis of the former. The rates of oxidation of aromatic ketone arylhydrazones are dependent on the polar effects of substituents both in the ketone and (more markedly) in the arylhydrazine moieties. For one series of compounds, the effects of substituents in the arylhydrazine are correlated by the Hammett equation ( p = -1 -95). the derived o-value forp-NO, being an I' exalted *' one (1 -1 7). These and related facts lead to the view that the rate-determining step of the oxidation involves the displacement of an acetate anion from the lead tetra-acetate by the NH-nitrogen ; this is followed by the uptake of an acetoxy or alkoxy group a t the ketonic carbon which, a t least for the acetoxylation, probably occurs intramolecularly within an intermediate lead derivative.THE monosubstituted hydrazones of ketones react with lead tetra-acetate, rapidly at 0-lo", to give good yields (55-90%) of azo-compounds of the type R1R2C(OAc)-N=NR3 which have been termed azo-acetates2 The following free-radial mechanism has been formulated for the reaction: slow