Lead tetraacetate oxidative rearrangement

Another example of this rearrangement has been used to prepare 1,2,3-triazole 146 from furazanic phenylhydrazone 147 (Scheme 84) [93JCS(P1)2491]. Interestingly, furoxanic Z-phenylhydrazones 150 underwent thermal recyclization to 1,2,3-triazole A-oxides 152, evidently through intermediate 151. Treatment of the hydrazone 150 with rerr-BuOK leads to the nitromethyl derivative 149 [OOOMIl] (Scheme 84). Lead tetraacetate oxidation of 147 with subsequent Lewis acid treatment of the initially formed intermediate afforded indazole 148 (Scheme 84) (85JHC29). 

Lead tetraacetate in acetic acid oxidizes phenolic 1-benzylisoquinolines to p-quinol acetates which usually rearrange to aporphines in trifluoroacetic acid (25). However, Blasko et al. (24) recently reported that lead tetraacetate oxidized ( )-A -norlaudanosine (34) to dibenzopyrrocoline 35 in 16% yield. 

A Favorskii-type rearrangement occurs on boron trifluoride promoted lead tetraacetate oxidation of enamines274 (Scheme 119). Five-membered rings may also be constructed by a Michael addition-radical cyclization process275 (Scheme 120). 

In addition to the Dimroth rearrangement of [l,2,4]triazolo[3,4-c][l,2,4]triazines discussed in the last section, derivatives of (42) would probably be accessible by the general application of a special case which involved the reaction of l-acetonyl-5-chloro-l,2,4-triazole (356) with hydrazine to produce the dihydro compound (357). Lead tetraacetate oxidation of (357) formed (358) (77JOC1018). 

Benzylisoquinoline 80b was also used in the first total syntheses of 4-hy-droxysarcocapnine, yagonine, and aristoyagonine (48). Lead tetraacetate oxidation of 80b gives o-quinol acetate 82, which after rearrangement gives a racemic mixture (1 9) of epimeric 4-acetoxy derivatives (83). Ullmann reaction of 83... 

Often the cyclized radical opens in the other direction to give rearranged products. This possibility has been used to explain the results of the pyrolysis of cyclopropyl acetates, of the lead tetraacetate oxidation of a-acetoxy alcohols,and of the stannane reduction of a-acetoxy chlorides. In all these cases free radical addition followed by -scission has been proposed, e.g., as in the 1,2-rearrangement of )ff-acyloxyl radicals 205 to 207 (Scheme 90) discovered independently by Teissier and by Tanner,and whose mechanism, while not completely elucidated, is presumed to proceed through 206, which must be considered as a transition state rather than an intermediate. 

Treatment of amides with bromine in alkaline medium promotes the Hofmann rearrangement, which may or may not involve a free nitrene intermediate." The oxidation of primary amides with lead tetraacetate and the resulting Lossen rearrangement also produces isocyanates, with the possible intervention of an acylni-... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

The rearrangement of cyclopropenyl azides (396) is used for the synthesis of monocyclic 1,2,3-triazines (397) (73JOC3149, 79CB1514). However, the most general method is the oxidation of N-aminopyrazoles (398) with lead tetraacetate or variety of other oxidants (92AHC(53)85). 

Both a- and P-pinenes are popular starting materials for the synthesis of other monoterpene chiral synthons such as carvone, terpineol, and camphor (vide infra). Reactions leading to other monoterpenes are briefly summarized in Figure 5.1. Treatment of a-pinene with lead tetraacetate followed by rearrangement gives trans-verbenyl acetate (7), which is hydrolyzed to yield trans-verbenol (8) 8 Subsequent oxidation of 8 gives verbenone (9), which can be reduced to give cw-verbenol... 

Adamantylideneadamantane has been prepared by (1) photolysis of 2-adamantylketene dimer,2 (2) reduction of 4< -chloroadaman-tylideneadamantane with sodium in liquid ammonia,3 (3) rearrangement of spiro[adamantane-2,4 -homoadamantan-5 -ol] with Lewis acids,4,5 (4) reduction of 2,2-dibromoadamantane with magnesium6 or zinc-copper couple,7 and (5) treatment of the azine of 2-ada-mantanone with hydrogen sulfide, followed by oxidation with lead tetraacetate and heating with triphenylphosphine.8... 

A version of this type of oxidation, which used a combination of lead tetraacetate and boron trifluoride etherate, allowed the transformation of enamines of cyclic ketones into esters of the corresponding contracted ring, as in the classical Favorskii rearrangement of a-haloketones under basic conditions72 (Scheme 51). 

The accessibility of position 9 becomes much higher when an activating group, such as an OH, is present in position 10. Although 10-hydroxycamptothecin (8) is available in small amounts from the plant material, two efficient preparations of this compound were developed, via catalytic reduction of CPT in acid medium to a tetrahydroquinoline, followed by selective oxidation with lead tetraacetate [8], or phenyliodonium diacetate [27], or via a photochemical rearrangement of camptothe-... 

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