Lead tetracetate, reactions

Treatment of 3-rncthyl A-cthoxycarbonyl-5-aryIidcnchydrazono-l//-pyrazolcs 194 with bromine, in the presence of sodium acetate, in acetic acid gives substituted ethyl 3-aryl-7-cthoxycarbonyI-6-rncthyl-1 //-pyrazolo[5,1 -/ -[l,2,4]triazoles 55 (Equation 35) <2002ARK133>. Analogously, the reaction of compounds 194 with lead tetracetate [Pb(OAc)4] in acetic acid also gives l//-pyrazolo[5,l-c][l,2,4]triazoles 55 (unreported yield) (Equation 35) <2002ARK133>. 

It is worth mentioning that in a few cases the (3-elimination of the silyl radical from the a-silyl alkoxyl radical (47) with the formation of corresponding carbonyl derivative was observed [63,64]. Evidently the fate of a-silyl alkoxyl radical depends on the method of radical generation and/or the nature of the substrate. Two examples that delineate the potentialities of this rearrangements are reported in Reactions (5.33) and (5.34). In the former, the 5-exo cyclization of secondary alkyl radical on the carbonyl moiety followed by the radical Brook rearrangement afforded the cyclopentyl silyl ether [65], whereas Reaction (5.34) shows the treatment of an a-silyl alcohol with lead tetracetate to afford the mixed acetyl silyl acetal under mild conditions [63]. 

Lassaigne test, 1039, 1040 defects in, 1043 Lead dioxide, 199 Lead tetracetate, 199, 951 Lessing rings, 92 Leuckart reaction, 561, 567 Liebermann nitroso reaction, 649 Light petroleum, purification of, 174 Ligroin, 174 Linseed oil, 445... 

Another 200 g. of the product may be obtained from the mother liquor following the directions of Colson.3 The liquid is returned to the original flask, heated to 80°C., with stirring, and a stream of dry chlorine passed into it. When the reaction is complete, the solution is filtered while hot, and the precipitated lead chloride is washed with hot glacial acetic acid. On cooling, lead tetracetate is deposited from the filtrate. This second crop is contaminated with lead chloride and must be recrystallized if a pure material is desired. 

Cupric acetate does not function as a reoxidant at the low temperature generally useful for the olefin arylation reaction. Mercuric acetate is sometimes useful but it may add to the olefins present and complicate the reaction. Lead tetracetate reoxidizes palladium but also causes the formation of acetates by a reaction apparently analogous to the cupric... 

Fig (5) The ketoalcohol (34) prepared from ketone (33) has been converted to methoxyabietatriene(38), by standard organic reactions. Hydroboration, oxidation of (38), followed by oxidation, yielded the ketone (39), which is converted to the alcohol (40), by metal hydride reduction. On subjection to transanular oxidation with lead tetracetate and benzene alcohol (40) fiimishes (41) whose conversion to pisiferic acid has already been described. 

The in situ Diels-Alder reaction of 4,4-diethylpyrazoline-3,5-dione (from 4,4-diethylpyrazolidine-3,5-dione (26) and lead tetracetate) with 1,3-dienes provides a convenient route to 5,8-dihydropyrazolo[I,2- ]-pyridazine-l, 3-(2// )-diones of type 27, which may be converted to perhy-dropyrazolo[l,2- ]pyridazin-l,3-diones (28) by catalytic hydrogenation over Pd/C [69JOC3I8I, 69LA(724)I50]. Conversion of 28 to 29 by lithium aluminum hydride reduction has been described (72JHC4I). 

Oxidation of phenols. Phenols are oxidized by lead tetracetate to o- and p-quinol acetates or biphenyls depending on reaction conditions. For example,... 

C. Substituted Products - The introduction of an acetate group into the 7 xigl position has been accomplished by treating the 7-hydrazone with lead tetracetate. The stereochemistry of the addition reaction of Grignard reagents to 20-keto steroids was shown in the synthesis of 17a, 20a-dihydroxycholesterol. In such a manner C-27 side chain compounds, including one or more hydroxyl groups, could be synthesized. ... 

Condensation of silylimine of (5)-lactic aldehyde with lithium enolate of t-butyl isovalerate affords the -lactam in 80% chemical yield and in a 97 3 diastereomeric ratio. The mixture was desilylated and treated with lead tetracetate to give, in one step, through a radical fragmentation reaction, the 4-acetoxy derivative as a 1 1 4(R) 4(S) imeric mixture. The lack oi stereospecificity is not easy to rationalize expecially if one considers that the analogous lead tetraacetate induced oxidative decarboxylation is completely trans stereoselective. Both reactions should have the same radical intermediate. However, this lack of stereospecificity is not important for the success of the synthesis since the mixture of diastereoisomers exclusively affords the trans 4-substituted azetidinone by the subsequent Merck procedure (Scheme 9). 

---