Acetals cleavage, oxidative

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

Other methods of molecular weight reduction include pentosanase treatment or formation of low-molecular-weight acetals. Oxidative cleavage of hemicellulose also could lead to reduced molecular weight. 

The reaction is based, on the one hand, on the oxidative cleavage of vicinal diols by lead(IV) acetate and, on the other hand, on the reaction of dichlorofluorescein with lead(IV) acetate to yield a nonfluorescent oxidation product. The dichlorofluorescein only maintains its fluorescence in the chromatogram zones where the lead(IV) acetate has been consumed by the glycol cleavage reaction [1],... 

These experiments verified that cleavage of the C—H bond is occurring in the rate-limiting step, but proof of the necessity of a trans relationship of the C—H to the nitrogen-mereury complex was lacking. Indications that such a relation was necessary are found in studies of the mercuric acetate oxidation of alkaloids, which will be discussed subsequently. 

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

Transformation of the amino nitriles to the corresponding amino acids, with removal of the dioxane ring, is carried out in two steps. Treatment with concentrated hydrochloric acid results in the hydrolysis of both the nitrile and the acetal group, and in cyclization to the corresponding 3-substituted 5-hydroxyniethyl-3-methyl-2-oxo-6-phenylmorpholinc hydrochlorides. Oxidative cleavage with 2 N sodium hydroxide solution, air and Raney nickel at 120 CC (ca. 30 h) delivers the hydrochlorides of the free a-methylamino acids in high yield. 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THF with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted alkynes.100 Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst.101... 

Examples of the oxidative cleavage/bis-hetero-Diels-Alder domino sequence of compounds of type 7-124 to give acetals of type 7-125 are listed in Scheme 7.34. This shows that the reaction is high-yielding and can be carried out under rather mild conditions. 

Piperidines bearing a masked aldehyde function in the e-position are easily transformed into quinolizidine compounds through intramolecular reductive amination after deprotection (acetal precursors) or oxidative cleavage (tv e-diols). Some examples are given below. 

FIGURE 6.21 (A) Removal of trityl and acetamidomethyl from sulfhydryl by oxidative cleavage by iodine. (B) Cleavage of terf-butylsulfanyl by mercury(II) acetate,88 followed by displacement of the metal ion by hydrogen sulfide. 

With a good route to the key meso diol 128 in hand, the authors turned their attention to desymmetrization, using the known asymmetric hydrolysis of meso diacetates by Lipase AK (Scheme 23). The meso diol 128 was first converted to diacetate 140, and then hydrolyzed with Lipase AK to cleave selectively one of the two acetates, producing chiral hydroxyester 141. Oxidation, cleavage of the acetate, and lactonization yielded the (3S,4.R) lactone 129. The corresponding lactol (3S,4 )-130 was found to be the enantiomer of the compound produced in the HLADH synthesis. 

I n contrast to the relative simplicity of the chromyl chloride oxidation of 2,2-disubstituted-l-alkenes to aldehydes, the rlimmyl acetate and chromic acid oxidations generally lead to epoxides, acids, and carbon-carbon double bond cleavage. For example, chromyl acetate oxidizes 4,4-dimethyl-2-neopentyl-I pentene primarily to l,2-epoxy-4,4-dimethyl-2-neopentyl-pentane in low yield,9 and chromic acid oxidizes the alkene principally to 4,4-dimethyl-2-neopentylpentanoic acid.6,10... 

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