Lead tetraacetate fragmentation

Lead tetraacetate fragmentation has not been applied to the 20-hydroxy-18, 20-cyclo steroids. However, preferential cleavage of the 17,20-bond would be expected, as was observed in the chromic acid oxidation of a saturated 20-hydroxy-18,20-cyclo steroid in hot acetic acid which affords the 18-acetyl-17-ketone in 50-60% yield. 

Cleavage of l-(trimethylsilyloxy)bicyclo[n.l.O]alkanes (8, 269-270). Lead tetraacetate fragmentation of the exo- and endo-methyl substituted silyl cyclopropyl ethers (1) is essentially stereospecific.2 Thus exo-1 is fragmented to the (E)-alke-noate 2 exclusively and endo-1 is converted into (Z)-2 exclusively. 

The carbonyl ylide precursor can be generated by lead tetraacetate oxidation of the hydrazone 58. Thermolysis of 59 in the presence of perdeuterated acetone led to a variety of products, some of which are shown above. An internal quench of the ylide via a 1,4-proton migration led to enol ether 61, while cycloaddition with perdeuterated acetone formed the dioxolane 62 and its regioisomer. Interestingly, the presence of products such as acetone and propene-t/s are proposed to indicate a reversible fragmentation of the ylide to a carbonyl derivative and a carbene. 

Succinic acids undergo bisdecarboxylation on exposure to lead tetraacetate [264], Contra-polarization at one of the a-carbons through fragmentation of the lead(IV) carboxylate moiety enables a smooth decarboxylation of the remaining functionality. 

Diethylamino)sulfur trifluoride, 110 Formylation (see also Carbonylation) Vilsmeier reagent, 341 Fragmentation reactions Cerium(IV) ammonium nitrate, 67 Copper(II) acetate-Iron(II) sulfate, 85 Lead tetraacetate, 155 Friedel-Crafts alkylation Aluminum chloride, 15... 

Although some radicals and cation radicals are postulated for chemical and electrochemical transformations of 2-benzopyrylium cations (Sections III,F,1 and IV,B)> attempts to record their electron spin resonance (ESR) spectra failed, obviously because of a low stability of these radicals. However, the structural combination of hydroxy aryl and 2-benzopyrylium fragments favors the formation of radical cations 301-303, and their ESR spectra were recorded on oxidation of the corresponding 2-benzopyrylium salts with lead tetraacetate (87RRC417). 

The oxidation of 1,2-O-isopropylidene-D-glucofuranose with lead tetraacetate was found by Criegee to yield formaldehyde,43 which confirmed the presence of a furanose ring in the sugar derivative. The major oxidation fragment, l,2-0-isopropylidene-5-oZde/q/do-D-a yZo-pentodialdose, was characterized subsequently by Iwadare,88 and has recently been shown to crys-... 

Treatment of 9-oxabicyclo[3.3.1]nonan-l-ols 1014 with a combination of lead tetraacetate and copper diacetate affords 3-allenyl tetrahydropyran-2-ones 1015 via an alkoxy radical accelerated ((-fragmentation pathway (Equation 396) <2001TL2047>. 

There is abundant information to support the contention that the lower-melting monoisopropylidene-mannitol (m. p. 85°) is the 3,4-derivative. For example, its tetrabenzoate is identical with that obtained by acetonation of 1,2,5,6-tetrabenzoyl-mannitol,11498 the structure of which is based on independent evidence.114 The larger fragment resulting from the oxidative scission of the D-enantiomorph of the isopropylidene-man-nitol with lead tetraacetate is 2,3-isopropylidene-D-//treo-dihydroxy-succinic dialdehyde, characterized by its subsequent conversion into D-i/ireo-tartaric acid.126 When methylated and hydrolyzed, the L-enantio-morph of the monoketal affords a tetramethyl-mannitol, which, in turn, yields dimethyl-L-glyceraldehyde with lead tetraacetate.127 Each of these facts is in itself proof that the acetone residue occupies the 3,4-position in the mannitol molecule. 

The monoacetone compound consumed three molecular equivalents of periodate, thus limiting the location of the ketal group to the 1,2- or 5,6-positions (see page 147, 1,6- improbable).148 The major fragment which resulted from its oxidation with lead tetraacetate was isopropyli-dene-L-glyceraldehyde, identified by conversion into L-glyceraldehyde 2,4-dinitrophenylhydrazone. This eliminated the 5,6-structure and clearly defined the parent compound as 1,2-isopropylidene-D-sorbitol.143... 

Fragments were obtained from the pentasaccharide by a Smith degradation involving oxidation with lead tetraacetate in acetic acid, followed by reduction with sodium borohydride and partial hydrolysis... 

Formation and /S-fission of bicyclic tertiary alkoxyl radicals from the corresponding alcohols are well known [38] [40]. The treatment of 5a-cholestane-3/3,5-diol-3-acetate, VII/70, and the 5/3-alcohol, VII/71, respectively (Scheme VII/15), with one molar equivalent of lead tetraacetate in the presence of anhydrous calcium carbonate gives radical fragmentation reactions. The products are the two (E)- and (Z)-3/3-acetoxy-5,10-seco-l(10)-cholesten-5-ones (VII/72 + VII/73) [40]. The ratio of VII/73 VII/72 is 63 10 [41] [42] [43]. 

As an alternative to the lead tetraacetate oxidation, (diacetoxyiodo)benzene can be used to initiate a fragmentation reaction which leads to unsaturated medium-sized lactones [110]. The structures of the starting materials are similar to those of compounds VII/157, VII/160, and VII/163. The same stereochemical consequences are observed as mentioned above. 

Bisenone 541 reacted with lead tetraacetate at the secondary a-carbons. Catalytic hydrogenation of this product gave 5466.428 A novel 1,6-fragmentation has been reported to occur upon sodium reduction of 541. Through use of a sodium... 

Tetrahydrofuranes. Cekovic, BoSnjak, and Mihailovic have reviewed their work on the oxidative cyclization of aliphatic alcohols to tetrahydrofuranes by oxidation with lead tetraacetate. The reagent should be free of acetic acid. Anhydrous calcium carbonate can be added to neutralize the acetic acid in the oxidant and that formed during the reaction. The reaction involves selective oxygenation of a 8-carbon the actual mechanism is uncertain, but is probably a radical reaction. Direct oxidation of the alcohol is generally of minor importance, but P-fragmentation can be a problem, particularly when a stable benzyl or allyl radical is formed. 

Barton oxidation was the key to form the 1,2-diketone 341 in surprisingly high yield, in order to close the five-membered ring (Scheme 38). The conditions chosen for the deprotection of the aldehyde, mercuric oxide and boron trifluoride etherate, at room temperature, immediately led to aldol 342. After protection of the newly formed secondary alcohol as a benzoate, the diketone was fragmented quantitatively with excess sodium hypochlorite. Cyclization of the generated diacid 343 to the desired dilactone 344 proved very difficult. After a variety of methods failed, the use of lead tetraacetate (203), precedented by work performed within the stmcmre determination of picrotoxinin (1), was spectacularly successful (204). In 99% yield, the simultaneous formation of both lactones was achieved. EIcb reaction with an excess of tertiary amine removed the benzoate of 344 and the double bond formed was epoxidized with peracid affording p-oxirane 104 stereoselectively. Treatment of... 

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