Allylic oxidation with manganese dioxide

Enhancing the utility of the allq lation-rearrangenient sequence in synthesis, the Evans group addressed the problems of a-versus-y alkylation as well as low anion reactivity by employing heterocyclic sulfides as the alleviation substrates tScheme IR.IfiE For instance, allylic imidazolyl sulfide 56 could be allqvlated efficiently, reaction at the a-position being favored by a chelated but reactive allyl lithium intermediate. Oxidation of 57 to the allylic sulfoxide and treatment with a secondary amine thiophile provided allylic alcohol 58 in high yield and with excellent stereoselectivity at the trisubstituted alkene. Allylic oxidation with manganese dioxide completed a synthesis of the sesquiterpene nuciferal (59). ... 

Oppenauer oxidation, 236 Oxidation of allylic alcohols with dichloro-dicyanobenzoquinone, 248 Oxidation of allylic alcohols with manganese dioxide, 247... 

To our mind, the enolate of 19 should exhibit a decided kinetic bias for kinetically controlled protonation on its a face because of the steric encumbrance associated with p proton delivery. In actual fact, rapid introduction of its lithium salt into a 1 4 mixture of water and tetrahydrofuran at -78 °C resulted in its quantitative conversion to 20 (Scheme III). Once the MOM groups had been removed, controlled oxidation with manganese dioxide led to 21, a very pivotal intermediate. To arrive at magellaninone (2), 21 was treated with methyllithium and the resulting unprotected diol 22 was directly reduced with lithium aluminum hydride. Subsequent Jones oxidation proceeded with the customary allylic rearrangement. 

This reaction was first reported by Corey and co-workers in 1968. It is an efficient synthesis of o ,j8-unsaturated esters by treatment of o, j0-unsaturated aldehydes (or allyl aicohois) with manganese dioxide and potassium cyanide in a proper alcohol. This reaction is thus known as the Corey-Gilman-Ganem oxidation in general or simply the Corey-Ganem oxidatin. ... 

The synthesis of several terpenoid natural lactones has been described. Butyrolactones are formed efficiently by the cyclization of hydroxy-acids, and acid olefin cyclization. The diester (123) has been converted independently into the isomeric lactones cinnamolide (124), and drimenin (125) (Scheme 71). Selective reduction of the a,/3-unsaturated ester was used in the latter case, whereas selective oxidation of the corresponding allylic alcohol with manganese dioxide was used in the former. Manganese dioxide has also been... 

Other Anhydrides.—The enones (42) and (43) have been prepared by oxidation of the corresponding allylic alcohols with manganese dioxide. Whereas the oxiran derivative (44) afforded the allylic alcohol (45) on treatment with butyl-lithium, the /yxo-isomer (46) gave the 2-substituted glycal (47). Enols with equatorially oriented hydroxy-groups were obtained when the enones (42) and (43) were reduced with sodium borohydride. 

Preparation of Propionaldehyde 7-2. Preparation of Acrolein by Oxidation of Allyl Alcohol with Manganese Dioxide... 

In comparison with manganese dioxide, the DDQ reagent has several advantages for the oxidation of allylic alcohols. The quinone method is more reproducible only one equivalent of oxidant need be added and generally fewer side reactions are observed. On the other hand, the workup of DDQ reactions often requires chromatography and in the simpler cases lower isolated yields may be realized. 

Chemoselective oxidation of the allylic alcohol in triol 865 with manganese dioxide followed by in situ cyclization and oxidation of the resulting 5,6-dihydropyran-2-ol provides the 5,6-dihydropyran-2-one subunit 866 of bryostatin (Equation 349) <20000L2189>. 

The next important reaction is a Michael addition so the alcohol must first be oxidized to a ketone. As it is an allylic alcohol, it can be oxidized by manganese dioxide. The ring is further flattened as three atoms are now trigonal. But-3-enyl Grignard reagent is next added with Cu(I)... 

We can illustrate the synthesis of allylic alcohols from allylic sulfoxides with this synthesis of the natural product nuciferal. We mentioned this route on p. 1257 because it makes use of a heterocyclic allyl sulfide to introduce an alkyl substituent regioselectively. The allyl sulfide is oxidized to the sulfoxide, which is converted to the rearranged allylic alcohol with diethylamine as the thiophile. Nuciferal is obtained by oxidizing the allylic alcohol to an aldehyde with manganese dioxide. 

Tritium-labelled disenedoylretronedne (64) required for studies of the biological properties of pyrrolizidine alkaloids, has been prepared by acylation of [9- H2]retronedne by senedoyl chloride. The label was introduced into retronedne following the Corey method of oxidation of the primary allylic alcohol function of unlabelled retronedne (65) with manganese dioxide in the presence of potassium... 

Mild oxidation of CC-2 with manganese dioxide afforded a conjugated enone, C21H25NO5, showing the allylic nature of the original hydroxy group. Furthermore, the H-NMR spectrum of the enone was identical with that of the oxidation product of 19 in particular, the signal for HA... 

Thus, reduction of androst-5-ene-l,17-dione (1-5) with lithium aluminum hydride reduced both 3- and 17-carbonyl groups, that at Cn cleanly to the /8-epimer and the ketone at C3 to a mixture of epimers (3-1) (Scheme 5.3). Treatment of the mixture of epimers with manganese dioxide oxidizes the allylic alcohol at C3, leaving that at C17 untouched, affording testosterone (3-2). 

For example, oxidation of the allylic alcohol 33 gave the a, -unsaturated aldehyde 34, used in a synthesis of the macrolactone bafilomycin Ai (6.30). Chemose-lective oxidation of allylic or benzylic alcohols can be achieved in the presence of aliphatic alcohols. Thus, in a synthesis of the alkaloid galanthamine, treatment of the diol 35 with manganese dioxide promoted selective oxidation of the benzylic alcohol to give the aldehyde 36 (6.31). 

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