Alcohols oxidative rearrangement

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

We had two possible routes in which alcohol 72 could be used (Scheme 8.19). Route A would involve rearrangement of tertiary alcohol 72 to enone 76. Deprotonation at C5 and generation of the enolate followed by exposure to an oxaziridine or other oxygen electrophile equivalents might directly afford the hydrated furan C-ring of phomactin A (see 82) via hydroxy enone 81. We had also hoped to make use of a chromium-mediated oxidative rearrangement of tertiary allylic alcohols. Unfortunately, treatment of 72 to PCC produced only unidentified baseline materials, thereby quickly eliminating this route. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Addition of isopropyl lithium to the surviving ketone followed by oxidative rearrangement of the resulting tertiary ailylic alcohol and concomitant oxidation of the secondary allylic alcohol gave the diketone 10. Selective addition of methyl lithium to the less hindered of the two ketones, again from the more open face, then gave 3. 

An alternative approach to the oxidative rearrangement uses chlorine as the oxidant in a one-pot synthesis (80JOC1109). Not only does this reagent convert the furan to the pyran-3-one, but it takes the latter to the 4-halogeno derivative via the labile 3,4-dihalogeno compound. The pyran-4-one is obtained by an in situ acid-catalyzed hydrolysis. The overall yield of maltol is over 60%, and ethylmaltol is obtained in a similar conversion from ethylfurfuryl alcohol. 

Collins reagent can transform tertiary allylic alcohols into rearranged enones,101 similar to PCC, which is routinely used for this purpose (see page 55). As this reaction is normally slower than the oxidation of primary and secondary alcohols, these can be oxidized with Collins reagent with no interference from tertiary allylic alcohols present in the same molecule.102... 

Isomerization of epoxides to allylic alcoholsThis rearrangement has been effected with strong bases and various Lewis acids. Enantioselective rearrangement to optically active allylic alcohols can be effected with catalytic amounts of vitamin B, at 25°. Thus cyclopentene oxide rearranges to (R)-2-cyclopentene-l-ol in 65% ee. The rearrangement of the as-2-butene oxide to (R)-3-butene-2-ol in 26% ee is more typical. 

The chroniium(VI) oxide-dipyridine complex also has beoi found to cause oxidative rearrangement of tertiary allylic alcohols to a,3-epoxy aldehydes and small amounts of a,3-unsaturated aldehydes (equation 6 and Table 3). This is potentially useful as a homologation sequence since the starting materials are readily available from vinyl metal addition to ketones. Use of pyridinium chlorochromate (PCC) for this transformation gives mosdy a,3 unsaturated aldehydes. 

In a similar fiashion to the Collins reagent, PCC will also induce oxidative rearrangement of tertiary allylic alcohols (Table S). PCC, and several other chromium oxidants, will also cause tertiary cyclopropyl alcohols to rearrange to give 3,y-unsaturated carbonyl compounds (equation 8). ... 

This reagent can be of value not only for its inherent chemoselectivity, but also because of the mild conditions under which oxidation occurs. For example the cyclohexylideneacetaldehydes (2) can be produced by manganese dioxide oxidation of the allylic alcohols despite the instability of (2) to air, acids and bases. Manganese dioxide is known to cleave 1,2-diols, and can cause oxidative rearrangement to... 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized stereospecifically with rearrangement to the allylic alcohol in good yield by simple exposure to a palladium/caibon catalyst, as depicted in equation (12). Adventitious oxygen preadsotbed on the catalyst seems the likely oxida The reaction fails on the parent ccphem or its monoxide, or on the free acid of the dioxide. This reaction would seem to hold some promise for furdier utility in the cephem field and odier related systems. 

The oxidative rearrangement of allylic alcohols to a -unsaturated kelmies or alddiydes is one of the most widely used synthetic reactions in this group, and forms part of a 1,3-carbonyl tran sition sequence. Scheme 7 shows this reaction and the related conversion of the allylic alcdiol to an a,p-epoxy carbonyl compound. Chromate reagents induce some allylic alcohol substrates to undergo a directed qmxidation of the alkene without rearrangement, but this reaction is beyond the scope of the present discussion. 

However, the conversion to the transposed a,p-unsaturated carbtxiyl compound is by far the more useful reaction. The fiill sequence serves both to form carbon-carbon bonds as well as to adjust the functional group array in tlw synthetic intermediate. Thus, starting with the enone (15), organometallic addition generates a tertiary allylic alcohol (16) and oxidative rearrangement yields a P-a]kyl-a,p-enone... 

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