Tertiary allylic alcohols, oxidative

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. 

Substituted allylic alcohols are carbonylated using the oxidizing system of PdCl2 and CuCU in the presence of HCI and oxygen at room temperature and l atm of CO to give the 7-lactone 16 in moderate yields[20], Carbonylation of secondary and tertiary allylic alcohols catalyzed by Pd2(dbah and dppb affords the 7-lactone 17 by selective attack of CO at the terminal carbon under fairly severe conditions[21]. 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

As the Jones-mediated transformation of tertiary allylic alcohols into enones is normally slower than the oxidation of secondary alcohols into ketones it is possible to selectively oxidize a secondary alcohol to ketone, without affecting a tertiary allylic alcohol present in the same molecule. 

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... 

Nevertheless, normally it is possible to selectively oxidize primary and secondary alcohols with PDC without affecting tertiary allylic alcohols.159... 

Sometimes, tertiary allylic alcohols interfere with the oxidation of primary and secondary alcohols with PDC, causing low-yielding transformations into the desired aldehydes and ketones.161 Secondary allylic alcohols occasionally suffer oxidative transposition to enones rather than a direct oxidation.162... 

PCC reacts with tertiary allylic alcohols, forming an intermediate chromate ester that evolves giving a conjugated enone or enal. Sometimes, the isomeric chromate ester produces the epoxidation of the alkene, giving an epoxy alcohol that can be further oxidized to an epoxy ketone. 

This oxidative transposition of tertiary allylic alcohols into enones or enals is carried out under mild conditions and has ample application in organic synthesis. Although, it can be carried out with other chromium-based reagents (see pages 16 and 35), PCC is the reagent of choice.272... 

Of course, using excess of PCC allows the operation of both, an oxidative transposition of a tertiary allylic alcohol and a normal oxidation of a primary or a secondary alcohol.276... 

The oxidation with PCC causes both, a normal oxidation of the primary alcohol and an oxidative transposition of the tertiary allylic alcohol. 

Although secondary allylic alcohols can suffer an oxidative transposition via the corresponding allylic chromate ester, in the same manner that the tertiary allylic alcohols normally, a direct oxidation to the corresponding enone with no transposition predominates.277 Nevertheless, minor amounts of enone, resulting from an oxidative transposition, can be formed.278 The formation of transposed enone may be minimized using the less transposing-prone PDC, instead of PCC.279... 

That was a [3,31 -sigmatropic reaction involving two nitrogens. There follows one with two oxygens and a chromium atom. When tertiary allylic alcohols are oxidized with CrC>3 in acid solution, no direct oxidation can take place, but a kind of conjugate oxidation occurs. 

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). ... 

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... 

The allyl alcohol in the seven-membered ring is oxidized to the corresponding a,P-unsaturated ketone as expected, whereas the allyl alcohol in the five-membered ring is oxidized to afford the transposed a,P-unsaturated ketone after [3,3]-sigmatropic rearrangement. This behavior is only observed with cyclic, tertiary allyl alcohols upon treatment with one equivalent of pyridinium chlorochromate. The desired oxidation product 34 is obtained in 90 % over two steps. 

Oxidation of Tertiary Allylic Alcohols (The Babler Oxidation) 97... 

OXIDATION OF TERTIARY ALLYLIC ALCOHOLS (THE BABLER OXIDATION)... 

J.-M. Bregeault, C. Lepetit, F. Ziani-Derdar, O. Mohammed i, L. Salles, A. Deloffre, Epoxidation of tertiary allylic alcohols and subsequent isomerization of tertiary epoxy-alcohols A comparison of some catalytic systems for demanding ketonization processes, in R. K. Grasselli, S. T. Oyama, A. M. Gaffney, J. E. Lyons (Eds.), 3rd World Congress on Oxidation Catalysis, Elsevier, Amsterdam, Stud. Surf. Sci. Catal. 110 (1997) 545. 

Ohler, E.. and Zbiral. Ei.. Oxidative rearrangement of phosphorus containing tertiary allylic alcohols. Synthesis of (3-oxo-l-cycloalkenyl)phosphonates, -methylphosphonates, -methyldiphenylphosphine oxides and their epoxy derivatives. Synthesis, 357, 1991. 

Since epoxidation at the vinyl double bond is unproductive, it is desirable to direct reaction on the al-lene moiety. This can be accomplished by taking advantage of the hydroxy-directed epoxidation of allylic alcohols using the t-butyl hydroperoxide/vanadium(V) system. The directing effects of both allylic and homoallylic type hydroxy groups have been examined at both positions of the vinylallene unit. " At the 1-position (64), primary, secondary and tertiary allylic alcohols are effective, while only primary homoallylic alcohols have b n examined (equation 35). Presumably the directing effect of the hydroxy groups favors formation of the intermediate allene oxide (65). A sample of the compounds prepared by this route is shown in Scheme 32. ... 

The formation of 3-methyl-2-buten-l-ol, 4, and of 3-methyl-2-butenal, 5, in some experiments can be related to the isomerization of la and subsequent oxidation of product 4, thus formed, to 5. We observed that the vanadium oxo-alkoxide, [OV(OC3H7)3], is a good precursor for the catalytic epoxidation of substrates la, lb and Ic with systems involving tert-butyl hydroperoxide and tertiary allylic alcohols. These systems proved to be more active than those involving [VO(acac)2] usually associated with anhydrous r-BuOOH in benzene or toluene (Figure 1 and Table 2). 

Recent Developments in the Pauson-Khand Reaction Oxidative Rearrangement of Tertiary Allylic Alcohols Other Methods... 

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