Alcohols, allylic examples

Sharpless epoxidations can also be used to separate enantiomers of chiral allylic alcohols by kinetic resolution (V.S. Martin, 1981 K.B. Sharpless, 1983 B). In this procedure the epoxidation of the allylic alcohol is stopped at 50% conversion, and the desired alcohol is either enriched in the epoxide fraction or in the non-reacted allylic alcohol fraction. Examples are given in section 4.8.3. 

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

As for allylic ethers, there are relatively few reports of highly enantioselective rhodium-catalyzed isomerizations of allylic alcohols (for example, >80% ee). Relative to the other processes described above, reactions of allylic alcohols are more atom economical [6] since they obviate the need for a separate hydrolysis step (Eq. 5). 

Radical cyclization of oximes or oxime ethers having allylic substituents or an aldehyde group to tetrahydropyrrole derivatives was described Thus, Sm -induced 5-exo-trig radical cyclization of oxime ethers containing a formyl group was found to be particularly effective for the preparation of cyclic trans-wimo alcohols. For example, oxime 96 in the system SmE/THF/f-BuOH at 25 °C or —78°C afforded pyrrolidin-3-ols 97 and 98 in a ratio 3 2 or 9 1 (equation 41) . Cyclization of oxime ether 99 in the... 

Rearranged fluoroalkenes are also the sole products of the reaction of sulfur tetrafluoride with tertiary allylic alcohols. For example, 1,1,l-trifluoro-2-(trifluoromethyl)but-3-en-2-ol gives exclusively 1,1,1,4-tetrafluoro-2-(trifluoromethyl)but-2-ene (12).57... 

Enone transposition. A new method for this transposition depends on the regio- and stereoselectivity of addition of C6HsSeCI to allylic alcohols. An example is shown in equation (I). 

Z)-Trisubstituted aRylic alcohols. The conditions used by Bestmann et al. (7, 329) for preparation of (Z)-disubstitutcd alkcncs via the Wittig reaction also can provide a stereoselective route to (Z)-trisubstiluted allylic alcohols. An example is the reaction of ethylidenetriphenylphosphorane with the THP ether of hydroxy-acetone (equation 1). The stereoselectivity is decreased with other protecting... 

Examination of the scope of the individual kinetic resolution systems leads to some general trends. Typically, oxidation proceeds most rapidly with activated alcohols, for example benzylic, allylic, and a-cyclopropyl alcohols. Occasionally, saturated alkyl alcohols can be oxidized, although reaction times are usually longer. Furthermore, sterically encumbered alcohols tend to be difficult to oxidize. Selectivity in the kinetic resolutions presumably relies on steric differences between the two alcohol substituents. 

Enantioselective isomerization can be advantageously used for the kinetic resolution of racemic allyl alcohols. For example treatment of 4-hydroxy-2-cyclopente-none (rac-28) in the presence of Rh[(R)-BINAP](MeOH)2 + gives rise to the enan-tiomerically enriched allyl alcohol (R)-29 (Scheme9) [13]. This unsaturated hydroxy ketone is an important building block for the synthesis of prostaglandins... 

In this case, a mixture of cis- and ra s-perfluoro-4,5-dihydro-2,3,4,5-tetramethyl-4-ethylfurans is obtained. This is a general reaction, which also works with other perfluoroolefins and alcohols. For example, when the tetrafluoroethylene pentamer reacts with allyl alcohol in the presence of bases, the initial reaction is nucleophilic addition at the multiple bond, forming olefin 44, followed by the formation of perfluoro-4-ethyl-2,3,4,5-tetramethyl-4,5-dihydrofuran 45 in the presence of KF (87ASCC31). 

Lithium amide deprotonation of epoxides is a convenient method for the preparation of allylic alcohols. Since the first deprotonation of an epoxide by a lithium amide performed by Cope and coworkers in 19585, this area has received much attention. The first asymmetric deprotonation was demonstrated by Whitesell and Felman in 19806. They enantioselectively rearranged me.vo-cpoxidcs to allylic alcohols for example, cyclohexene oxide 1 was reacted with chiral bases, e.g. (S,S) 3, in refluxing TFIF to yield optically active (/ )-2-cyclohexenol ((/ )-2) in 36% ee (Scheme 1). 

Homoallylic alcohols. Aldehydes or ketones react with an allylic halide in the presence of tin and aluminum in an aqueous medium to give homoallylic alcohols.1 Example ... 

Oganoselenium reagents have been observed to exhibit selectivity for the oxidation of allylic alcohols, for example a catalytic amount of dimesilyl diselenide with r-butyl hydroperoxide as cooxidant will oxidize benzyUc and allylic alctdiols in the presence of saturated alcohols, as in the case of the diol (7 equation 4). 

The procedure is commendable for its sinq>licity, reduced toxicity (chromium in all its oxidation states is carcinogenic) and achieves good yields of ketones from alcohol, for example, octan-2-ol is oxidized into octan-2-one (92%), cyclohexanol into cyclohexanone (90%) and menthol into menthone (98%). Pyridinium chromate is also a well-known oxidant for allylic oxidations. As a silica gel supported reagent, this is turned into an efficient alcohol oxidant that will leave acid-labile functions unscathed. Another advantage of the reagent is the long shelf-life of more than a year. These solid-supported oxidants also greatly facilitate pr uct work-up, when compared with their solution counterparts. 

The substrates for allyl vinyl Claisen rearrangements are conveniently obtained by transetherification of vinyl alkyl ethers with allylic alcohols. Typical examples of this rearrangement are represented in Scheme 2.155. The transformation of allyl vinyl ether 482 into aldehyde 483 illustrates the unique potential of the Claisen rearrangement as a method to prepare angularly substituted derivatives from readily available precursors such as 484, a goal hardly achievable by other routes. Products of this type are used as key intermediates in the syntheses of many natural compounds. ... 

Propanol doped on dehydrated alumina reduces various aldehydes and ketones at room temperature to the corresponding alcohols. a,p-Unsaturated aldehydes are selectively reduced under these conditions to the corresponding allylic alcohols. For example, citral is converted to geraniol in 88% yield. a,p-Unsaturated nitriles are reduced to saturated nitriles with triethylamine-formic acid azeotrope in... 

The metal catalysis method has been used for the preparation of simple enols, by isomerization of allylic alcohols, for example, these enols are stable enough for isolation (see p. 231), but slowly tautomerize to the aldehyde or ketone, with half-lives ranging from 40 to 50 min to several days. ... 

A further interesting extension of the allyl alcohol epoxidation reaction is the metal-catalyzed direct hydroxy-epoxidation of alkenes. In such a reaction an ene reaction with singlet oxygen, 02, or Schenk reaction forming an ally lie hydroperoxide is combined with metal-catalyzed intramolecular epoxidation to form epoxy alcohols For example, with 2,5-dimethyl-2,4-hexadiene (83) two different isomers, 84 and 85, were obtained with Ti(OPr-/)4 and VO(acac)2, respectively (equation 16). Similar reactions were also car-... 

The chiral bis(sulfonamide) catalyzed cyclopropanation was extended successfully to y-silyl-and y-stannyl-substituted allylic alcohols, for example, giving cyclopropanes 90 and 91.109... 

Both enantiomers of the racemic 11,17-diketone (428) were reduced by Rhodotorula ntucilaginosa to 17S-alcohols, but the antipode having 13S-chirality was further reduced to the 11-alcohol Reoxidation then afforded the enantiomeric forms of (428). Similar high stereospecificity was observed in the microbiological reduction of 6,7-dihydro-(428), where reduction of the 11-ketone in the 13S,17S-antipode occurred to give the allylic alcohol Further examples of the differential metabolism of racemic 19-nortestosterone and its 13-ethyl homologue by Curvularia lunata have been described. ... 

Allylic alcohols, for example geraniol, 2-methylallyl alcohol, 3,3-dimethylallyl alcohol, 3-buten-2-ol, l-octen-3-ol, and l-hexen-3-ol, are epoxidized with tert-butyl hydroperoxide in the presence of a vanadyl salen oxo-transfer catalyst in supercritical CO2. The metal catalyst was prepared in a simple two-step, Schiff base-type reaction to form the salen ligand, followed by complexation to the vanadyl group. The use of non-toxic supercritical CO2 in the presence of the new epoxidation vanadium catalyst led to yields and diastereoselectivities that were comparable to those resulting from the use of environmentally hazardous solvents such as CH2CI2 [59]. 

Isomerization of vinylcarbinols to ally lie alcohols. Treatment of a tertiary vinylcarbinol, available by the reaction of a ketone with a vinyhnagnesium halide, with acetic acid-acetic anhydride and a trace of p-toluenesulfonic acid at 20° (cooling) for several hours results in isomerization to the acetate of the isomeric primary allylic alcohol. An example is the preparation of 3,3-dimethylallyl alcohol from 2-methyl-3-butene-2-ol. 

Cyclohexenones with an alkyl group at the -position are reduced to mixtures of allylic alcohols for example, the carbonyl group of (5) is reduced in 83% yield... 

Conversion of ailylic alcohols into rearranged allylic amines. This transposition can be accomplished by conversion of an allyhc alcohol, for example geraniol (1), into the trichloroacetimidate (2) by treatment with sodium or potassium hydride and then with trichloroacetonitrile (ether, -10° and then 20°). This allylic imidate undergoes a [3.3] sigmatropic rearrangement when... 

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