Alkenes allyl/homoallyl alcohols

Alkene isomerization. Homoallylic alcohols are converted to allylic alcohols, and in the presence of ethanediol, monoethers are obtained. ... 

The asymmetric epoxidation of all four isomeric allylic-homoallylic alcohols of the type 26 and the subsequent hydride reduction of each epoxide to both possible dideoxyheptitols has been reported. " Only three isomers of 26 undergo a diastereoselective epoxidation and it was concluded that the direction of epoxidation for E-alkenes was controlled by the chirality of the allylic alcohol, whereas for Z-configurated olefins the relative stereochemistry between the two alcohols is important. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Employing protocol V with the methanesulfonamide catalyst 122, a 93 7 er can be obtained in the cyclopropanation of cinnamyl alcohol. This high selectivity translates well into a number of allylic alcohols (Table 3.12) [82]. Di- and tri-substi-tuted alkenes perform well under the conditions of protocol V. However, introduction of substituents on the 2 position leads to a considerable decrease in rate and selectivity (Table 3.12, entry 5). The major failing of this method is its inability to perform selective cyclopropanations of other hydroxyl-containing molecules, most notably homoallylic alcohols. 

Recently, Oshima et al. developed the conversion of acid chlorides into the corresponding homoallylic alcohols catalyzed by in r(/ -prepared hydridozirconium allyl reagents (Scheme 41),147 147a The proposed mechanism suggests an initial hydride transfer from the zirconocene crotyl hydride species, in equlibrium with its Cp2Zr(l-alkene),147a to the acid chloride with subsequent allylation to afford the corresponding homoallylic alcohols. 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

Solutions of acetyl nitrate at subambient temperature can react with alkenes to yield a mixture of nitro and nitrate ester products. Cyclohexene forms a mixture of 2-nitrocyclohexanol nitrate, 2-nitrocyclohexanol acetate, 2-nitrocyclohexene and 3-nitrocyclohexene. This illustrates one of the problems of allylic and homoallylic alcohol 0-nitration with this reagent. 

An alternate approach has been developed by Charette and coworkers in which chiral iodomethylzinc phosphates were prepared and tested in the cyclopropanation of unfunctionalized alkenes. Although these reagents were not sufficiently reactive to convert aryl-substituted alkenes (such as indene) to the corresponding cyclopropane, they reacted nicely with protected aryl-substituted allylic and homoallylic alcohols (equation 92) °. Several 3,3 -disubstituted binols were tested and ligand 23 stood out as being the most effective with this class of compounds. The active reagent in this case is a chiral iodomethylzinc phosphate. 

Ab initio calculations also confirm that the use of an allyl magnesium alkoxide in place of the alcohol functionality will lead to high or complete stereoselectivity (138). When homoallylic alcohols are used, the Kanemasa protocol afforded the respective isoxazolines with poor stereoselectivity ( 55 45) in the case of terminal alkenes, but with very high diastereoselectivity (up to 96 4) in the reaction of cis-1,2-disubstituted olefins (136). Extension of this concept to the reaction of a-silyl allyl alcohols also proved feasible and produced the syn (threo) adducts as nearly pure diastereomers (>94 6) (137). Thus, the normal stereoselectivity of the cycloaddition to the Morita-Baylis-Hillman adducts (anti > syn, see above) can be reversed by prior addition of a Grignard reagent (176,177). Both this reversal... 

When either an alcohol or an amine function is present in the alkene, the possibility for lactone or lactam formation exists. Cobalt or rhodium catalysts convert 2,2-dimethyl-3-buten-l-ol to 2,3,3-trimethyl- y-butyrolactone, with minor amounts of the 8-lactone being formed (equation 51).2 In this case, isomerization of the double bond is not possible. The reaction of allyl alcohols catalyzed by cobalt or rhodium is carried out under reaction conditions that are severe, so isomerization to propanal occurs rapidly. Running the reaction in acetonitrile provides a 60% yield of lactone, while a rhodium carbonyl catalyst in the presence of an amine gives butane-1,4-diol in 60-70% (equation 52).8 A mild method of converting allyl and homoallyl alcohols to lactones utilizes the palladium chloride/copper chloride catalyst system (Table 6).79,82 83... 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

For alkenes 87, 88, and 91, the regiochemistry is solely dependent on the steric hindrance of the allylic substituent. It is obvious that the regioselectivity trend for the homoallylic alcohols 85, 86, and 89 is different from that of compounds 87, 88, and 91, respectively, although the hydroxyl group exerts approximately the same steric hindrance as a methyl group. 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

Allylic diethylboranes.1 These boranes (2) can be prepared from 1-methyl-cycloalkenes and 2-alkenes by metallation with trimethylsilylmethylpotassium2 followed by reaction with 1. The products react with acetaldehyde to form homoallylic alcohols (3), which can be converted into a,(3- and p,y-unsaturated ketones. 

In contrast with unreactive, unfunctionalised terminal alkenes, allylic and homoallylic ethers (22, 24) and alcohols (20) from which the product organolithiums (21, 23, 25) can be chelated in a (preferably) five-membered, oxygen-containing ring, carbolithiate rapidly and cleanly.23 Coordination overrides any preference for the lithium to be bonded to the primary carbon, but cannot overcome the unfavourability of forming a tertiary organolithium - 26 gives 27, but 28 cannot be carbolithiated. Coordination to sulfur in similar thioethers 29 works too. 

In contrast to allylic alcdiols, the asymmetric epoxidation of homoallylic alcohols shows the following three general characteristics (i) the rates of epoxidation are slower (ii) enantiofacial selectivity is reversed, i.e. oxygen is delivered to the opposite face of the alkene when the same tartrate ester is used and (iii) the of oiantiofacial selectivity is lower with enantiomeric excesses of the epoxy alcohols... 

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