Alcohols optically pure allylic

The optically pure ligands C6H5C H(Me)N(CH2CH2PPh2)2 (R)-(+)- and (A)-(-)-pnp, in the presence of [Ir(cod)(0 Mc)]2 and propan-2-ol, selectively reduce a,/3-unsaturated ketones to optically pure allylic alcohols.604... 

Still other epoxide cleavage processes can be nucleophile induced, but without direct attack at a heterocyclic carbon. A good illustration of this phenomenon is seen in the alkylative elimination of epoxy tosylhydrazones (62). Optically pure allyl alcohols have been prepared... 

The method is a very promising one it has been demonstrated to be the best route to date for the synthesis of optically pure allyl alcohols. The Ti-alkoxide-tartrate complex is more erythro- lective than VO(acac)2-TBHP in the absence of tartrate, the reaction is generally fhreo-selective. Allyl alcohol epoxide intermediates of great use in the nthesis of natural products may be pre-... 

Baker s yeast was employed in the reduction of 21 to the optically pure allylic alcohol 22. Pd(2) catalyzed hydride transfer of the chiral intermediate gave the indenone 23 required as an intermediate in the synthesis of a putative antihypertensive drug. At variance with the example cited above, the double bond was unaffected by the yeast [48] (Scheme 8). 

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

Related catalytic enantioselective processes It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [27] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [28]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect the resolution of five- and six-membered allylic carbinols [29] in this instance, as with the Ti-catalyzed procedure, the presence of an unprotected hydroxyl function is required. Perhaps the most efficient general procedure for the enantioselective synthesis of this class of cyclic allylic ethers is that recently developed by Trost and co-workers, involving Pd-catalyzed asymmetric additions of alkoxides to allylic esters [30]. 

One of the earliest and most important discoveries in metal-catalyzed asymmetric synthesis is Sharpless s Ti-catalyzed epoxidation of allylic alcohols. A mere mention of all the total syntheses that have used this technology would require a separate review article Here, we select Trost s masterful total synthesis of solamin (100, Scheme 14), for its beautiful and multiple use of Sharpless s asymmetric epoxidation.1161 Optically pure epoxy alcohol 95 is converted to both epoxy iodide 96 and diol 97 The latter two intermediates are then united to give 98, which is oxidized and converted to dihydrofuran 99 by a Ramberg-Backlund transformation. The Re catalyzed butenolide annulation that is used to afford the requisite unsaturated lactone only adds to the efficiency of this beautiful total synthesis. 

Enantioselective epoxidation of allylic alcohols using t-butyl peroxide, titanium tetra-wo-propoxide, and optically pure diethyl tartrate. 

Substituted acrylates (which reseitible the enamide substrates employed 1n asymmetric hydrogenation) may be deracemized by reduction with an optically active catalyst, especially DIPAMPRh . Selectivity ratios of 12 1 to 22 1 have been obtained for a variety of reactants with compounds of reasonable volatility, separation of starting material and product may be effected by preparative GLC. Recovered starting material can then be reduced with an achiral catalyst to give the optically pure anti product. Examples of kinetic resolutions by this method are given in Table II. More recently very successful kinetic resolutions of allylic alcohols have been carried out with Ru(BINAP) catalysts. 

An overview of the yields and enantioselectivities obtained in the asymmetric epoxidation of allylic alcohols with different published known optically pure hydroperoxides is given in Table 19. Here, only the results for the best hydroperoxide of one type (highest asymmetric induction) are presented. 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allylic alcohols 39 (Scheme 69).235 Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (S,S)-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity dr 92 8) as racemic mixture. The ( )-allylic alcohols react totally unselectively threoIerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threo/erythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S)- and (/ ,S)-146 ee >99%) and the... 

The Bayhs-Hillman reaction of optically pure azetidine-2,3-diones [235, 236] with methyl vinyl ketone in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) in acetonitrile at -20°C for 1 h have been reported to give functionalized allylic alcohols, having the p-lactam scaffold, in good yields (80%) and complete diastereoselectivity [237]. In terms of achieving good yields with a reasonable rate of reaction, 50 mol% of DABCO seemed to be the catalyst amount of choice for this reaction. No significant solvent effect was observed in the overall yield (Scheme 108). 

The ability of the Sharpless epoxidation catalyst to differentiate between the two enantiomers of an asymmetric allyl alcohol affords a powerful synthetic tool to obtain optically pure materials through kinetic resolution.43 Because the procedure relies on one enantiomer of a secondary allyl alcohol undergoing epoxidation at a much faster rate than its antipode, reactions are usually run to 50-55% completion.22 In this way, resolution can often be impressive.18 25 26 44-46 An increase in steric bulk at the olefin terminus increases the rate of reaction.46 47... 

Diastereoselective allylation of optically pure sulfinyl dienal complexes using tributyl allyltin can be obtained (Scheme 138). 2,4-Hexadien-1,6-dial iron tricarbonyl complex (88) undergoes nucleophilic addition reactions with diaUcylzincs in the presence of a catalytic amount of an optically active amino alcohol (Scheme 139). Very high enantio-and diastereoselectivity is observed. Related reactions of (88) with chiral allyl boronic esters give allylated alcohols in very high enantiomeric excess. 

Yamamoto and Maruoka investigated the reaction of chiral acetals with organoaluminum reagents. Unprecedented regio- and stereochemical control was observed in the addition of trialkylaluminums to chiral a,/3-unsaturated acetals derived from optically pure tartaric acid diamide [83]. The course of the reaction seemed to be highly influenced by the nature of substrates, solvents, and temperature. These findings provide easy access to optically active a-substituted aldehydes (84), /3-substituted aldehydes (85), a-substituted carboxylic acids (86), or allylic alcohols (87). Because optically pure RJi)- and (5,5)-tartaric acid diamides are both readily available, this method enables the predictable synthesis of both enantiomers of substituted aldehydes, carboxylic acids, and allylic alcohols from a,/3-unsaturated aldehydes (Sch. 54). 

A modified aluminum hydride prepared by treating LiAlH4 in THE with equimolar amounts of ethanol and optically pure S-(-)-2,2 -dihydroxy-1,1 -binaphthyl, gives allylic alcohols of very high optical... 

Elimination of silyl groups with p-oxy groups, i.e., Peterson-type elimination, is a useful method for preparing stereodefined alkenes [101,102]. The synthetically useful allylsilanes are effectively synthesized in geometrically and enantio-merically pure forms through the Peterson-type elimination of organosilanes prepared by palladium-catalyzed bis-silylation (Eq. 57) [72, 73]. The intramolecular bis-silylation of optically active allylic alcohols in refluxing toluene af-... 

The Simmons-Smith cyclopropanation of the same 3-(2-phenylcydopropyl)prop-2-enol (110) in its racemic form afforded an inseparable mixture of and a t/-bicyclopropanes 111 in a 1.3 1 ratio. Therefore, the Charette protocol (p 286) was used first to prepare the required allylic alcohol in its optically pure form and the cyclopropanation was carried out in the presence of the tartrate derived dioxaborolane 93. By using (+)- and (— )-tartrate derived dioxaborolane 93, both the syn- and antf-bicyclopropyls 111 were obtained. The diastereoselectivities observed in their formation were consistently greater than 12 1. ... 

The lipase-catalysed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated by Itoh et al. [16] (Fig. 7.7). They found that the acylation rate was strongly dependent on the counter anion of the imidazolium salt, while the lipase-catalysed acylation proceeded with high enantioseleclivity in all ionic liquid tested. Good results were obtained when the reaction was carried out in [bmimT [PFg ] or [bmun" ][BF ]. Other examples of kinetic resolution of allylic alcohols catalysed by lipases in ionic liquids were also reported by these authors [71, 72]. The transesterification of 5-phenyl-l-penten-3-ol under reduced pressure at 27 hPa and 40°C was carried out using methyl phenylthioacetate as acyl donor in [bmim+] [PF ] and [bdmim ][BF ], for obtaining the corresponding acylated compound in optically pure form [71], The acetylation of methyl mandelate catalysed by immobilised P5L in [bdmim ][BF ] is another example reported by these authors about the successful application of ionic liquids as reaction media in racemic resolutions... 

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