Alcohols, acetylenic, resolution

In practice, oxidations of this type have been observed and generally have been carried out with a substrate bearing a racemic secondary alcohol so that kinetic resolution is achieved. Although these oxidations are not strictly within the scope of this chapter, they are summarized briefly in Eqs. 6A.7-6A.9 to acquaint the reader with other potential uses for the Ti-tartrate catalytic complex. In the kinetic resolutions shown in Eqs. 6A.7 and 6A.8, the oxidations are controlled by limiting the amount of oxidant used to 0.6 equiv. Only modest resolution was attained for the acetylenic alcohol (Eq. 6A.7, 21% ee) [77] and the allenic alcohol (Eq. 6A.8, 40% ee) [77]. Resolutions of the furanols [142] or the thiophene alcohols [143] of Eq. 6A.9 generally are excellent (90-98% ee except when Rj is a I-butyl group). Only in the kinetic resolution of the furanols has the oxidation product been identified and, in that case, is a dihydropyranone. 

Resolution of tert-acetylenic alcohols. Brucine forms stable 1 1 molecular complexes with only one enantiomer of several terr-acetylenic alcohols. In some liivorublc eases, complete resolution can be achieved by only one complexation in oilier eases, repetition of complexation is necessary for complete resolution. The complexes are decomposed by dilute HC1. Complexation involves a hydrogen bond between the OH group and the N atom of brucine in addition, the linearity of the acetylene group may be involved.1... 

Toda, F., and Tanaka, K. (1981) ANew Optical Resolution Method of Tertiary Acetylenic Alcohol Utilizing Complexation with Brucine, Tetrahedron Lett., 22, 4669-4672. 

This procedure required but one resolution of the acetylenic alcohol 40 which then served to resolve the remaining chiral portion of the molecule. The resolution of octyn-3-ol 40 therefore was the start of the synthesis of the optically active 7-oxaprostanoids. Reaction of the racemic octyn-3-ol 40 with phthalic anhydride gave the phthalyl acid 41 which formed the crystalline salt 42 by reaction with ( )-o -phenethylamine with the absolute configuration shown. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

Brucine is recommended as a convenient and effective reagent for the resolution of tertiary acetylenic alcohols. The evidence available to date indicates that the acetylenic alcohol must have two aryl groups, or one aryl group and one bulky alkyl group, attached to the hydroxyl-bearing carbon atom. The X ray crystal structure analysis of one such brucine complex was also reported. 

ABSTRACT. Novel optical resolutions of guest compounds by inclusion complex formation with optically active host compound are reviewed Tertiary acetylenic alcohols, cyanohydrins, and secondary alcohols were resolved by complexation with alkaloids such as brucine or sparteine. Cycloalkanones, 2,3 -epoxycyclohexanones, and some other neutral compounds were resolved by complex formation with optically active diacetylenic diol. Mutual optical resolution of bis-g-naphthol and sulfoxides by complex formation was also reviewed. 

Optical Resolution of Tertiary Acetylenic Alcohols by Complex Formation with Brucine... 

An optically active acetylenic alcohol is an useful starting material to prepare various chiral compounds, because it has two functional groups. However, the optical resolution of an acetylenic alcohol by the diastereomeric method for its phthalic acid half-ester is complicated and successful only in a few cases,1 Recently, the preparation of optically active secondary acetylenic alcohol by the enantioselective reduction of ethynyl ketone or by the enantioselective addition of lithium acetylide to aldehyde has been reported. However, these methods are not applicable to the preparation of optically active tertiary acetylenic alcohols. 

We found that some tertiary acetylenic alcohols form a 1 1 complex with brucine, and that the acetylenic alcohols v ere easily resolved by utilizing the complexation. As an example, the experimental detail of the resolution of 1,1-di-methyl-2-phenyl-3-pentyn-2-ol is described. A solution... 

We also found that optically active sparteine can be used instead of brucine for the optical resolution of tertiary acetylenic alcohols and 2), and that sparteine can also be resolved by complexation with optically active acetylenic alcohols,5Efficiency of the optical resolution of acetylenic alcohols with sparteine was compared to that with brucine (Table 2), In some cases, optical resolution by complexation with sparteine is more efficient than that with brucine, and the use of the much less poisonous sparteine also has advantages over the use of the poisonous brucine. 

The method used for the optical resolution of acetylenic alcohols by complexation with brucine was found to be applicable to cyanohydrins ( ) 7 and some secondary alcohols ( ). Surprisingly, it was also found that racemic cyanohydrins are converted into one optically active isomer in yields of more than 50% in the presence of brucine. For example. When a solution of racemic (1.0 g) and an equimolar amount of brucine (2.1 g) in MeOH (2 ml) was kept in an uncapped flask at room temperature for 24 h, a brucine complex of (+) J crystallized out. Decomposition of the complex gave 100% ee (+)- in almost quantitative yield. This is not a simple optical resolution method but a novel preparation method of optically active cyanohydrins. This eantiomerization method can be applied to... 

The first example of chiral polymer from a disubstituted acetylene is a polyd-trimethylsilyl-l-propyne)-based polymer, poly(46), which was synthesized in moderate yields using TaCls-PhaBi (112). Poly(46) displays small optical rotations, and its molar ellipticities of the Cotton effects are up to a few hundreds. The main chain of poly(46) is, therefore, not a well-ordered helix. This is probably because of the less controlled geometrical structure (cis and trans) of the polymer backbone. However, the free-standing film of this polymer achieves an enantioselective permeation of various racemates including alcohols and amino acids. For example, the concentration-driven permeation of an aqueous solution of tryptophan by poly(46) gives 81% enantiomeric excess (ee) of the permeate at the initial stage. A characteristic of the membrane of poly(46) is its ability to enantioselectively recognize 2-butanol and 1,3-butanediol, because the direct resolution of these racemates by hplc is impossible. 

Coke and Richon have constructed the 8-lactone framework of n-hexadecalactone, the proposed pheromone isolated from Vespa orientalis, through lactonization of a hydroxy acid intermediate [39] (Scheme 3). The optically pure amino alcohol 22 obtained by resolution was converted to the optically active epoxide 23 by quatemization, followed by Hofmaim elimination. Addition of the dianion of propiolic acid to epoxide 23, and subsequent reduction of the resulting acetylenic hydroxy acid with hydrogen and palladium, provided the saturated hydroxy acid 25, which spontaneously cycUzed to afford 8-lactone 12. In a similar way, the enantiomer of amino alcohol 22 was also transformed into the antipode of lactone 12. Furthermore, using this method, any terminal epoxide can easily be converted to the corresponding saturated 8-lactone in two steps. 

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