Alcohols, acetylenic, resolution chiral

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. 

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. 

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. 

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. 

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