Optically pure 3-carbon compounds

Optically pure 3-carbon compounds from carbohydrates... 

There are several methods for the preparation of optically pure 3-carbon compounds from carbohydrates. These fall into 3 major categories namely the cleavage of carbohydrate chains to form the desired compounds directly, the transformation of a (larger) carbohydrate-derived fragment to a 3-carbon one and the use of carbohydrates as a chiral auxiliaiy to induce chirality into a pro-chiral 3-carbon fragment. We will review the first approach and discuss our efforts in the other two areas more fully. 

An interesting historic parallel (91) can be drawn to van t Hoffs proposal of the tetrahedrd geometry for groups bound to a tetra-valent carbon atom because its original appearance was also in the form of an obscure pamphlet in Dutch (Utrecht, 1874) however, van t Hoffs work was quickly translated into French (1875) and German (1876) for publication in vehicles having considerably wider circulation. In Zeelen s case, only the synthetic aspects of his dissertation were subsequently published in the journal literature (92). However, the later paper does provide a cautionary note that the surfactant compounds employed in the film balance studies may not have been optically pure because racemization during their synthesis was possible. 

By using either one of these photosystems, one-electron (3-activation of a,(3-unsaturated carbonyl compounds produced carbon-centered radical precursors which cyclize efficiently and stereoselectively to tethered activated olefins or carbonyl groups. The 1,2-anti-stereochemistry observed contrasts with the general trend of syn-stereochemistry expected in 5-hexenyl radical cyclizations. Application of this methodology was successfully demonstrated by the stereoselective synthesis of optically pure C-furanoside, starting from L-tartaric acid (Scheme 38) [57,58]. 

The obtained compounds were purified and characterized by H, C and Sn NMR, using CDCI3 with tetramethylsilane as the standard. The results indicated that the compounds were optically pure, without epimerization in the carbon atom of the menthyl group bonded to the tin atom. 

Hydrolysis of ( )-colchicine with 0.1 N HC1 affords ( )-colchiceine, and ( )-deacetylcolchiceine is obtained on hydrolysis of ( )-colchicine with 20% H2SO4 and AcOH. Treatment of ( )-deacetylcolchiceine with trifluoroacetic anhydride afforded a trifluoroacetamide which, on methyl-ation with diazomethane, gave a mixture of ( )-trifluoroacetyldeacetyl-colchicine and ( )-trifluoroacetyldeacetylisocolchicine. The latter compounds were separated by chromatography and hydrolyzed with potassium carbonate in acetone/water to give ( )-deacetylcolchicine and ( )-deacetylisocolchicine (37). An easy chemical resolution of ( )-de-acetylcolchicine with 10-camphorsulfonic acid in methanol afforded the optically pure amines, and (+)- and (-)-colchicine after N-acetylation. 

A noteworthy synthetic application for the reaction of J with a,g-unsaturated phosphoryl compounds is represented by the addition involving hitherto unknown (-)-(Sp)-methylphenylvinylphosphine oxide J 2. The resulting tertiary phosphine oxides 13 with saturated carbon chains and known stereochemistry at phosphorus constitute attractive starting materials for the preparation of optically pure phosphines. The organophosphorus substrate 12 was obtained by decarbomenthoxylation of the enantiomeric ester... 

Enantiomeric separations of amino acids and short peptides are performed using either a direct or the indirect approach [10]. The indirect approach employs chiral reagents for diasteromer formation and their subsequent separation by various modes of CE. The direct approach uses a variety of chiral selectors that are incorporated into the electrolyte solution. Chiral selectors are optically pure compounds bearing at least one functional group with a chiral center (usually represented by an asymmetric carbon atom) which allows sterically selective interactions with the two enantiomers. Among others, cyclodextrins (CDs) are the... 

Optically active organometallic compounds, especially pseudotetrahedral half-sandwich complexes of the type Cp(OC)(Ph3P)Fe-R with iron as the center of chirality have been extensively investigated in the past. However, synthetic utilization of stereocontrol by the chiral iron has been limited to systems with a-bonded carbon ligands [2]. In contrast, silicon-iron complexes have not yet found analogous application. In context with our studies concerning metallo-silanols we have established simple routes to isolate diastereomerically pure derivatives with a chiral iron fragment. 

The solid state X-ray structure will also give information about the configurational, stereochemical or tautomeric information about the studied structure. In the case of optically pure compounds, the absolute configuration of an unknown compound can be obtained, if the molecule studied contains at least one heavier element (like Cl, S, or metal ion) per 20 carbon atoms. If compound 1 would be optically active, the chlorine atom would enable the determination of the absolute configuration. 

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