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Wybrane zastosowania bornan-10,2-sultamu (sultamu Oppolzera) w asymetrycznej syntezie organicznej

Szymańska A., Wieczerzak E., Łankiewicz L.

Wiadomości Chemiczne

2000

[Z] 54, 9-10

759-791

Among many methods of asymmetric synthesis, the second-generation reactions i.e. the ones applying chiral auxiliary to generate new chiral centers are the most popular and widely used. In the last two decades many new and highly effective chiral auxiliaries were described. Bornane-10,2-sultam, introduced by Oppolzer and co-workers, is one of the most popular and most effective in generating new chiral centers. Utility of bornane-10,2-sultam is due to: - simplicity of a method of its synthesis from the natural, cheap source - camphore, - high yield of connecting to prochiral substrates, - almost quantitative asymmetric induction, - straight-forward removal of the formed product from bornane-sultam, which does not cause the loss of chirality at a new chiral center , - its high recovery after removal of product and possibility of multiple applications. One of the most popular reactions studied in asymmetric synthesis is aldol condensation. This reaction was also studied with application of bornane-10,2-sultam. Depending on the type of used base and Lewis acid either pure "cis" or "trans" isomer can be obtained. The chemical yields of the reactions were also high (50÷90%) N-acylated derivatives of bornane-10,2-sultam have been revealed also as facile precursors of chiral alkylated carboxylic acids, ketones, aldehydes and alcohols. Diastereoisomeric excesses were in many cases above 99% and chemical yields were also very good (in most cases above 80%). Alkylation step can be also applied for asymmetric synthesis of a-amino acids (building blocks of peptides) but it is necessary to use N-[bis(methylthio)methylene]glycine methyl ester for acylation of the sultam. Quite nice selection of amino acids in optically pure form can be obtained by this method. Other possibilities of the sultam usage arise from nucleophilic and electrophilic amination reactions. These methods were applied to obtain amino, azido, hydrazino and N-hydroxyamino acids with high chemical yield and optical purity. Applications of bornane-10,2-sultam derivatives with a,b-unsaturated side chain are also very wide in asymmetric organic synthesis. One of the most popular reaction involving unsaturated compounds is Diels-Alder's cycloaddition. Cyclic compounds formed during this addition are mostly endo-structures. Stereoselectivity is very high (higher than 90 %) and depends on a type of Lewis acid used. Another interesting precursor in this kind of addition reaction seems to be the sultam derivative obtained by the modification with glyoxalic acid. Its application can lead to piran derivatives and substrates of sugar moieties. N-acylated with a,b-unsaturated carboxylic acids sultam derivatives are also good substrates for carbonyl compounds stereoselectively substituted on a- or/and b-carbons (aldehydes, ketones, carboxylic acids) as well as alcohols. For preparation of optically active a,b-disubstituted compounds either Grignard's reagents or Gilman's reagents can be applied with diastereoselectivity in range 70÷95 %. Other possibilities of the "unsaturated" sultam derivatives usage arise from reactions with "hydrogen sources" or oxidation agents. Asymmetric hydride addition or catalytic hydrogenation leads to a,b-disubstituted chiral compounds with good yield and stereoselectivity. Oxidation of double bond, on the other hand, is facile method for obtaining 1,2-diols - superior substrates for ketones, acetals and sugars.