Search results for "CHIRAL AUXILIARY"
showing 10 items of 28 documents
ChemInform Abstract: Carbohydrate Auxiliaries in Stereoselective Syntheses of Decahydroquinoline Alkaloids.
2010
Using tetra-O-pivaloyl-β-D-galactopyranosylamine as the chiral auxiliary, both trans- and cis-annelated decahydroquinoline alkaloids can be synthesized stereoselectively. This methodology of asymmetric synthesis is based on the effect that both enantiomers of 2,6-disubstituted piperidin-4-ones are selectively and alternatively accessible using the auxiliary as the identical stereodifferentiating tool. In addition, the carbohydrate auxiliary controls the stereoselective protonation of enolates formed by conjugate addition of cuprates to N-galactosyl octahydroquinolin-4-ones. The syntheses of trans-4a-epi-pumiliotoxin C and cis-4a-epi-perhydro-219A illustrate this concept of asymmetric synthe…
Recent Advances in Asymmetric [3,3]-SigmatropicRearrangements
2003
The synthesis of new complex structures is still a challenge in preparative organic chemistry. Focusing on the generation of defined stereogenic centers, the [3,3]-sigmatropic rearrangements are known as reliable reactions. Always, a highly ordered transition state must be passed through, which allows the shift of chiral information from the reactant into the nascent product. Generally, the complete [1,3]- and, frequently, the [1,4]-chirality transfer enables one to predict the configuration of the new centers. This review focuses on Claisen and Cope rearrangements, which adopt the chiral information via a so termed asymmetric induction. This means, that the directing chiral subunit is plac…
ChemInform Abstract: Enantioselective Syntheses of Dopaminergic (R)- and (S)-Benzyltetrahydroisoquinolines.
2010
Optically pure (1S,R)- and (1R,S)-benzyltetrahydroisoquinolines (BTHIQs), 12a,b as the major diastereomers, were prepared by stereoselective reduction of the isoquinolinium salt possessing (R)- and (S)-phenylglycinol as the chiral auxiliary, respectively. The absolute configurations of (1S,R)-13a hydrochloride (O-debenzoylated derivative from 12a) and (1R,S)-12b diastereomers were unambiguously determined by single-crystal X-ray analysis. Reductive removal of the chiral auxiliary group, subsequent N-propylation, and cleavage of the methylenedioxy group furnished the optically active catecholamines (1S)-16a and (1R)-16b in good overall yield. We have separately prepared for the first time pa…
Carbohydrate Auxiliaries in Stereoselective Syntheses of Decahydroquinoline Alkaloids
2002
Using tetra-O-pivaloyl-β-D-galactopyranosylamine as the chiral auxiliary, both trans- and cis-annelated decahydroquinoline alkaloids can be synthesized stereoselectively. This methodology of asymmetric synthesis is based on the effect that both enantiomers of 2,6-disubstituted piperidin-4-ones are selectively and alternatively accessible using the auxiliary as the identical stereodifferentiating tool. In addition, the carbohydrate auxiliary controls the stereoselective protonation of enolates formed by conjugate addition of cuprates to N-galactosyl octahydroquinolin-4-ones. The syntheses of trans-4a-epi-pumiliotoxin C and cis-4a-epi-perhydro-219A illustrate this concept of asymmetric synthe…
Synthesis of a polymer-bound galactosylamine and its application as an immobilized chiral auxiliary in stereoselective syntheses of piperidine and am…
2004
A 2,3,4-tri-O-pivaloylated beta-D-galactopyranosyl azide bearing a hydroxy-functionalized spacer unit at the C-6 position of the galactose was synthesized and immobilized on the solid phase by using a polymer-bound chlorosilane. The azide was reduced to the corresponding galactopyranosylamine, which served as a versatile chiral auxiliary in highly diastereoselective Ugi four-component condensation reactions at ambient temperature. Fluoride-induced cleavage from the polymeric support furnished N-glycosylated N-acylated alpha-amino acid amides. The reaction of the immobilized galactosylamine with aldehydes gave rise to the corresponding aldimines, which underwent a domino Mannich-Michael cond…
Synthesis, configuration, and calcium modulatory properties of enantiomerically pure 5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylates.
1992
Enantiomerically pure hexahydroquinolinones of the structural type 9 were prepared by a variation of the Hantzsch synthesis in which an optically active acetoacetate served as a chiral auxiliary reagent. Determinations of the de and ee values are described. The absolute configurations of the optically pure products were characterized by single-crystal X-ray analysis. The antipodes 9a and 9b exhibited calcium antagonistic activities on smooth musculature; the (S)-(-)-enantiomer 9b was the more potent compound with regard to the EC50 values which differed by a factor of 100; the intrinsic activity of 9b was 1.2, compared with a value of 0.54 for 9a. On the other hand, R-(+)-9a exerted positiv…
Enantioselective syntheses of dopaminergic (R)- and (S)-benzyltetrahydroisoquinolines.
2001
Optically pure (1S,R)- and (1R,S)-benzyltetrahydroisoquinolines (BTHIQs), 12a,b as the major diastereomers, were prepared by stereoselective reduction of the isoquinolinium salt possessing (R)- and (S)-phenylglycinol as the chiral auxiliary, respectively. The absolute configurations of (1S,R)-13a hydrochloride (O-debenzoylated derivative from 12a) and (1R,S)-12b diastereomers were unambiguously determined by single-crystal X-ray analysis. Reductive removal of the chiral auxiliary group, subsequent N-propylation, and cleavage of the methylenedioxy group furnished the optically active catecholamines (1S)-16a and (1R)-16b in good overall yield. We have separately prepared for the first time pa…
ChemInform Abstract: Cross-Metathesis Reactions as an Efficient Tool in the Synthesis of Fluorinated Cyclic β-Amino Acids.
2009
The synthesis of enantiomerically pure, cyclic, gamma,gamma-difluorinated beta-amino acids with various ring sizes has been carried out with a cross-metathesis (CM) reaction being one of the key steps, followed by a Dieckmann-type condensation to bring about the cyclization. Subsequent catalytic hydrogenation under microwave irradiation with (-)-8-phenylmenthol as a chiral auxiliary led to the successful chemo- and diastereoselective chemical reduction of the resulting cyclic beta-enamino esters. The efficiency and scope of the CM reaction with different types of fluorinated imidoyl chlorides and unsaturated esters has also been studied in order to determine the optimal reaction conditions …
Diastereoselective Intramolecular Additions of Allyl- and Propargylsilanes to Iminium Ions: Synthesis of Cyclic and Bicyclic Quaternary Amino Acids
2010
Chiral imino lactones derived from (R)-phenylglycinol containing an allyl- or propargyltrimethylsilyl group in the side chain readily cyclized in the presence of acidic reagents to afford spirocyclic compounds with high diastereoselectivity. Removal of the chiral auxiliary produced 2-substituted 1-aminocycloalkanecarboxylic acids, whereas further cyclizations by means of metathesis or hydroamination reactions led to bicyclic derivatives of pipecolic acid and proline.