15 Fakultätsübergreifend / Sonstige Einrichtung
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Item Open Access Diastereoselective synthesis of α-Hydroxy- and α-amino-indolizidines and -quinolizidines : evidence for a novel cyclization/hydride migration mechanism in the TiCl4-induced reaction of prolinalbenzylimines by deuterium labeling studies(1994) Laschat, Sabine; Grehl, MatthiasLewis acid-catalyzed cyclization of prolinal and 2-piperidine-carbaldehyde benzylimines 11, 12 results in the diastereoselective formation of α-amino-β-alkyl-substituted indolizidines 15, 17, 19, 21 and -quinolizidines 16, 18, 20, respectively. Both diastereoselectivity and constitution depend on the Lewis acid. FeCl3 yields α,β- trans-α-(benzylamino)-β-isopropenyl derivatives 15 and 16, probably by a cationic cyclization via carbenium ions 32a, b. In contrast, TiCl4 yields ,α,β- cis-α-(benzylideneamino)-β-isopropyl derivatives 19 and 20 by a novel cyclization/intermolecular hydride transfer mechanism, which was supported by deuterium labeling studies. Compounds 15, 16, 19, and 20 were converted to the diastereomeric acetamides 24, 25 and 28, 29. By an analogous cyclization of the aldehydes 8 and 9 only α ,β-cis-α-hydroxy--isopropenylindolizidines 51 and -quinolizidines 52 were obtained irrespective of the Lewis acid used. The structures of 30 and 52 were elucidated by X-ray analysis.Item Open Access Carbohydrates as chiral templates : diastereoselective synthesis of N-glycosyl-N-homoallylamines and β-amino acids from imines(1991) Laschat, Sabine; Kunz, HorstComplexing properties and chirality of carbohydrates were utilized in diastereoselective reactions of 0-pivaloylated N-galactosylimines with allylsilanes and -stannaries. With dyltrimethylsilane in the presence of SnCl4, imines 2 of aromatic and heteroaromatic aldehydes were converted to homdylamines 3, giving ratios of diastereomers higher than 7:1. No addition products derived from α-anomeric aromatic imines were formed. Aliphatic homdylamines 3 were synthesized by using allyltributylstannane in the presence of SnCl4. Both α- and β-anomeric aliphatic imines reacted with the allylstannane. They gave the same ratio of diastereomers and showed the same sense of asymmetric induction.Item Open Access Carbohydrates as chiral templates : stereoselective synthesis of chiral homoallyl amines and β-amino acids(1990) Laschat, Sabine; Kunz, HorstEnantiomerically pure homoallyl amines and β-amino acids are synthesized using the diastereoselective Lewis acid induced addition of allylsilanes and allylstannanes to Schiff bases of O-aryl protected galactopyranosylamine.Item Open Access Structure of two octahydroacridines(1994) Fröhlich, Roland; Grehl, Matthias; Kramm-Glade, Sigrid; Laschat, SabineA cis-decalin conformation with congestion at the central heterocyclic ring is found for cis-5,9,9-trimethyloctahydroacridine, C16H23N, and a slightly twisted trans-decalin conformation is found for (3RS,-4aRS,9aSR)-3,5,9,9-tetramethyloctahydroacridine, C17-H25N.Item Open Access Diastereoselektive Synthese von Amino-substituierten Indolizidinen und Chinolizidinen aus Prolinalimin bzw. 2-Piperidincarbaldimin durch intramolekulare Hetero-En-Reaktion(1994) Laschat, Sabine; Grehl, MatthiasWir haben uns mit der Frage beschäftigt, inwieweit mit solchen Hetero-En-Reaktionen diastereoselektiv substituierte Indolizidine und Chinolizidine hergestellt werden können, insbesondere im Hinblick darauf, daß eine Reihe dieser Alkaloid-Derivate wirksame Glucosidase-Inhibitoren sind und wegen ihrer Anti-HIV-Aktivitat zunehmendes Interesse gewinnen.Im folgenden berichten wir über eine neue formal intramolekulare Lewis-Säure-katalysierte Imino-En-Reaktion zum diastereoselektiven Aufbau von 7-Alkenyl-8-amino-substituierten Indolizidinen und von 2-Alkyl-1-amino-substituierten Chinolizidinen, bei der sich die relative Konfiguration der beiden neugebildeten Stereozentren durch die Wahl der Lewis-Säure steuern läßt.Item Open Access Carbohydrates as chiral templates : stereoselective synthesis of (R)- and (S)-α-aminophosphonic acid derivatives(1992) Laschat, Sabine; Kunz, HorstThe stereoselective synthesis of diethyl (S)- or (R)-α-[(O-pivaloyl-hexapyranosyl) amino]benzylphosphonates is achieved via Lewis acid catalyzed addition of diethyl phosphite to O-pivaloylated N-benzylidene -β-D-galactosylamine or N-benzylidene-α-D-arabinopyranosylamine. The process can also be performed by a one-pot procedure selectively giving (S)-aminophosphonic acid derivatives from galactosylamine and (R)-aminophosphonic acid derivatives from β-L-fucosylamine as the chiral auxiliaries.Item Open Access Carbohydrates as chiral templates : stereoselective synthesis of (R)-homoallyl amines using L-fucose as the auxiliary formally enantiomeric to D-galactose(1990) Laschat, Sabine; Kunz, Horst(R)-Homoallyl amines are synthesized with high asymmetric induction using the diastereoselective Lewis acid induced addition of allylsilanes and allylstannanes to Schiff bases of O-acyl protected fucopyranosylamine.Item Open Access Intramolecular hetero-Diels-Alder reaction of N-arylimines : applications to the synthesis of octahydroacridine derivatives(1993) Laschat, Sabine; Lauterwein, JürgenA new intramolecular Lewis acid catalyzed hetero-Diels-Alder reaction of N-arylimines 5 with nonactivated olefins tethered to the 2-azadiene system was developed in order to prepare 1,2,3,4,-4a,9,9a,l0-octahydroacridined erivatives 6. Both reactivity and cis/ trans selectivity of 6 were mainly dependent on the substitution pattern in the 3-position of the cyclization precursor 5. The type of Lewis acid plays only a minor role in determination of the cis/trans ratio. The synthetic utility of this new cyclization was increased by performing it as a one-pot reaction. Both absolute configuration and preferred conformation were assigned by detailed NMR studies.Item Open Access Stereoselective synthesis of amino acid derivatives using carbohydrates as templates(1993) Kunz, Horst; Sager, Wilfried; Pfrengle, Waldemar; Laschat, Sabine; Schanzenbach, DirkGlycosylamines contain the easily cleavable semi-aminal-type N-glycosidic bond. O-Protected glycosylamines, therefore, can advantageously be used as a form of "asymmetric ammonia", for instance, in Strecker syntheses and in Ugi reactions to give amino acid amides as well as in modifications of the Mannich reaction.Item Open Access Novel (imino-η6-arene)chromium-complexes and their diastereoselective intramolecular hetero-Diels-Alder reactions(1993) Laschat, Sabine; Noe, Ralf; Riedel, Michael; Krüger, CarlNovel (imino-η6-arene)chromium complexes 3 with nonactivated olefins tethered to the 2-azadiene system were prepared and further used in a new diastereoselective intramolecular Lewis-acid-catalyzed hetero-Diels-Alder reaction to give (η6-octahydroacridine)chromium complexes 4. The trans selectivity of the cyclization reaction is mainly controlled by the Cr- (CO)3 fragment and to a minor extent by the catalyst, solvent, and substituents. Complexes 4 also could be obtained by a one-pot reaction starting from aldehyde 1, (o-toluidine)Cr(CO)3 (2), and Lewis acid. The stereochemistry of 4a was established by single-crystal X-ray structure analysis.