1,721,148 research outputs found
Enantioselective synthesis of epi-emetine analogues: Control of the facial selectivity in a three-component domino Knoevenagel-hetero-Diels-Alder reaction
No full textThe domino Knoevenagel-hetero-Diels-Alder reaction of the aldehyde rac-8, Meldrum's acid 2 and enol ether 3 leads to the cycloadduct rac-17 as the main product which in a second domino process was transformed into the benzoisoquinolizidine rac-18 by solvolysis, hydrogenolysis, condensation and hydrogenation; rac-18 was used as a substrate for the synthesis of the two diastereomeric epiemetine analogues 9 and 10 with > 96% ee (9) and 80% ee (10), respectively, by condensation with the phenylethylamine 23, Bischler-Napieralski reaction and "enantioselective" hydrogenation using the chiral catalyst (R,R)-26
Stereoselective synthesis of novel 19-nor-steroids by a double Heck reaction
No full textThe estrane 4 was synthesized by two successive Heck reactions starting from enantiopure 2 and the cyclohexenone 5, which contains a (Z)-bromovinyl group. The first intermolecular Pd-catalyzed reaction leads to 10 in a highly regio-and diastereoselective manner. Transformation of the enone 10 to give the corresponding enol triflate 14 followed by an intramolecular Heck reaction affords the cyclized product 4 with an unusual cis-junction of the rings B and C in high yield
Stereoselective total synthesis of a novel D-homosteroid by a twofold Heck reaction
No full textThe D-homosteroid 1 was synthesized by two successive Heck reactions starting from enantiopure 3 and the bromoarene 2 containing a (Z)-bromovinyl group. The first intermolecular Pd-catalyzed reaction leads to 6 in a highly regio- and diastereoselective way which forms 1 with an unusual cis-junction of the rings B and C by a second intramolecular Heck reaction
A novel palladium-catalyzed domino Tsuji-Trost-Heck process for the synthesis of tetrahydroanthracenes
No full textA novel type of palladium-catalyzed domino reaction is described combining Tsuji-Trost and Heck reactions. This method allows efficient access to tetrahydroanthracene derivatives 1 in up to 89% isolated yield in a one-pot process starting from the diketone 3. The tetrahydroanthracene structural motif is found in many natural products, such as in the antibiotic tetracycline
Highly regio- and enantioselective Heck reactions of N-substituted 2-pyrroline with the new chiral ligand BITIANP
No full textThe intermolecular Heck reaction of N-substituted 2-pyrroline 4 with aryl triflates 5a-d in the presence of the chiral ligand (S)-BITIANP 1 gives the N-substituted 5-aryl-2-pyrrolines la-d highly regioselectively with excellent enantioselectivity (93-95% ee) and good yields (80-92%). In addition, cyclohexenyl triflate 8 can be transformed with 91% ee
Regioselective silane-terminated intramolecular Heck reaction with alkenyl triflates and alkenyl iodides
No full textOne of the main problems of the Heck reaction using acyclic substrates which lead to Pd intermediates with beta- and beta'-hydrogens is the lack of selectivity in the formation of the double bond as the last step of the catalytic cycle. The use of allylsilanes as the alkene moiety permits control of the elimination Step. Thus, the Pd-0-catalysed reaction of 1a-e and 2 leads exclusively to the corresponding bicyclic compounds 11a-c. In contrast, reaction of 7 yields a mixture of 12a, (E)-12b and (Z)-12b
AN EFFICIENT DOMINO SONOGASHIRA/DOUBLE CARBOPALLADATION/C-H-ACTIVATION REACTION LEADING TO FLUORESCENT POLYCYCLIC AROMATIC HYDROCARBONS
No full textA facile synthesis of fluorescent polycyclic aromatic hydrocarbons through a highly productive palladium-catalyzed fourfold domino Sonogashira/double carbopalladation/C-H-activation process was developed. Dedicated to Professor Isao Kuwajima on the occasion of his birthda
Multicomponent domino reactions for the synthesis of biologically active natural products and drugs
No full textA main issue in modern synthetic organic chemistry, which deals with the preparation of natural products, pharmaceuticals, diagnostics, agrochemicals, and other important materials, is the improvement of efficiency, the avoidance of toxic reagents, the reduction of waste, and the responsible treatment of our resources. One of the ways to fulfill these goals is the development and use of domino processes, which consist of several bond-forming reactions and which allow the highly efficient synthesis of complex molecules starting from simple substrates, Herein, the combination of several catalytic bond-forming transformations is clearly most appropriate. The synthesis of the enantiopure alkaloid (-)-hirsutine 22, which has a strong inhibitorial effect on influenza A viruses, was accomplished using a biomimetic domino Knoevenagel-hetero-Diels Alder-solvolysis-hydrogenation process. In a similar way the alkaloids (+)-dihydrocorynantheine 23 and (-)-dihydroantirhine 24 as well as heterosteroids 62, D-homosteroids 65 and 68, and azasteroids 25 are prepared. In addition, novel steroid alkaloids 26 are accessible by a combination of the formation of an iminium salt, a hydride shift, and an alkylation. The antileukemic pentacyclic (-)-cephalotaxine 27 is obtained by a combination of two Pd-catalyzed reactions. (C) 2000 John Wiley & Sons. Inc
Ecologically benign and economically useful transformations: Research and education at the Georg-August-University Goettingen, Germany
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