Helvetica Chimica Acta
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Polymer‐ and Dendrimer‐Bound Ti‐TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species? Abstract α,α,α′,α′‐T etraa ryl‐1,3‐d ioxolane‐4,5‐d imethanol s (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α‐position, were prepared in the usual way (18–22, 24, 25 ). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33–40 , Scheme 3 ) as beads with a rather uniform particle‐size distribution (150–45 μm), swellable in common organic solvents. HOCH2 ‐ and BrCH2 ‐substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules (23, 26–32 ). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4 ) which can be used as polymer‐ or dendrimer‐bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R2 Zn and RTi(OCHMe2 )3 ; Scheme 5, Table 1 ) and to ketones (of LiAlH4 , Table 2 ); enantioselective ring opening of meso ‐anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3‐crotonyl‐1,3‐oxazolidin‐2‐one to cyclopentadiene and to (Z )‐N ‐benzylidenephenylamine N ‐oxide ( → 48, 49 , Scheme 7, Tables 3, 4 , and Fig. 5 ). The enantioselectivities reached with most of the polymer‐bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer‐bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4 ). The poorer selectivity in the Diels‐Alder reaction (Scheme 7a ), induced by the polymer‐bound Cl2 Ti‐TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis ‐acid catalysis, and to propose a cationic, trigonal‐bipyramidal complex as the catalytically active species (Fig. 6 ). It is suggested that similar cations may be involved in other Ti‐TADDOLate‐mediated reactions as well.
Helvetica Chimica Acta - Tập 79 Số 6 - Trang 1710-1740 - 1996
Die Frage nach der Aufrollung von Fadenmolekeln in strömenden Lösungen Abstract Fadenmolekeln hochpolymerer Stoffe, welche in ruhender Lösung eine statistisch wahrscheinlichste Gestalt (loses Knäuel) haben, erfahren in strömender Lösung nicht nur Richtungsänderungen der Fadenachse; vielmehr werden durch die von der strömenden Flüssigkeit ausgeübten Kräfte auch Änderungen der Gestalt, insbesondere Änderungen des Abstandes h zwischen Molekelanfangs und ‐endpunkt erzwungen.
Helvetica Chimica Acta - Tập 26 Số 5 - Trang 1394-1465 - 1943
Enantioselective 1,4‐Addition of Aliphatic <i>Grignard</i> Reagents to Enones Catalyzed by Readily Available Copper(I) thiolates derived from TADDOL. Preliminary communication Abstract Two simple thiols derived from the parent TADDOL, α,α,α′,α′ tetraphenyl‐2,2‐dimethyl‐1,3‐clioxolan‐4,5‐dimethanol, are used to prepare Cu1 complexes C and D to catalyze (0.05 equiv.) 1,4‐additions of Grignard reagents RMgCl to cyclic enones with enantioselectivities which are comparable to or better than previously reported (enantiomer ratios up to 92:8).
Helvetica Chimica Acta - Tập 80 Số 8 - Trang 2515-2519 - 1997
New C<sub>20</sub>‐Diterpenoid Alkaloids from <i>Delphinium anthriscifolium</i> var. <i>savatieri</i> Abstract magnified image Five new vakognavine‐type C20 ‐diterpenoid alkaloids, anthriscifolmines D–H (1 –5 , resp.), were isolated from the whole herb of Delphinium anthriscifolium var. savatieri. The structures of these new alkaloids were determined by spectroscopic techniques, including HR‐ESI‐MS, 1D‐, and 2D‐NMR experiments.
Helvetica Chimica Acta - Tập 92 Số 4 - Trang 745-752 - 2009
Diterpenoid Alkaloids from <i>Delphinium tatsienense</i> Abstract Three new C20 ‐diterpenoid alkaloids, along with twenty‐two known alkaloids, were isolated from the whole herbs of Delphinium tatsienense. The new alkaloids include a vakognavine‐type C20 ‐diterpenoid alkaloid, designated as tatsienenseine A (1 ), and two hetisine‐type C20 ‐diterpenoid alkaloids, designated as tatsienenseines B (2 ) and C (3 ). Their structures were elucidated by IR, HR‐ESI‐MS, 1D‐ and 2D‐NMR analyses.
Helvetica Chimica Acta - Tập 94 Số 5 - Trang 853-858 - 2011
Cadinane‐Type Sesquiterpenoids, Strobilols E – K, from the Liquid Culture of <i>Strobilurus ohshimae</i> Abstract New cadinane sequiterpenoids, strobilols E–K (5 –11 , resp.) have been isolated from a liquid culture of Strobilurus ohshimae. Their structures have been established on the basis of spectral analyses.
Helvetica Chimica Acta - Tập 91 Số 8 - Trang 1595-1603 - 2008
Metal Complexes with Macrocyclic Ligands. XI. Ring size effect on the complexation rates with transition metal ions Abstract The 12‐16 membered tetraazamacrocycles 1 ‐6 were synthesized, their protonation constants and complexation kinetics measured at 25° and I = 0.50. The results of Table 1 Show that pK K K The kinetics of complex formation between the macrocyclic ligands and several transition metal ions have been studied by pH‐stat and stopped‐flow techniques and the results have been analyzed as bimolecular reactions between the metal ion and the different protonated species of the ligands. The rate constants, given in Table 2 , show that the macrocycles react less rapidly than analogous open chain amines. However, for a given protonated species of the ligand the rate of complexation follows the order Cu2+ > Zn2+ > Co2+ > Ni2+ which parallels the sequence of their water exchange rates. For the diprotonated tetraamines LH2+ the slower rates seem to be mainly a consequence of electrostatic interactions, since a correlation between logk K + , however, the complexation rates of a metal ion with the different macrocycles are all in one order of magnitude and do not depend in a regular way on the ring size or the basicity of the ligand. It is therefore suggested that in this case other factors such as unfavourable preequilibria must be considered as important.
Helvetica Chimica Acta - Tập 61 Số 7 - Trang 2296-2306 - 1978
Metal Complexes with Macrocyclic Ligands, VI. The role of substituents on the complexation rate of transition metal ions with several 1,4,7,10‐tetraazacyclotridecanes Abstract 12,12‐Dimethyl‐1,4,7,10‐tetraazacyclotridecane (I), 11,13‐dimethyl‐1,4,7,10‐tetraazacyclotridecane (II), 11,11,13‐trimethyl‐1,4,7,10‐tetraazacyclotridecane (III) and 1,4,7,10,12,12‐hexamethyl‐1,4,7,10‐tetraazacyclotridecane (IV) have been synthesized and their properties are described. While the Ni2+ and Cu2+ complexes of I–III have square planar geometries, those of IV are pentacoordinate according to their absorption spectra. Similarly, while the Co2+ complex of I is octahedral and readily oxygenated, the analogous complex with IV is pentacoordinate and not sensitive to oxygen. The rate of complexation of these ligands with Cu2+ and Ni2+ decreases in the order I > II > III ≫ IV, indicating that the number as well as the position of the methyl groups are important. Finally for Cu2+ the formation of the thermodynamic stable end product is slown down by methyl substitution in α‐position to the coordinating nitrogen atoms (ligand II and III) so that an intermediate can be observed, whereas with I Cu2+ directly forms the end product.
Helvetica Chimica Acta - Tập 58 Số 5 - Trang 1358-1366 - 1975
Carbocyclische Verbindungen aus Monosacchariden. I. Umsetzungen in der glucosereihe Carbocyclic Compounds from Monosaccharides. 1. Transformations in the Glucose Series A method for the preparation of pentasubstituted cyclopentanes from monosaccharides is presented, involving two crucial steps, viz. the reductive fragmentation of 5‐bromo‐5‐deoxyglucosides (such as 10, 17 and 23 , see Scheme 3 ) with Zn or butyl lithium yielding 5,6‐dideoxy‐hex‐5‐enoses (such as 11 and 24 , see Schemes 3 and 4 ), and the subsequent cyclization of these hexenoses with N ‐methyl‐ or N ‐(alkoxyalkyl)hydroxylamines (via the corresponding nitrones) to form cyclopentano‐isoxazolidines (see Scheme 2 ). Thus, the glucosides 17 and 23 were converted diastereoselectively and in good yields into the cyclopentano‐isoxazolidines 27 and 45 (Schemes 5 and 7 ), which were characterized by their transformation into various derivatives. 27 and 45 were correlated through the common derivative 62 . The configuration of the cyclization products were established by pyrolysis of the N ‐oxide 65 to the enol ether 67 (Scheme 10 ).
Helvetica Chimica Acta - Tập 62 Số 6 - Trang 1990-2016 - 1979
Synthesis of 2‐Acetamido‐2‐deoxy‐<scp>D</scp>‐gluconhydroximolactone‐ and Chitobionhydroximolactone‐Derived <i>N</i>‐Phenylcarbamates, Potential Inhibitors of β‐<i>N</i>‐Acetylglucosaminidase Abstract The N ‐phenylcarbamate 7 , derived from 2‐acetamido‐2‐deoxy‐D ‐gluconhydroximo‐1,5‐lactone (3 ) and the analogous N ‐phenylcarbamate 14 , derived from chitobionhydroximo‐1,5‐lactone (10 ) have been prepared as potential inhibitors of β‐N ‐acetylglucosaminidases. The unambiguous synthesis of the hydroximo‐1,5‐lactone 3 involves oxidation of the oxime 1 , followed by deprotection with Na/NH3 .
Helvetica Chimica Acta - Tập 73 Số 7 - Trang 1918-1922 - 1990
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