Calixarenes are molecular bowls or baskets that have been demonstrated to serve as hosts for cations, anions, and neutral molecules. The central cavity and scaffolding of various functionalities on the upper and lower rims establishes this class of compounds as extremely important in supramolecular chemistry studies. In earlier studies, calixsugars (molecules that have sugar molecules appended to the upper rim of the calix) were prepared. Among the potential advantages of these molecules are increased water solubility and enhanced possibilities that these chiral attachments can promote enantiomeric selection. Computational studies, however, have indicated that the chosen calixsugars had significantly encumbered upper rims, suggesting that host-guest associations would be limited. In an attempt to understand those factors responsible for the favored conformations of calixsugars, a number of 5,17-disubstituted and tetrasubstituted calix[4]arenes were prepared and studied experimentally as well as by molecular mechanics conformational searching techniques.
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Upper rim guanidinocalix[4]arenes as artificial phosphodiesterases.
J Org Chem
April 2012
Dipartimento di Chimica Organica e Industriale, Università degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy.
Calix[4]arene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared with the behavior of a monofunctional model compound, the catalytic superiority of the calix[4]arene derivatives points to a high level of cooperation between catalytic groups. Combination of acidity measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator.
View Article and Find Full Text PDFCalix[4]arene derivatives monosubstituted at all four methylene bridges.
J Org Chem
April 2008
Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
The scope of the reaction of the tetrabromocalixarene derivative 2b with alcohols under solvolytic conditions in trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) was explored. The reaction proceeded readily with MeOH, EtOH, n-PrOH, ethylene glycol and i-PrOH affording preferentially the rccc isomer of the tetrasubstituted product. The methoxy derivative 6a undergoes isomerization upon attempted recrystallization from CHCl3/MeOH and its rcct and rctt forms were characterized by X-ray crystallography.
View Article and Find Full Text PDFThe nucleophilic substitution route. A facile method for the fourfold functionalization of the methylene bridges of calix[4]arene.
Org Lett
July 2007
Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
The radical bromination of p-tert-butylcalixarene tetramethyl ether was reinvestigated and the product (2b) characterized by X-ray crystallography. The tetrabromo calixarene derivatives 2a or 2b react with alcohols (TFE, EtOH), azide, and 2-methylfuran under solvolytic conditions affording calixarene derivatives functionalized at the four bridges. The reaction with alcohols and the aromatic compound proceeds in stereoselective fashion and affords the rccc isomer of the tetrasubstituted product.
View Article and Find Full Text PDFChloride selective calix[4]arene optical sensor combining urea functionality with pyrene excimer transduction.
J Am Chem Soc
July 2006
Adaptive Sensors Group, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland.
A neutral 2-site chloride selective compound has been developed (3), based on a 1,3-alternate tetrasubstituted calix[4]arene providing a preorganized supramolecular scaffold. The resultant supramolecular cavity is among the first to combine urea functional groups bridged with single methylene spacers to pyrene moieties. It combines a naturally and synthetically proven H-bonding system with the elegant ratiometric fluorescent signaling properties of an intramolecular pyrene excimer system, triggered by conformational changes upon anion coordination.
View Article and Find Full Text PDFComputational and experimental studies of di- and tetrasubstituted calix[4]arenes.
Chirality
April 2002
U.S. Army Edgewood Research, Development and Engineering Center, APG Maryland, USA.
Calixarenes are molecular bowls or baskets that have been demonstrated to serve as hosts for cations, anions, and neutral molecules. The central cavity and scaffolding of various functionalities on the upper and lower rims establishes this class of compounds as extremely important in supramolecular chemistry studies. In earlier studies, calixsugars (molecules that have sugar molecules appended to the upper rim of the calix) were prepared.
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