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Shaping Calix[8]arene Framework By Intramolecular Bridging - Synthesis of Conformationally Blocked Calix[8]arene Derivatives
No full textThe first examples of 1,5-intrabridged calix[8]arene derivatives are described, which are conformationally frozen in a double-conical shape suitable for ditopical recognition
Diester intrabridging of p-tert-butylcalix[8]arene and unexpected formation of the monospirodienone derivative
No full textResolution of inherently chiral calix[4]arenes with AABB and CDCD substitution patterns at upper and lower rim, respectively
No full textMethylation of p-tert-butylcalix[8]arene. Products obtained in the presence of weak bases
No full textMethylation of p-tert-butylcalix[8]arene with Mel in the presence of K2CO3 or CsF in acetone or THF/DMF has been investigated. Depending on the experimental conditions, different CH2Cl2-soluble derivatives and Variable amounts of a material insoluble in common organic solvents are formed. The composition of the reaction mixture was determined by exhaustive benzylation which originates soluble, more tractable compounds. With both bases methylation mainly follows the so-called alternate alkylation route, proximal substitution also occurring to some extent, a behaviour similar to benzylation under comparable conditions. The CH2Cl2-soluble portion of the methylation mixtures, when obtained under appropriately chosen conditions, is enriched in a particular component and thus convenient procedures are now available for the preparation of mono-, 1,3-di-, 1,2,4-tri-, 1,2,3,4-tetra-, and heptamethoxy derivatives of p-tert-butylcalix[8]arene
Study On the Alkylation of P-tert-butylcalix[8]arene - Partially O-alkylated Calix[8]arenes
No full textReactions of p-tert-butylcalix[8]arene (1) with various alkylating agents, including para-substituted benzyl bromides, tert-butyl bromoacetate, and 2-chloro-N,N-dimethylacetamide, were carried out in THF-DMF in the presence of a base. With strong bases (Nail or BaO/Ba(OH)(2)) octasubstituted derivatives were obtained, usually in good yield, while in the presence of weak bases (K2CO3 or CsF) more or less complicated reaction mixtures were obtained, wherefrom 1,3,5,7-tetraethers with C-4 symmetry were isolated in yields up to 49%. The eight equivalent ArCH2Ar groups of these compounds give rise to a singlet in the H-1-NMR spectrum, indicating a conformational freedom of the macrocycle, as confirmed by VT-NMR studies. The origin of the 1,3,5,7-regioselectivity can be explained, assuming that alkylation proceeds via a sequence of alternating monodeprotonation and alkylation steps. In each individual monodeprotonation step, those phenoxide monoanions are formed preferentially which are stabilized by two flanking hydrogen bonds. However, this cannot be the whole explanation. In fact, when methyl iodide was used as the electrophile in the reaction with 1, the main products were the 1,2,4-trimethoxy and 1,2,3,4-tetramethoxy derivatives, whereas the 1,3,5,7-tetramethyl ether was not detected in the reaction mixture. This finding leads us to believe that in the reaction with Mel some factor other than stability of oxyanions prevails, possibly the molecular dimension of the electrophile. This seems to be confirmed by the observation that n-butyl iodide, of intermediate dimension between MeI and arylmethyl bromides, gives alkylation products typical of both reaction courses
Methylation of p-tert-butylcalix[8]arene. Products obtained in the presence of strong bases
No full textMethylation of p-tert-butylcalix[8]arene with Mel in the presence of strong bases (NaH, BaO/Ba(OH)(2), or Cs2CO3) in THF has been investigated. Composition of the reaction mixtures differs remarkably from those obtained using weak bases and described in a preceding paper. Several previously unreported methylated products [two dimethoxy- (2(1,5) and 2(1,4)), a trimethoxy- (3(1,2,5)), a tetramethoxy- (4(1,2,3,6)), two pentamethoxy- (5(1,2,3) and 5(1,2,4)), and three hexamethoxy- (6(1,2), 6(1,3), and 6(1,4))] have been isolated. In many cases the substitution pattern was assigned unambiguously on the basis of spectroscopic evidence only, while in others chemical correlation with known compounds was required
Double-two-point and four-point intramolecular bridging of p-tert-butylcalix[8]arene
No full textCalix[8]arenes bis-bridged at the lower rim with ortho- or meta-xylene unit (3a-b and 4a-b) have been obtained in high yields by reaction of 1,3,5,7-tetra-O-substituted calix[8]arenes with 1,2- or 1,3-bis(bromomethyl)benzene. The use of 1,2,4,5-tetrakis(bromomethyl)benzene afforded the four-point intrabridged calix[8]arenes 5a-b in yields up to 41%. VT NMR studies indicated that the flipping motion of aromatic rings in these compounds is inhibited by the intramolecular bridging. Molecular modeling suggested an unprecedented pseudo pleated loop conformation for compounds 5a-b
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