Silvio E. Biali

Calix[4]arene derivatives monosubstituted at all four methylene bridges.

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. Incorporation of hydroxy groups on the bridges was accomplished via solvolysis in AcOH, followed by LiAlH4 reduction of the acetoxy groups. Reaction of the tetra-(2-methylfuranyl)calixarene derivative 11 with benzyne followed by deoxygenation with Me3SiCl/NaI afforded in low yield the tetra-(4-methylnaphthyl)calix[4]arene derivative 12. Reaction of de-tert-butylated tetrabromo derivative 2a with m-xylene in HFIP followed by methylation of the crude product afforded the tetraxylyl derivative 14.

Calix[4]arenes with alkylidene bridges, synthesis and conformational properties

Abstract Calix[4]arenes with one or two alkylidene bridges were synthesized by “2+2” fragment condensation. As predicted by molecular mechanics calculations aliphatic residues at the bridging carbon atom(s) prefer the equatorial position.

Calix[4]arenes with Two Different Chemical Modifications at the Bridges

Hexabromocalix[4]arene derivatives (3a and 3b) possessing pairs of dibromomethylene and bromomethylene groups located at distal or proximal positions of the calix scaffold were prepared via photochemical bromination of tetramethoxycalix[4]arene. The dibromomethylene groups undergo hydrolysis to afford calix[4]arenes possessing pairs of carbonyl groups and bromomethylene groups located at distal or proximal bridges (4a and 4b, respectively). The bromomethylene groups of 4 undergo reactions with nucleophiles under S(N)1 conditions to afford a wide array of derivatives possessing two different chemical modifications at the bridges.

Selective Functionalization of a Single Methylene Bridge of a Calix[6]arene

The monohydroxycalix[6]arene derivative 4a was prepared via photochemical bromination of hexamethoxycalix[6]arene 3a in aq THF. Monohydroxycalix[6]arene 4a and its chloro derivative 5 are useful synthetic intermediates for the preparation of structurally diverse calix[6]arenes functionalized at a single methylene bridge.

Preparation, Structure and Stereodynamics of Phosphorus-Bridged Calixarenes

The pyrolysis of several dialkylphosphate ester derivatives of calix[4]arenes yielded the same phosphorus bridged compound7. Under the pyrolytic conditions the phosphate groups may be cleaved or intermolecularly transferred. X-ray crystallography of the bridged calixarenes7 and8 shows that they exist in a chiral ‘flattened cone’ (fc) conformation. The bridged calixarenes undergo in solution a dynamic process with a barrier of about 10.1 kcal mol−1 for7 and8 and 13.1 kcal mol−1 for10, respectively. The dynamic processes result in enantiomerization of the systems. Pyrolysis of partially phosphorylated calix[6]arenes resulted in the formation of two products (11 and12), each consisting of two subunits of three proximal rings bridged by a phosphate group. The rotational barriers for11 and12 are 14.4 and 8.8 kcal mol−1, indicating that the bridged calix[6]arene system12 is appreciably more flexible than11.

Solution conformation and inversion barrier in p-tert-butyl-25,27-dihydroxycalix[4]arene

Abstract Spectroscopic data indicate that the solution conformation of the dihydroxycalix[4]arene 3 (1,3-alternate) is intramolecularity hydrogen bonded and is different from the crystallographic conformation of its MeOH solvate (1,2-alternate).

Incorporation of Substituents at the Methylene Linkages of the Calix[5]arene Skeleton

Under S(N)1 reaction conditions, the bromine atoms of the pentabromo p-tert-butylcalix[5]arene 6 were replaced by alkoxy, electron-rich aryl, and acetylacetonate groups.

Calix[6]arenes incorporating functionalised substituents at the methylene bridges

The hexabromocalix[6]arene 5 reacts with p-tert-Bu-phenol, thymol and propargyl alcohol to afford the corresponding hexasubstituted methylene-functionalised calixarenes. The substituents at the bridges were further functionalised via acetylation or click chemistry. Reaction of the hexahydroxy derivative 10 with DAST afforded 11, a first example of a calixarene with all bridges monofluorinated.

NMR diffusion coefficients of p-tert-butylcalix[n]arene systems

The pulsed gradient spin echo (PGSE) NMR technique was used for measuring the diffusion coefficients of neutral and charged calix[n]arenes and for probing their association in solution with potential guests.

Stable simple enols. 8. Synthesis and keto .dblharw. enol equilibria of the elusive 2,2-dimesitylethanal and 1,2,2-trimesitylethanone. Conformations of 1,2,2-trimesitylethanone and 1,2,2-trimesitylethanol

Der Alkohol (I), der durch Reduktion des entsprechenden Saurechlorids mit LiAlH4 erhalten werden kann, ergibt bei seiner Oxidation mit dem neutralen Reagens Pyridiniumdichromat den leicht zu (III) enolisierenden Aldehyd (ll).