Synthesis and molecular recognition studies of crown ethers

This short review summarizes the synthesis and molecular recognition studies of crown ether type macrocycles accomplished at the Institute for Organic Chemistry of Budapest University of Technology and Economics in the last few years. The research work reported here belongs to the areas of proton-ionizable crown ethers and chiral macrocycles. Proton-ionizable crown ethers at higher pHs than their pK_a values are mostly ionized to ligand anions which increase the cation-ligand complex stability with enhancement of selectivity and avoid the need for a counter anion to accompany the cation transport or solvent extraction. The latter factor is not only advantageous from energetical point of view, but is also important when counter anions are not required to be transported. Enantiopure chiral macrocycles have also drawn the attention of many researchers, because owing to their enantioselective complexation they are excellent candidates for effective sensors and selectors of the enantiomers of biologically important chiral compounds such as protonated primary organic amines, amino acids and the derivatives of the latters

Synthesis and enantiomeric recognition studies of a novel 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups

A novel optically active 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups was synthesized and its enantiomeric recognition properties were examined towards the enantiomers of tetrabutylammonium salts of chiral α-hydroxy and N-protected α-amino acids using UV–vis spectroscopy

Membrane-Supported Recovery of Homogeneous Organocatalysts: A Review

As catalysis plays a significant role in the development of economical and sustainable chemical processes, increased attention is paid to the recovery and reuse of high-value catalysts. Although homogeneous catalysts are usually more active and selective than the heterogeneous ones, both catalyst recycling and product separation pose a challenge for developing industrially feasible methods. In this respect, membrane-supported recovery of organocatalysts represents a particularly useful tool and a valid option for organocatalytic asymmetric synthesis. However, catalyst leaching/degradation and a subsequent decrease in selectivity/conversion are significant drawbacks. As the effectivity of the membrane separation depends mainly on the size of the catalyst in contrast to the other solutes, molecular weight enlargement of small organocatalysts is usually necessary. In the last few years, several synthetic methodologies have been developed to facilitate their recovery by nanofiltration. With the aim of extending the possibilities for the membrane-supported recovery of organocatalysts further, this contribution presents a review of the existing synthetic approaches for the molecular weight enlargement of organocatalysts

Synthesis and Complexation Studies of Optically Active Aza- and Diazacrown Ethers Containing a Pyrene Fluorophore Unit

Novel enantiopure azacrown [(R,R)-1 and (S,S)-1] and diazacrown [(R,R)-2–(R,R)-4 and (S,S)-2–(S,S)-4] ethers containing a pyrene fluorophore unit and two phenyl groups at their chiral centers were obtained in multistep syntheses. The action of these chemosensors is based on the photoinduced electron transfer (PET) process, thus they show fluorescence enhancement in the presence of protonated primary amines and amino acid esters. Their recognition abilities toward the enantiomers of 1-phenylethylamine hydrogen perchlorate (PEA), 1-(1-naphthyl) ethylamine hydrogen perchlorate (NEA), phenylglycine methyl ester hydrogen perchlorate (PGME), and phenylalanine methyl ester hydrogen perchlorate (PAME) were examined in acetonitrile using fluorescence spectroscopy

Királis (tio)négyzetamid organokatalizátorok előállítása és alkalmazása: Synthesis and application of chiral (thio)squaramide organocatalysts

During my research work cinchona-, and proline-based chiral squaramide, and thiosquaramide organocatalysts were synthesized. The promising new organocatalyst class, the thiosquaramides, were gained by the thionation of the appropriate squaramide with pyridine complex of phosphorus pentasulfide. In 2017, Rawal and his co-workers were the first ones who successfully managed to apply the double hydrogen-bond donor thiosquaramides in asymmetric reactions. They recognized the great potential of this new type of catalyst. If the carbonyl groups of squaramide are converted to thiocarbonyl groups, the acidity of the hydrogen-bond unit increases which can result in stronger hydrogen bond between the substrate(s) and the catalyst. Moreover, thiosquaramides are found to be more soluble in nonpolar organic solvent than their squaramide counterparts. The synthesized organocatalysts were applied in asymmetric C–C coupling reactions, furthermore, the proline-based (thio)squaramide were proved to be excellent organocatalysts for the enamine catalysed Diels–Alder reaction. Kivonat Kutatómunkám során cinkona alkaloid, valamint prolin alapú királis négyzetamid, illetve tionégyzetamid organokatalizátorokat állítottam elő. A tionégyzetamidokat a megfelelő négyzetamidokból foszfor-pentaszulfid piridin komplexével történő tionálás segítségével alakítottam ki. 2017-ben Rawal és munkatársai elsőként alkalmazták sztereoszelektív szintézisekben a kétszeres hidrogénkötés donor típusú tionégyzetamid organokatalizátorokat. Azt találták, hogyha a karbonilcsoportokat tiokarbonil csoportokra cserélik, akkor a katalizátor savas karaktere növekszik és ezáltal jobb hidrogénkötés donorként viselkedhet. Ezenfelül a tionégyzetamidok további előnye a négyzetamidokkal szemben, hogy nagyobb lipofilitásuk révén gyakran jobb oldhatósággal rendelkeznek a szerves oldószerekben. Az előállított organokatalizátorokat C–C kapcsolási konjugált addícióban vizsgáltam, továbbá, a prolin alapú organokatalizátorokat sikeresen alkalmaztam enamin katalizált Diels–Alder-reakcióban is

Larger Groups, Smaller Enantioselectivity? Two Anthracene-Containing, Pyridino-Crown Ether-Based Fluorescent Sensor Molecules

(R,R)- and (S,S)-enantiomers of anthracene-containing pyridino-18-crown-6 ether having tert-butyl groups at the stereogenic centers were prepared with the aim of achieving higher enantioselectivity than for the reported (S,S)-analogue having isobutyl groups. The enantiomeric recognition abilities of the new sensor molecules toward chiral protonated primary amines and amino acid esters were studied in acetonitrile by UV–vis and fluorescence spectroscopies. The pKa values of these pyridino-crown ethers and their reported (S,S)-analogues having methyl or isobutyl groups have also been determined in acetonitrile