Enantiomeric recognition of organic ammonium salts by chiral dialkyl-, dialkenyl-, and tetramethyl-substituted pyridino-18-crown-6 and tetramethyl-substituted bispyridino-18-crown-6 ligands: comparison of temperature-dependent proton NMR and empirical force field techniques

Stereoselective synthesis of (-) Cephalosporolide D

“…Accordingly, the known D-glyceraldehyde derivative 9, prepared from a literature procedure from D-mannitol [13] was subjected to Wittig olefination afforded ester 8 in 81% yield. Ester 8 was subjected to catalytic hydrogenation in the presence of 10% Pd-C in methanol under hydrogen atmosphere to give saturated ester 10 in 84%.…”

“…Organic solutions were dried over anhydrous Na2SO4 and concentrated below 40 °C in vacuum. 1 H NMR spectra were acquired at 300 MHz, while, 13 C NMR at 75 MHz with TMS as internal standard for solutions in CDCl3. J values were given in Hz.…”

Synthesis of new optically active bis-pyridino- and bis pyridono-18-crown-6 type ligands containing four lipophilic chains attached to the chiral centers by "two-to-two" type macrocyclization

“…These yields can be considered as satisfactory for template assisted "two-to-two" type cyclizations. 2,3,4,19 Preparation of the optically active diols (S,S)-16 20 and (R,R)-17, 21 and of the di-p-tosylates 18, 22 and 19 22 was performed as reported.…”

“…Among these, the tetramethyl-substituted ligand (R,R,R,R)-3 has been studied most. Although it failed to show appreciable enantiomeric selectivity in complexing chiral primary organic ammonium salts, 4,6,7 ligand (R,R,R,R)-3 turned out to be very effective in inducing high enantio-and diastereoselectivity in rare earth metal mediated asymmetric aldol reactions in aqueous media. Reported optically active bis-pyridino-and bis-p-substituted pyridino-18-crown type ligands and also the reported mixture of racemic-and meso-bis-pyridono-dicyclohexo-18-crown-6 macrocycles.…”

Application of Flow Chemistry to Macrocyclization of Crown Ethers

“…The factors influencing enantioselectivity of these ligands toward the enantiomers of the abovementioned organic ammonium salts have been extensively studied [9][10][11][12][13][14][15][16][17]. These studies have established a tripod-like hydrogen bonding involving the pyridine nitrogen and two alternate oxygen atoms of the pyridino-crown ether and the three ammonium protons of the organic ammonium salt (attractive interaction) [15], π-π stacking between the aromatic rings of the host and the guest (attractive interaction) [17,18] as well as the steric repulsion caused by the substituent at the stereogenic center of the ligand and certain hydrogens of the organic ammonium salt (repulsive interaction), respectively.…”