Showing papers by "J. Fraser Stoddart published in 1975"

The stereochemistry of 2,4- and 2,3-disubstituted-γ-butyrolactones

Abstract: Relative configurational assignments have been made to the 2,4-dimethyl-(7), 2,4-di-t-butyl-(8), 2,4-diphenyl-(9), 4-methyl-2-phenyl-(10), and 2-methyl-3-phenyl-(31)γ-butyrolactones on the basis of the stereoselective synthesis of their cis-isomers, from the corresponding disubstituted but-2-en-4-olides (2)–(5) and 2-methylene-3-phenyl-γ-butyrolactone (41) by hydrogenation over palladium. The characteristic features from the 1H n.m.r. spectra of the cis- and trans-isomers of (7)–(10) have been employed to assign relative configurations to the 2-methyl-4-phenyl-(11), 2,4-diethyl-(12), 2-ethyl-4-methyl-(13), and 2-butyl-4-methyl-(14)γ-butyrolactones. Equilibration studies on seven 2,4-disubstituted γ-butyrolactones [(7)–(10) and (12)–(14)] indicate that (i) the free energy differences between cis- and trans-isomers are small, and (ii) the cis- is thermodynamically more stable than the trans-isomer in all cases. The opposite situation is true of the 2-methyl-3-phenyl-γ-butyrolactones (31) where the trans-isomer is found to predominate at equilibrium. 1H N.m.r. spectroscopic data suggest that the conformational behaviour of the five-membered ring in 2,4-disubstituted γ-butyrolactones is different for diastereoisomers but is not influenced to any great extent by the nature of the substituent groupings.

Synthesis of configurationally chiral cryptands and cryptates from carbohydrate precursors

Abstract: Optically pure configurationally chiral 18-crown-6 and 9-crown-3 cryptands have been synthesised from L-tartaric acid and D-mannitol and some of the 18-crown-6 derivatives have been shown to form cryptates with metal and primary alkylammonium cations.

Chiral recognition by configurationally chiral cryptands

Abstract: 1,1′,4,4′-Tetra-O-triphenylmethyl-2,2′:3,3′-bis-O- oxydiethylenedi-L-threitol [LL-(3)] and 1,2:1′,2′:5,6:5′,6′-tetra-O-isopropylidene-3,3′:4,4′- bis-O-oxydiethylenedi-D-mannitol [DD-(4)] exhibit enantiomeric differentiation in complexation equilibria towards (±)-(RS)-α-phenylethylammonium hexafluorophosphate.

Isomerism in bicyclic diacetals. Part I. 1,3:2,4- and 1,4:2,3-Di-O-methylene-erythritol

Abstract: Acid-catalysed methylenation of erythritol affords 1,3:2,4- and 1,4:2,3-di-O-methylene-erythritol and a small amount of 1,4-anhydro-2,3-O-methylene-erythritol. Constitutional assignments have been made to the diacetals on the basis of their 1H n.m.r. and mass spectra. Deuteriation studies and the lanthanide shift reagent, Eu(fod)3, have been employed to investigate the conformational behaviour of the 1,4:2,3-diacetal in solution by 1H n.m.r. spectroscopy. Acid-catalysed equilibration of the 1,3:2,4- and 1,4:2,3-diacetals indicates that there is a free energy difference of 1·37 kcal mol–1 in favour of the former at room temperature. The significance of these results is discussed in terms of electronic effects associated with the –O–[graphic omitted]–O– fragments as well as steric effects.

Isomerism in bicyclic diacetals. Part II. Bicyclic methylene diacetals in the galacto, arabino, and ribo series

Abstract: Studies of the acid-catalysed methylenation of tetritols with the galacto, arabino, and ribo configurations have led to the following observations: (i) dimethyl galactarate affords dimethyl 2,3:4,5- and 2,5:3,4-di-O-methylene-galactrate, (ii) methyl-D-arabinonate affords methyl 2,3:4,5-, 2,4:3,5-, and 2,5:3,4-di-O-methylene-D-arabinonate, and (iii) methyl D-ribonate affords methyl 2,4:3,5- and 2,5:3,4-di-O-methylene-D-ribonate. Vicinal coupling constant data obtained by 1H n.m.r. spectroscopy show that dimethyl 2,5:3,4-di-O-methylenegalactarate and methyl 2,5:3,4-di-O-arabinonate exist predominantly in gauche,gauche conformations in solution whereas the gauche,anti conformation is highly populated in solutions of methyl 2,5:3,4-di-O-methylene-D-ribonate. The relative stabilities of the constitutional isomers are discussed in terms of electronic effects associated with gauche oxygen–oxygen interactions in O–[graphic omitted]–O fragments as well as in terms of steric effects. There is no strong evidence to support the view that such gauche oxygen–oxygen interactions are an important stabilising feature in cis-fused 3,5,8,10-tetraoxabicyclo[5.3.0]decanes.