Experimental Comparative Study of Dynamic Behavior in Solution Phase of C-Tetra(phenyl)resorcin[4]arene and C-Tetra(phenyl)pyrogallol[4]arene

The synthesis of phenyl-resorcinarenes and pyrogallolarenes is known to produce a conformational mixture of cone and chair isomers. Depending on the synthesis conditions the composition of the conformational mixture is variable; however, the cone conformer is the greatest proportion of phenyl-resorcin[4]arenes and chair conformer of pyrogallol[4]arenes. The experimental evidence suggests that phenyl-substituted resorcinarene and pyrogallolarene exist as a dynamic boat in solution.

Crystal structures of two chiral piperidine derivatives: 1-[(1R)-2-hydroxy-1-phenylethyl]piperidin-4-one and 8-[(1S)-1-phenylethyl]-1,4-dioxa-8-azaspiro[4.5]decane-7-thione

The crystal structures of the two title piperidine derivatives show different conformations for the six-membered heterocycle. TheN-substituted 4-piperidinone 1-[(1R)-2-hydroxy-1-phenylethyl]piperidin-4-one, C13H17NO2, (I), has a chair conformation, while the piperidine substituted in position 2 with a thiocarbonyl group, 8-[(1S)-1-phenylethyl]-1,4-dioxa-8-azaspiro[4.5]decane-7-thione, C15H19NO2S, (II), features a half-chair conformation. Comparison of the two structures, and data retrieved from the literature, suggests that the conformational flexibility is mainly related to the hybridization state of the C atom α to the piperidinic N atom: a Csp3atom favours the chair conformer, while a Csp2atom distorts the ring towards a half-chair conformer. In the crystal structure of (I), weak C—H...O hydrogen bonds link the molecules into supramolecular chains propagating along theb-axis direction. In the crystal of (II), the molecules are linked by weak C—H...S contacts into supramolecular chains propagating along theb-axis direction.

Formation of Ring A in the Biosynthesis of Hopanoids from Squalene. A Density Functional Study

Density functional calculations were carried out on the ring closure of the 2,6-dimethyloct-6-en-2-yl cation to a chair conformer of the 1,2,3,3-tetramethylcyclohexyl cation as a model for the first step in the biosynthetic cyclization of squalene to the triterpene hopanoids. The concerted reaction was found to have an activation energy of 4.6 kcal/mol and to be exothermic by 11.5 kcal/mol.