Structural Transition of Bombyx mori Liquid Silk Studied with Vibrational Circular Dichroism Spectroscopy.

We investigated the structural transition from liquid silk to silk fibers with vibrational circular dichroism spectroscopy. Liquid silk showed a major right-handed optically active band at around 1650 cm(-1) and a minor one at around 1680 cm(-1). The former disappeared over time, while the intensity in the latter increased. With the former wavenumber, liquid silk mainly adopted a random-coil structure. In contrast, the latter may reflect an intermediate structure in the transition. Furthermore, two right-handed bands at around 1630 and 1660 cm(-1) appeared with the disappearance of the major band, and then the wavenumber of the former shifted to around 1620 cm(-1). The shift results from the decrease in the frequency of the CO stretching mode due to the stacking of the ß-sheet that comprises fibers. The band at 1660 cm(-1) may reflect another intermediate structure due to its strong correlation with that at 1620 cm(-1) in terms of their temporal change in intensity.

Organocatalytic, enantioselective intramolecular [6+2] cycloaddition reaction for the formation of tricyclopentanoids and insight on its mechanism from a computational study.

Diphenylprolinol silyl ether was found to be an effective organocatalyst for promoting the asymmetric, catalytic, intramolecular [6 + 2] cycloaddition reactions of fulvenes substituted at the exocyclic 6-position with a δ-formylalkyl group to afford synthetically useful linear triquinane derivatives in good yields and excellent enantioselectivities. The cis-fused triquinane derivatives were obtained exclusively; the trans-fused isomers were not detected among the reaction products. The intramolecular [6 + 2] cycloaddition occurs between the fulvene functionality (6π) and the enamine double bond (2π) generated from the formyl group in the substrates and the diphenylprolinol silyl ether. The absolute configuration of the reaction products was determined by vibrational circular dichroism. The reaction mechanism was investigated using molecular orbital calculations, B3LYP and MP2 geometry optimizations, and subsequent single-point energy evaluations on model reaction sequences. These calculations revealed the following: (i) The intermolecular [6 + 2] cycloaddition of a fulvene and an enamine double bond proceeds in a stepwise mechanism via a zwitterionic intermediate. (ii) On the other hand, the intramolecular [6 + 2] cycloaddition leading to the cis-fused triquinane skeleton proceeds in a concerted mechanism via a highly asynchronous transition state. (iii) The fulvene functionality and the enamine double bond adopt the gauche-syn conformation during the C-C bond formation processes in the [6 + 2] cycloaddition. (iv) The energy profiles calculated for the intramolecular reaction explain the observed exclusive formation of the cis-fused triquinane derivatives in the [6 + 2] cycloaddition reactions. The reasons for the enantioselectivity seen in these [6 + 2] cycloaddition reactions are also discussed.