Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.     

Stereochemistry of the Reactions of Sulfur-Stabilized Carbanions on Cyclopropane and Cyclohexane Rings

Stereochemistry in alkylations and H-D exchange of configurationally defined α-sulfonyl- and α-sulfinyl-carbanions generated from the compounds I-IV, are discussed.     

Synthetic Zinc and Magnesium Chlorin Aggregates as Models for Supramolecular Antenna Complexes in Chlorosomes of Green Photosynthetic Bacteria

Abstract— A comparison of the spectra of in vitro (3-hydroxymethyl-13¹-oxometallochlorin) and in vivo chlorosomal (bacterio-chlorophyll- c ) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte-riochlorophyll- c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Q_y absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm⁻¹ (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 3¹-hydroxy and a 13¹-keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five-coordinated metallochlorin macrocycle with an axial ligand (bands at 1500-1630 cm⁻¹), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e . C=O…H-O…M (=Zn or Mg).