title: Diastereoisomeric assignment in a pacifenol derivative using vibrational circular dichroism.
abstract: The configuration of a chiral center in semisynthetic (-)-(2R,5R,5aR,8zeta,9aS)- 2,8-dibromo-2,5,9,9a-tetrahydro-5-hydroxy-5,8,10,10-tetramethyl-6H-2,5a-methano-1-benzoxepin-7(8H)-one (3 or 4), prepared in two steps from (-)-(2R,5R,5aR,7S,8S,9aS)-2, 7-dibromo-8-chloro-2,5,7,8,9,9a-hexahydro-5,8,10,10-tetramethyl-6H-2,5a-methano-1-benzoxepin-5-ol, known as pacifenol 1, has been determined using vibrational circular dichroism (VCD) measurements. The vibrational spectra (IR and VCD) of diastereoisomers 3 and 4 were calculated using density functional theory (DFT) at the B3LYP/DGDZVP level of theory for the two conformers that in each case account for the total energetic distribution found in the first 10 kcal/mol range. The DFT conformational optimization of the 8R diastereoisomer 3 indicates the cyclohexanone exists almost exclusively in a boat conformation with a beta-equatorial bromine atom and an alpha-axial methyl group at the chiral center alpha to the carbonyl group, while for the 8S diastereoisomer 4 a 5:1 conformational distribution in favor of a chair conformation with an alpha-axial bromine atom and a beta-equatorial methyl group is calculated, suggesting due to well-known chair versus boat relative stabilities that the plausible diastereoisomer would be the 8S molecule. A comparison of the IR spectrum of the reaction product with the calculated spectra of 3 and 4 provided no means for the diastereoisomeric assignment, while from comparison of the VCD spectra it became immediately evident that the rearranged molecule possesses the 8R absolute configuration as shown in 3, in concordance with a single crystal X-ray diffraction study that could be refined to an R-factor of 2.9%.
authors.affiliation: Instituto de Química, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile.
authors.forename: Marcelo A
authors.initials: MA
authors.lastname: Muñoz
authors.forename: Cristina
authors.initials: C
authors.lastname: Chamy
authors.forename: Alvaro
authors.initials: A
authors.lastname: Carrasco
authors.forename: Juana
authors.initials: J
authors.lastname: Rovirosa
authors.forename: Aurelio San
authors.initials: AS
authors.lastname: Martín
authors.forename: Pedro
authors.initials: P
authors.lastname: Joseph-Nathan
mesh_terms: D002942:Circular Dichroism / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D012717:Sesquiterpenes / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism; D014732:Vibration; D014961:X-Ray Diffraction; D014965:X-Rays
chemical_list: D012717:Sesquiterpenes; C094445:pacifenol