title: Conformational analysis of the cis- and trans-adducts of the Pictet - Spengler reaction. Evidence for the structural basis for the C(1) - N(2) scission process in the cis- to trans-isomerization.
abstract: The stable conformations of both the trans- and cis-1,3-disubstituted Nb-benzyl stereoisomers of the Pictet - Spengler reaction have been determined by NMR spectroscopy and X-ray crystallography in order to better understand the C(1) -N(2) cis- to trans-isomerization process. In the Na-H series, the chair conformation was preferred for the trans-isomer 3a, while the cis-isomer 3b existed predominantly in the boat form. However, in the Na-methyl series (1a, 1b, 2a, 2b), both the cis (1b, 2b) and trans (1a, 2a) diastereomers existed in the chair conformation to relieve the A(1,2)-strain between the Na-methyl function and the substituent at C(1). The difference in the preferred conformations of the cis-isomers in the Na-H and Na-methyl series (as compared to the preferred conformations in the trans-isomers) can be employed to understand the reduced rate of epimerization of cis-2b into trans-2a as compared to 3b into 3a. This provides the structural basis for the carbocation-mediated intermediate in the C(1) - N(2) scission process.
authors.affiliation: Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, USA.
authors.forename: Dongmei
authors.initials: D
authors.lastname: Han
authors.forename: F Holger
authors.initials: FH
authors.lastname: Försterling
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Deschamps
authors.forename: Damon
authors.initials: D
authors.lastname: Parrish
authors.forename: Xiaoxiang
authors.initials: X
authors.lastname: Liu
authors.forename: Wenyuan
authors.initials: W
authors.lastname: Yin
authors.forename: Shengming
authors.initials: S
authors.lastname: Huang
authors.forename: James M
authors.initials: JM
authors.lastname: Cook
mesh_terms: D001593:Benzyl Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D001593:Benzyl Compounds