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.
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.
Han
Dongmei
D
Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, USA.
Försterling
F Holger
FH
Deschamps
Jeffrey R
JR
Parrish
Damon
D
Liu
Xiaoxiang
X
Yin
Wenyuan
W
Huang
Shengming
S
Cook
James M
JM
D001593:Benzyl Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism
D001593:Benzyl Compounds