Stereocontrol within Confined Spaces: Enantioselective Photooxidation of Enecarbamates Inside Zeolite Supercages
作者:J. Sivaguru、Thomas Poon、Roberto Franz、Steffen Jockusch、Waldemar Adam、Nicholas J. Turro
DOI:10.1021/ja046115a
日期:2004.9.1
Dye-exchanged Y zeolite is shown to be an effective medium to control the stereoselectivity in the photooxygenation of chiral oxazolidinone-functionalized Z/E-1 enecarbamates. An enantioselectivity (ee) as high as 80% was observed in the methyldesoxybenzoin (MDB) product, obtained in the methylene-blue-exchanged NaY zeolite at room temperature. The efficacy of the asymmetric induction in the MDB product depends on the Z/E geometry of the alkene, the Z-isomer being more effective than the corresponding E-isomer. The stereoselectivity is rationalized in terms of conformational effects through cationic interactions between the zeolite and the substrate.
Stereochemical Features of the Physical and Chemical Interactions of Singlet Oxygen with Enecarbamates
作者:Thomas Poon、Nicholas J. Turro、Jessica Chapman、P. Lakshminarasimhan、Xuegong Lei、Steffen Jockusch、Roberto Franz、Ilyas Washington、Waldemar Adam、Sara G. Bosio
DOI:10.1021/ol035856y
日期:2003.12.1
[GRAPHICS]Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in O-1(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with O-1(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects.