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.
Enecarbamates as Selective Substrates in Oxidations: Chiral-Auxiliary-Controlled Mode Selectivity and Diastereoselectivity in the [2+2] Cycloaddition and Ene Reaction of Singlet Oxygen and in the Epoxidation by DMD and <i>m</i>CPBA
作者:Waldemar Adam、Sara G. Bosio、Nicholas J. Turro、Barbara T. Wolff
DOI:10.1021/jo035745c
日期:2004.3.1
The stereochemical course of the oxidation of chiral oxazolidinone-substituted enecarbamates has been studied for singletoxygen (1O2), dimethyldioxirane (DMD), and m-chloroperbenzoic acid (mCPBA) by examining of the special structural and stereoelectronic features of the enecarbamates. Valuable mechanistic insight into these selective oxidations is gained. Whereas the R1 substituent on the chiral
手性恶唑烷酮取代enecarbamates氧化的立体化学过程已经研究了单重态氧(1 Ò 2),二甲基二(DMD),和中号氯过苯甲酸(米通过的enecarbamates的特殊的结构和立体电子特征检查CPBA) 。获得了对这些选择性氧化的有价值的机理见解。而手性助剂上的R 1取代基负责双键的空间屏蔽,并决定π面非对映选择性的感觉,Z / E构型等结构特征和R 2的性质双键上的基团负责非对映选择性的程度。在1 O 2的情况下,通过乙烯基氮官能团进行的立体电子控制可控制模式选择性(烯反应与[2 + 2]环加成反应)。