TY  - JOUR
A1  - Bauch, Marcel
A1  - Fudickar, Werner
A1  - Linker, Torsten
T1  - Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates
JF  - Molecules : a journal of synthetic chemistry and natural product chemistry
N2  - Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.
KW  - singlet oxygen
KW  - photooxygenation
KW  - naphthalenes
KW  - carbohydrates
KW  - stereoselectivity
KW  - auxiliary control
KW  - [4+2] cycloaddition
Y1  - 2021
U6  - https://doi.org/10.3390/molecules26040804
SN  - 1420-3049
VL  - 16
IS  - 4
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Bauch, Marcel
A1  - Krtitschka, Angela
A1  - Linker, Torsten
T1  - Photooxygenation of oxygen-substituted naphthalenes
JF  - Journal of physical organic chemistry
N2  - The reaction of oxygen-substituted naphthalenes with singlet oxygen (O-1(2)) has been investigated, and labile endoperoxides have been isolated and characterized at -78 degrees C for the first time. Low-temperature kinetics by UV spectroscopy revealed that alkoxy and silyloxy substituents remarkably increase the rate of photooxygenations compared to 1,4-dimethylnaphthalene, whereas acyloxy-substituted acenes are inert towards O-1(2). The reactivities nicely correlate with HOMO energies and free activation energies, which we determined by density functional theory calculations. The lability of the isolated endoperoxides is due to their very fast back reaction to the corresponding naphthalenes even at -20 degrees C under release of O-1(2), making them to superior sources of this reactive species under very mild conditions. Finally, a carbohydrate-substituted naphthalene has been synthesized, which reacts reversibly with O-1(2) and might be applied for enantioselective oxidations in future work.
KW  - kinetics
KW  - labile peroxides
KW  - low-temperature experiments
KW  - naphthalenes
KW  - singlet oxygen
Y1  - 2017
U6  - https://doi.org/10.1002/poc.3734
SN  - 0894-3230
SN  - 1099-1395
VL  - 30
SP  - 6803
EP  - 6813
PB  - Wiley
CY  - Hoboken
ER  -