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Stoichiometric Reactions of Enamines Derived from Diphenylprolinol Silyl Ethers with Nitro Olefins and Lessons for the Corresponding Organocatalytic Conversions – a Survey
Authors:Dieter Seebach  Xiaoyu Sun  Marc‐Olivier Ebert  W Bernd Schweizer  Albert K Beck  Jörg Duschmalé  Helma Wennemers  Yujiro Hayashi  Markus Reiher
Institution:1. Laboratorium für Organische Chemie, Departement Chemie und Angewandte Biowissenschaften, ETH‐Zürich, H?nggerberg HCI, Wolfgang‐Pauli‐Strasse 10, CH‐8093 Zürich, (phone: +41‐44‐632‐2990;2. fax: +41‐44‐632‐1144);3. Tokyo University of Science, Department of Industrial Chemistry, Faculty of Engineering, Kagurazaka, Shinjuku‐ku, Tokyo 162‐8601, Japan and Department of Chemistry, Tohoku University, 6‐3 Aramaki‐Aza, Aoba‐ku, Sendai, Miyagi 980‐8578, Japan (phone: +81‐22‐795‐3554;4. fax: +81‐22‐795‐6566);5. Laboratorium für Physikalische Chemie, Departement Chemie und Angewandte Biowissenschaften, ETH‐Zürich, H?nggerberg HCI F 235, Wolfgang‐Pauli‐Strasse 10, CH‐8093 Zürich, (phone: +41‐44‐633‐4308;6. fax: +41‐44‐633‐1595)
Abstract:The stoichiometric reactions of enamines prepared from aldehydes and diphenyl‐prolinol silyl ethers (intermediates of numerous organocatalytic processes) with nitro olefins have been investigated. As reported in the last century for simple achiral and chiral enamines, the products are cyclobutanes ( 4 with monosubstituted nitro‐ethenes), dihydro‐oxazine N‐oxide derivatives ( 5 with disubstituted nitro‐ethenes), and nitro enamines derived from γ‐nitro aldehydes ( 6 , often formed after longer reaction times). The same types of products were shown to be formed, when the reactions were carried out with peptides H‐Pro‐Pro‐Xaa‐OMe that lack an acidic H‐atom. Functionalized components such as alkoxy enamines, nitro‐acrylates, acetamido‐nitro‐ethylene, or hydroxylated nitro olefins also form products carrying the diphenyl‐prolinol silyl ether as a substituent. All of these products must be considered intermediates in the corresponding catalytic reactions; the investigation of their chemical properties provided useful hints about the rates, the conditions, the catalyst resting states or irreversible traps, and/or the limitations of the corresponding organocatalytic processes. High‐level DFT and MP2 computations of the structures of alkoxy enamines and thermodynamic data of a cyclobutane dissociation are also described. Some results obtained with the stoichiometrically prepared intermediates are not compatible with previous mechanistic proposals and assumptions.
Keywords:Catalysis  Nitro olefins  Aldehydes  Cyclobutanes  Oxazine N‐oxides  Oseltamivir  Density‐functional theory (DFT)
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