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N Probst A Madarász A Valkonen I Pápai K Rissanen A Neuvonen PM Pihko 《Angewandte Chemie (International ed. in English)》2012,51(34):8495-8499
Hold them tight: Guided by X-ray structures, bifunctional thiourea catalysts containing an activating intramolecular hydrogen bond were redesigned. The new catalysts were used to effect a highly enantioselective Mannich reaction between malonates and both aliphatic and aromatic imines (see scheme; Boc=tert-butoxycarbonyl). 相似文献
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Nicolaou KC Pihko PM Bernal F Frederick MO Qian W Uesaka N Diedrichs N Hinrichs J Koftis TV Loizidou E Petrovic G Rodriquez M Sarlah D Zou N 《Journal of the American Chemical Society》2006,128(7):2244-2257
Syntheses of the three key building blocks (65, 98, and 100) required for the total synthesis of the proposed structure of azaspiracid-1 (1a) are described. Key steps include a TMSOTf-induced ring-closing cascade to form the ABC rings of tetracycle 65, a neodymium-catalyzed internal aminal formation for the construction of intermediate 98, and a Nozaki-Hiyama-Kishi coupling to assemble the required carbon chain of fragment 100. The synthesized fragments, obtained stereoselectively in both their enantiomeric forms, were expected to allow for the construction of all four stereoisomers proposed as possible structures of azaspiracid-1 (1a-d), thus allowing the determination of both the relative and absolute stereochemistry of the natural product. 相似文献
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[structure: see text] A concise synthesis of both AB ring spiroisomers of the pectenotoxins is described. The nonanomeric AB spiroketal ring system of the pectenotoxins-1, -2, -3, and -6 is formed under very mild, kinetic spiroketalization conditions, along with the anomeric isomer. Only catalytic asymmetric transformations were used as the source of chirality in the synthesis route. 相似文献
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Pápai I Hamza A Pihko PM Wierenga RK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(10):2859-2866
Protein crystallographic analysis of the active sites of enolizing enzymes and structural analysis of hydrogen-bonded carbonyl compounds in small molecule crystal structures, complemented by quantum chemical calculations on related model enolization reactions, suggest a new stereoelectronic model that accounts for the observed out-of-plane orientation of hydrogen-bond donors (HBDs) in the oxyanion holes of enolizing enzymes. The computational results reveal that the lone-pair directionality of HBDs characteristic for hydrogen-bonded carbonyls is reduced upon enolization, and the enolate displays almost no directional preference for hydrogen bonding. Positioning the HBDs perpendicular to the carbonyl plane induces strain in the catalyst-substrate complex, which is released upon enolization, resulting in more favorable kinetics and thermodynamics than the in-plane arrangement of HBDs. 相似文献