Abstract:The highly enantioselective construction of a quaternary carbon center of dihydroquinazoline by an asymmetric Mannich reaction and chiral recognition are described. The key transformation was to establish the chiral trifluoromethyl quaternary carbon center by a diamine-Brønsted acid-catalyzed enantioselective and regioselective Mannich reaction of a methyl ketone and 4-trifluoromethyldihydroquinazoline. An unusual phenomenon of self-discrimination of enantiomers in hydrogen-bonded dimers was observed. A valuable intermediate was transformed into the enantiopure HIV reverse transcriptase inhibitor DPC 083 (> 99.9 ee) simply by reduction of the carbonyl group and elimination of the hydroxy group in hexamethylphosphoric tramide (HMPA).

This study presented a failure analysis of a 52.3 m composite wind turbine blade under static loading. Complex failure characteristics exhibited at the transition region of the blade were thoroughly examined and typical failure modes were indentified. In order to predict multiple failure modes observed in the tests and gain more insights into the failure mechanisms of the blade, a Finite Element (FE) simulation was performed using a global-local modeling approach and Progressive Failure Analysis (PFA) techniques which took into account material failure and property degradation. Failure process and failure characteristics of the transition region were satisfactorily reproduced in the simulation, and it was found that accumulated delamination in spar cap and shear web failure at the transition region were the main reasons for the blade to collapse. Local buckling played an important role in the failure process by increasing local out-of-plane deformation, while the Brazier effect was found not to be responsible for the blade failure.

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