Abstract:The diastereospecific formation of dihydropyrimidines (DHPMs) has been achieved in moderate to high yields with up to 99% ee by a Biginelli reaction. The reaction was performed by using a combined catalyst consisting of a chiral bifunctional primary amine-thiourea 9f and a Brønst-ed acid with tert-butylammonium trifluoroacetate (t-BuNH 2 ·TFA) as additive in dichloromethane at room temperature. The possible mechanism for the reaction has been proposed to explain the origin of the activation and the asymmetric induction.
We
have developed an efficient and mild enantioselective palladium-catalyzed
(5+3) cycloaddition of vinylcyclopropanes and oxidopyrylium ylides
generated in situ from benzopyranones, in the presence of a chiral
PHOX ligand. These reactions afford various highly functionalized
bridged oxa-[3.3.1]carbocycles with three stereogenic centers that
are challenging to synthesize, in moderate to good yields and enantioselectivities.
The organic volatile flavor compounds in fermented stinky tofu (FST) were studied using SPME-GC/MS. A total of 39 volatile compounds were identified, including nine esters, seven alcohols, five alkenes, four sulfides, three heterocycles, three carboxylic acids, three ketones, two aldehydes, one phenol, one amine and one ether. These compounds were determined by MS, and conformed by comparison of the retention times of the separated constituents with those of authentic samples and by comparison of retention indexes (RIs) of separated constituents with the RIs reported in the literature. The predominant volatile compound in FST was indole, followed by dimethyl trisulfide, phenol, dimethyl disulfide and dimethyl tetrasulfide. In order to find a better extraction time, the extraction times was optimized for each type of SPME fiber; the results show that the best extraction time for Carboxen/PDMS is 60 min, for PDMS/DVB 30 min, for DVB/CAR/PDMS 60 min and for PDMS 75 min. Of the four fibers used in this work, Carboxen/PDMS is found to be the most suitable to extract the organic volatile flavor compounds in fermented stinky tofu.
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