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Enantioselective total syntheses of both enantiomers of the recently isolated decanolide natural product seimatopolide A are described. The C-2-symmetric building blocks (R,R)-hexa-1,5-diene-3,4-diol (derived from D-mannitol) and its enantiomer (derived from L-(+)-tartrate) serve as key starting materials, which are elaborated in a bidirectional way using a selective mono-cross-metathesis, regio- and stereoselective epoxidation, and regioselective reductive epoxide opening to furnish the first fragment. Both enantiomers of the second fragment, 3-hydroxypent-4-enoic acid, were conveniently obtained through a lipase-catalyzed kinetic resolution and merged with the first fragment via Shiina esterification. An E-selective ring-closing metathesis was used to access the 10-membered lactone. A comparison of the specific optical rotations of synthetic seimatopolides with those reported for the natural product suggests that the originally assigned (3R,6R,7R,9S)-configuration should be corrected to (3S,6S,7S,9R).
A sequential ruthenium-catalyzed ring-closing metathesis-transfer hydrogenation sequence has been established as a synthesis of chromanes starting from 2-(allyloxy)styrenes. The sequence requires only one precatalyst, the first-generation Grubbs catalyst, which is converted into a ruthenium hydride species in situ. Propan-2-ol serves as a chemical trigger for the formation of the ruthenium hydride and as hydrogen source.
The synthesis of 7-methoxy-8-(4-methyl-3-furyl)-2H-chromen-2-one, a natural product with antileishmanial activity recently isolated from the plant Galipea panamensis, is described. The key step is a Suzuki-Miyaura coupling of a furan-3-boronic acid and an 8-halocoumarin, which is advantageously synthesized using a ring-closing metathesis reaction. Several non-natural analogues are also available along these lines.
Novel substituted pyrimidines were synthesized from methyl 2,4-dioxo-4-phenyl-butanoate (I-A) and urea, followed by Mitsunobu coupling of I-A with benzyl or allyl alcohol to give the corresponding 2-hydroxypyrimidine ethers in good yields. Saponification of I-A, followed by reaction with benzyl or allyl amines in the presence of TBTU yielded 2-hydroxy-6-phenyl-pyrimidine 4-carboxamides. AChE and BuChE assays revealed 2-hydroxy-6-phenyl-pyrimidine-4-carboxyallyamide as the most active compound, IC50=90 mu M, with no inhibition of BuChE.
A one-flask reaction sequence comprising ring closing metathesis (RCM) of butenoates derived from allylic alcohols and a base-mediated ring opening gives 2Z,4E-configured dienoic acids in high yields and stereoselectivities. Application of the method to the synthesis of the natural product fusanolide A suggests that the originally published structure was erroneously assigned and should be revised.
An assisted tandem catalytic transformation of diallyl amines and diallyl ethers into N-aryl pyrroles and furans, respectively, is described. The sequence relies on ring closing metathesis followed by dehydrogenation of the initially formed dihydropyrroles and dihydrofurans. Both steps are Ru-catalyzed, but the sequence requires only one precatalyst, because conversion of the metathesis catalyst into the dehydrogenation catalyst is achieved in situ, triggered by the oxidant tert-butyl hydroperoxide.
alpha,beta-Unsaturated d-lactones are accessible via a sequential ring-closing metathesis (RCM) double-bond migration reaction starting from butenoates of allyl alcohols. This approach proceeds efficiently with lower catalyst loadings and higher initial substrate concentrations compared to the alternative RCM of acrylates derived from homoallylic alcohols.