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A modular synthesis for pochonin C and radicicol is reported. The two natural products were prepared in seven and eight steps, respectively, from three readily available fragments. Alternative syntheses of these compounds were achieved using a combination of polymer-bound reagents and solid phase reactions. The conformation of the two natural products was studied and compared by using 2D NMR spectroscopy.

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Back to its origins: Pochonin C was synthesized in 11 steps from the four fragments shown in the scheme. This asymmetric synthesis and the conversion of pochonin C into radicicol defines the configuration at the center bearing the chlorine atom as S. Ipc=isopenocampheyl, PG=protecting group

Hyptatic acid-A (32), a 2alpha,3beta,24-trihydroxyolean-12-en-28-oic acid, previously isolated from Hyptis capitata, was obtained from maslinic acid (2). The regioselective cyclopalladation of the axial methyl group on C-4 of maslinic acid afforded the C-24 hydroxymethylene group due to the presence of a C-2-OR substituent. Nevertheless, hederagenin (7) (23-hydroxy derivative) was formed when this oxygenated group was not present.

The crystal structure of the diethyl 2-benzimidazol-1ylsuccinate-picric acid (1 / 1) molecular complex has been determined by X-ray diffraction analysis. Diethyl 2benzimidazol-l-ylsuccinate molecules form channels along the a axis, in which the picric acid molecules are located. The benzimidazole moiety and the phenol group are held together by hydrogen bonding between the hydrogen of the phenol and the N3 atom of benzimidazole. Additionally, this hydrogen forms an intramolecular hydrogen bond with one O atom of the ortho-nitro group, thus producing a bifurcated hydrogen bond. I H NMR spectra in DMSO-d6 solution and CP/MAS solid 13C NMR studies of this 2-benzimidazol-1-ylsuccinate-picric acid (1/1) molecular complex, as well as those of dimethyl, diethyl, di-n-butyl and 1-n-butyl-4-ethyl 2imidazol-1-ylsuccinates, diethyl 2-pyrazol-1-ylsuccinate, ethyl imidazol-1-ylacetate, ethyl pyrazol-1-ylacetate and ethyl pyrazol-l-ylsuccinate, suggest that the picric acid linkage depends on the nature of the azole. Actual proton transfer is deduced for the imidazole derivatives, but only weak hydrogen bonding could be inferred for pyrazole derivatives.

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