A synthesis of the AB‐ring substructure of granaticin A was developed. The pyranolactone moiety was stereoselectively accessed by Sharpless asymmetric dihydroxylation and subsequent oxa‐Pictet–Spengler cyclization. The use of BF3·OEt2 resulted in the formation of the cis pyranolactone, whereas the combination of BF3·OEt2 with trifluoroacetic acid led to the trans isomer. The resulting hydroquinones were cleaved selectively by ozonolysis to dicarboxylic acids. An aryl Grignard reagent could be regioselectively added to unsymmetrical anhydrides. As an alternative strategy for the construction of the B‐ring, a benzyne–furan cycloaddition could be established.
Key indicators: single-crystal X-ray study; T = 300 K; mean (C-C) = 0.013 Å; R factor = 0.080; wR factor = 0.173; data-to-parameter ratio = 15.2.The packing of the title compound, [Na(C 9 H 5 ClNOS 2 )-(H 2 O)] n , in the crystal structure occurs by pairwise attachment of +sc-and Àsc-arranged 4-(4-chlorophenyl)-2-thioxo-2,3-dihydrothiazol-3-olate subunits via S to sodium. Water molecules that are bound in the axial position of the distorted octahedral coordination octahedron give rise to a stereogenic center at sodium.
Related literatureFor related literature, see: Allen et al. (1987); Hartung et al. (1996 Hartung et al. ( , 1999 Hartung et al. ( , 2007; Nardelli (1999).
ExperimentalCrystal data
The BCD‐ring substructure of granaticin A was synthesised following a new approach for the construction of the naphthoquinone moiety. The 2‐oxabicyclo[2.2.2]oct‐5‐ene substructure was accessible stereoselectively using a Sharpless asymmetric dihydroxylation and a diastereoselective ketone reduction in combination with Yoshii's route. The naphthoquinone B‐ring was prepared by addition of an aryllithium intermediate to an anhydride followed by a Friedel–Crafts cyclisation mediated by AlCl3 and Mg(OTf)2. The success of the Friedel–Crafts cyclisation relied on the conversion of the ketone‐carboxylic acid into a lactone acetal.
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