The boron enolate of pyrone 2 undergoes asymmetric aldol reactions with aldehydes to give protected anti 1,2-diols 3. The pyrone is readily available from trans stilbene using asymmetric dihydroxylation. Yields for the aldol reaction range from 62 to 92% and the selectivities from 6:1 to >20:1 for the anti isomers. Protection and hydrogenolysis of the products can be used to remove the pyrone, giving differentially protected diol intermediates 12 that are amenable to multistep synthesis.
[figure: see text] A synthesis of the left-hand portion of the ansamycin antitumor natural product geldanamycin is reported. An advanced intermediate incorporates the methoxyquinone precursor as a pentasubstituted benzene with a 10-carbon chain that contains 4 of the 6 stereocenters. The key reaction is a novel anti glycolate aldol reaction with a new diaryl-4-oxapyrone used to generate the C-11, C-12 hydroxy, methoxy functionality.
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