Add time:09/26/2019         Source:sciencedirect.com

Publisher SummaryThe twenty-five carbon backbone of zincophorin contains interesting structural elements, such as a propionic acid unit branched at C2 with a trisubstituted tetrahydropyran (C3–C7), alternate methyl and hydroxyl groups (C8–C13 and C18–C 19), one disubstituted double bond (C16–C17) and a trisubstituted bond (C20–C21) both of (E) configuration, and a remote isolated stereocenter (C22). Zincophorin is less oxygenated than most of the other ionophore antibiotics and contains a single oxygen heterocycle. The structure of zincophorin suggests the involvement of propionic acid subunits, followed by extensive reduction or dehydration reactions after the assembly of the polyketide chain. The synthesis highlights the potential of Lewis acid-promoted cyclocondensation between silyloxydienes and aldehydes as a route toward oxygen heterocycles, as well as polypropionate units after subsequent manipulation of the dihydropyrone adducts. The silylation of the primary lithium alkoxide has been efficiently carried out by the addition of a mixture of tert-butyldiphenylsilyl chloride and imidazole to afford the silyl ether. The major diastereomeric homopropargylic alcohol formed during the course of the allenylzinc addition, which possesses the required absolute configurations at C9 and C10 for the synthesis of zincophorin, has been isolated in acceptable yield (63%) and partially hydrogenated to the homoallylic alcohol.

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