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[摘要]：New model compds. were investigated within our efforts to synthesize the natural product heliquinomycin. We could provide good evidence that electronic effects prevent the spiroketal formation of compds. As a consequence, precursors I and II were prepd., which do not exert the strong electron-withdrawing effect. Dihydroisocumarin deriv.III and a phthalide were coupled by Heck reactions with an enone to provide the required precursors I and II. After reductive debenzylation the resulting intermediates were treated with anhyd. hydrochloric acid in alcs. to give spiroketals in reasonable yields. The spiroketalizations occur under thermodn. control affording products with the hydroxyl group in a trans-relationship with respect to the pyran oxygen atom. Attempts to oxidatively convert a dihydroisocumarin into an isocoumarin deriv. failed. In one attempt a ring contraction to new spiroketal IV was obsd. |
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