Expeditious diastereoselective synthesis of medium ring heterocycle-fused chromenes <i>via</i> tandem 8/9-<i>endo-dig</i> and 8-<i>exo-dig</i> hydroalkoxylation-formal-[4 + 2] cycloaddition

Abstract

The first examples of highly diastereoselective tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition are described for the synthesis of medium ring heterocycle-fused chromenes. TMS-alkynols preferred the exo-dig mode of hydroalkoxylation over the endo-dig mode leading to spiro-cyclic chromenes. The method could be used for the synthesis of linearly-fused ladder-like polyethers. A thia-heterocycle-fused chromene could be transformed into a complex bridged tricyclic ketal by a tandem carbene-insertion-[2,3]-sigmatropic shift.