A versatile and highly stereoselective synthesis of
spiro ethers and spiro ketals is presented. The
key step in the developed synthetic sequence is based on
diastereoselective intramolecular
photoaddition of dihydro-4-pyrones to chiral 1,3-dioxin-4-ones.
Subsequent fragmentation of the
produced four-membered ring provides spiro ether structures. The
spiro ethers can be transformed
to their corresponding spiro ketals, with retention of configuration at
the spiro center, via Baeyer−Villager oxidation. The configuration of the spiro center is
defined by the facial selectivity at the
photocycloaddition step. Two examples of complete stereofacial
selectivity were achieved. The
unique and important application of the developed sequence was
demonstrated in enantioselective
synthesis of a less thermodynamically stable spiro ketal
43.
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