Regioselective Synthesis of Trisubstituted 2,3-Dihydrofurans from Donor−Acceptor Cyclopropanes or from Reaction of the Corey Ylide with α-Sulfenyl-, α-Sulfinyl-, or α-Sulfonylenones

[reactions: see text] Regioselective synthesis of 2,4,5- or 3,4,5-trisubstituted 2,3-dihydrofurans has been realized by using donor-acceptor cyclopropanes or by a Corey ylide reaction with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones. The method allowed a straightforward synthesis of the natural product calyxolane B.