2,3-DIHALO-1-(PHENYLSULFONYL)-1-PROPENES AS VERSATILE REAGENTS FOR THE SYNTHESIS OF ANNULATED FURANS AND CYCLOPENTENONES

2,3-Dihalo-1-(phenylsulfonyl)-1-propenes (DBP and DIP) are conveniently prepared by treating 1-(phenylsulfonyl)-1,2-propadiene with the appropriate halogen. These novel reagents undergo reaction with a variety of simple beta-dicarbonyl anions to give substituted and annulated furans. When the reaction is carried out in polar solvents, 2,3,4-trisubstituted furans are formed. The reaction proceeds by an initial addition-elimination of the carbanion onto the vinyl carbon of the unsaturated sulfone which is followed by intramolecular ring closure on the enolate oxygen atom. When sodium methoxide is used as the base, the initially produced adduct undergoes deacylation and subsequent cyclization to give a 2,4-disubstituted furan. The synthetic utility of the method is demonstrated by a synthesis of (R)-menthofuran. Treatment of DIP with various trimethylsilyl enol ethers in the presence of silver tetrafluoroborate give alkylation products derived from S(N)2 displacement of the terminal halide. These compounds readily cyclize with base to produce an isomeric set of furans. Anions derived from 1,3-dicarbonyls substituted in the C-2 position are found to induce a complete reversal in the mode of ring closure. The major products obtained are 3-[(phenylsulfonyl)methyl]-substituted cyclopentenones. The internal displacement reaction leading to the furan ring apparently encounters an unfavorable A1,3-interaction in the transition state when a substituent group is present at the 2-position of the dicarbonyl compound. This steric interaction is not present in the transition state leading to the cyclopentenone ring. An efficient synthesis of cis-jasmone was carried out using this methodology.