[reaction: see text] In the presence of a Lewis or protic acid and at low temperature, 1,3-dioxy-1,3-dienes add to sulfur dioxide generating zwitterionic intermediates that can react with carbon nucleophiles such as allylsilanes. After a retro-ene elimination of SO(2), valuable polyketide precursors are obtained.

At low temperature and in the presence of an acid catalyst, SO 2 adds to 1,3-dienes equilibrating with the corresponding 3,6-dihydro-1,2-oxathiin-2-oxides (sultines). These compounds are unstable above -60°C and equilibrate with the more stable 2,5-dihydrothiophene 1,1-dioxides (sulfolenes). The hetero-Diels-Alder additions of SO 2 are suprafacial and follow the Alder endo rule. The sultines derived from 1-oxy-substituted and 1,3-dioxydisubstituted 1,3-dienes cannot be observed at -100°C but are believed to be formed faster than the corresponding sulfolenes. In the presence of acid catalysts, the 6-oxy-substituted sultines equilibrate with zwitterionic species that react with electron-rich alkenes such as enoxysilanes and allylsilanes, generating β,γ-unsaturated silyl sulfinates that can be desilylated and desulfinylated to generate polypropionate fragments containing up to three contiguous stereogenic centers and an (E)-alkene unit. Alternatively, the silyl sulfinates can be reacted with electrophiles to generate polyfunctional sulfones (one-pot, four-component synthesis of sulfones), or oxidized into sulfonyl chlorides and reacted with amines, then realizing a one-pot, four-component synthesis of polyfunctional sulfonamides. Using enantiomerically enriched dienes such as 1-[(R)-or 1-(S)-phenylethyloxy]-2-methyl-(E,E)-penta-1,3-dien-3-yl isobutyrate, derived from inexpensive (R)-or (S)-1-phenylethanol, enantiomerically enriched stereotriads are obtained in one-pot operations. The latter are ready for further chain elongation. This has permitted the development of expeditious total asymmetric syntheses of important natural products of biological interest such as the baconipyrones, rifamycin S, and apoptolidin A.

No abstract

Herein, a major breakthrough in a sulfur dioxide-mediated oxyallylation cascade reaction is reported that allows the preparation of complex long-chain polyketide fragments with more than ten stereogenic centers through a carefully designed desymmetrization process. An allylbissilane is combined, under the appropriate reaction conditions, with two different 1,3-dioxy-1,3-dienes permitting the construction of a 13-membered polypropionate precursor in one pot. Four stereocenters are selectively created during this process. The so-obtained pseudo-C(2)- or -C(S)-symmetric products are desymmetrized through selective deprotection and can be selectively elongated in both directions using aldol chemistry.

Multicomponent reactions O 0359Sulfur Dioxide Mediated One-Pot, Four-Component Synthesis of Polyfunctional Sulfones and Sulfonamides, Including Medium-Ring Cyclic Derivatives. -Further studies into the recently published multi-component synthesis of sulfones and sulfonamides are presented including application to the preparation of thiazonine and thiocine derivatives. In the oxyallylation of enoxysilanes the diastereoselectivity can be increased up to 9:1 depending on the nature of the reactands while the E/Z-selectivity is controlled by the amount of the acid promoter. -(BOUCHEZ, L. C.; TURKS, M.; DUBBAKA, S. R.; FONQUERNE, F.; CRAITA, C.; LACLEF, S.; VOGEL*, P.; Tetrahedron 61 (2005) 48, 11473-11487; Lab. Glycochem. Asymmetric Synth., Ec. Polytech. Fed. Lausanne, CH-1015 Lausanne, Switz.; Eng.) -Nuesgen 12-066