| Summary: | For a long time, the organic chemistry of sulfur dioxide (SO<sub>2</sub>) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO<sub>2</sub> generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO<sub>2</sub> have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO<sub>2</sub> are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO<sub>2</sub>). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl<sub>2</sub>, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C<sub>1</sub>–C<sub>11</sub>-fragment and Koert’s C<sub>16</sub>–C<sub>I</sub> fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.
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