Sulfur dioxide

Summary

Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula SO2. It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activity and is produced as a by-product of copper extraction and the burning of sulfur-bearing fossil fuels. Structure and bonding SO2 is a bent molecule with C2v symmetry point group. A valence bond theory approach considering just s and p orbitals would describe the bonding in terms of resonance between two resonance structures. The sulfur–oxygen bond has a bond order of 1.5. There is support for this simple approach that does not invoke d orbital participation. In terms of electron-counting formalism, the sulfur atom has an oxidation state of +4 and a formal charge of +1. Occurrence Sulfur dioxide is found on Earth and exists in very small concentrations in the atmosphere at about 15 ppb. On other

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https://en.wikipedia.org/wiki/Sulfur_dioxide

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Sulfur dioxide mediated syntheses of polyene containing natural products

Laure Bouchez

The goal of this thesis was to open a new route to the synthesis of conjugated trienes using a new SO2-based C-C bond forming reactions developed in Lausanne. In addition, the potential of SO2 for applications in synthetic organic chemistry has been exploited. Firstly, we discovered that under suitable conditions sulfur dioxide is able to undergo ene reaction with different building blocks such as enoxysilanes and allylsilanes to form the corresponding sulfinates. The latter can be used as key intermediates for the preparation of β-ketones, β-keto-sulfonamides and β-keto-sulfonic esters. This approach allows preparing β-keto-sulfonamides in one-pot operations. Up to now, the same type of compounds would require multi-steps syntheses. Considering the importance of sulfonamides and sulfonic esters in medicinal chemistry we prepared representative examples of such compounds. Secondly, our studies demonstrate, for the first time, that sulfones containing (E)-alkene units can be obtained through the reaction of (E,E)-1-silyloxy-2-methylpenta-1,3-diene with substituted enoxysilanes and an excess of sulfur dioxide, followed by subsequent quenching with electrophiles. Importantly, this four-component reaction yields (E)-alkenes only in the presence of a protic acid promoter and only with 1-silyloxypentadienes. In all cases mixtures of α,β-syn and α,β-anti ketones are obtained. The α,β-diastereoselectivity of the oxyallylation (face selectivity of the enoxysilane addition) does not surpass 6:1 (3R,4S vs.3S,4S). Interestingly only β,ε-like substituted ε-sulfonylketones are formed, in contrast with the (Z)-isomers which present unlike β,ε-relative configuration. Thirdly, sulfur dioxide-based chemistry was applied for the asymmetric synthesis of the conjugated (E,E,E)-triene moiety of apoptolidinone, a highly potent apoptosis inducing agent with a daunting molecular architecture. Two main synthetic routes towards the synthesis of the trienic fragment of apoptolidinone were developed, one leading to the racemic fragment and a second route leading to the enantiomerically enriched C(1)-C(9) fragment. The latter was transformed further into the fragment already synthesized by Nicolaou and co-workers, demonstrating that this second route is significantly shorter than the route previously reported in the literature. The same methodology yields also facile access to the trienic part C(5)-C(14) fragment of restricticin. This underlines further the general applicability and the high flexibilities of our method toward the synthesis of trienes.

EPFL2004

Total Asymmetric Syntheses of beta-Hydroxy-delta-lactones via Umpolung with Sulfur Dioxide

Claudia Jana Exner, Sylvain Claude Laclef, Florent Poli, Maris Turks, Pierre Vogel

Cyclic stereotriads and stereotetrads of the beta-hydroxy-delta-lactone type, e.g. prelactones B and E, common in polyketides and polypropionates, are prepared via SO2-induced oxyallylations of enoxysilanes with (1E,3Z)-1-(1-phenylethoxy)penta-1,3-dien-3-yl carboxylates. Using (Z)- or (E)-enoxysilanes both 4,5-cis- or 4,5-trans-delta-lactones are obtained. Depending on the reduction method applied to the obtained aldol intermediates 5,6-trans or 5,6-cis-derivatives are formed. The delta-lactones can be prepared in both their enantiomeric forms depending on the (1R)- or (1S)-configuration of the starting 1-(1-phenylethoxy)penta-1,3-dienes.

2011

Polyketides show interesting biological activities and therefore represent an important class of natural products. The necessity of their diastereo- and enantioselective synthesis led to the development of numerous methodologies. A brief overview of this topic is presented. The research carried out in our group has led to the development of a C-C-bond forming reaction involving Umpolung of a chiral, electron rich 1,3-dioxy-1,3-diene due to hetero-Diels-Alder reaction with sulphur dioxide. The so-formed six-membered sultines (6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides) can be ionized into a zwitterion in the presence of a Lewis acid promoter, which can be trapped by nucleophiles, e.g. enoxy- or allylsilanes, to generate the corresponding alkenylsulfinates. Protolysis of the latter to sulfinic acids initiates retro-ene desulfitation to produce polyketide- or polypropionate fragments. Studies towards the influence of electronic and steric effects on the diastereoselectivity of the SO2-reaction cascade between 1,3-dioxy-1,3-dienes and enoxysilanes have been performed. A novel series of chiral, p-substituted 1-arylethoxy-3-acyloxy-1,3-dienes has been synthesised and their reactivity and diastereoselectivity in the SO2-cascade has been studied. Enoxysilanes and silyl-based Lewis acid promoters of different steric bulk have been employed in the SO2-cascade with the aim to improve the diastereoselectivity of the reaction. The utility of the SO2-methodology is demonstrated by the short and selective synthesis of several natural and unnatural δ-lactones with various relative configurations from the same starting materials (stereodiversity). The stereoselectivity of the reaction cascade has been verified to be governed exclusively by the chiral auxiliary, even in the presence of additional stereocentres. The methodology has been extended to the synthesis of pseudo-C2- or -CS-symmetrical, long-chain polyketide precursors via bidirectional elongation strategy, either through multistep or one-pot operations employing 1,3-dienes of the novel series and allylsilanes in the first case or allylbissilanes in the second. The so-obtained products are readily desymmetrised and have been shown to undergo selective chain elongations in both directions, which allow for the selective formation of advanced polyketide fragments.

EPFL2010

Sulfur dioxide mediated syntheses of polyene containing natural products

Laure Bouchez

EPFL2004

EPFL2010