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1 2003

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Synteza wielofunkcyjnych pochodnych tiopiranu w reakcjach cykloaddycji Dielsa-Aldera

Bogdanowicz-Szwed K., Budzowski A.

Wiadomości Chemiczne

2003

[Z] 57, 5-6

435--457

The hetero-Diels-Alder (HDA) methodology employing -unsaturated thiocarbonyl compounds represents a straightforward and efficient approach to sulfur-containing six-membered heterocycles. This review presents applications of α,β-unsaturated thiocarbonyl compounds in heterodiene reactions and covers the literature published in the last two decades. The α,β-unsaturated thiocarbonyl compounds, formally present 1-tia-1,3-butadiene system and include the following groups of compounds: thiochalcones, enaminothiones, α,β-unsaturated thioamides and compounds containing thiocarbonyl groups conjugated with carbon-carbon double bonds of carbo- and heteroaromatic rings. In general, reactions of 1-tia-1,3-butadiene systems with dienophiles belong to cycloadditions with normal electron demand, although there are some examples of cycloadditions with inverse electron demand. The first chapter concerns the cycloadditions of thiochalcones. Most of them are synthesized from chalcones in reactions with Lawesson reagent. Thiochalcones are unstable, at room temperature they are in equilibrium with two dimeric forms: thiopyran and 3,4-dihydro-1,2-dithiin. Both compounds are formed by heterodiene cycloaddition of two molecules of thiochalcone. At higher temperatures the dimers undergo decomposition to thiochalcones. Thiochalcones generated in situ by heating the dimers were successfully used in cycloadditions to acryloamide, styrene and enol ethers well as in an asymmetric hetero Diels-Alder (AHDA) reactions with (-) dimenthyl fumarate and in the reactions catalyzed by ytterbium triflate. The next chapter deals with the reactions of enaminothiones. These compounds reacted smoothly with dienophiles containing electron withdrawing groups yielding cycloadducts in high yields. Some cycloadducts easily eliminated amine furnishing stable 2H-thiopyran derivatives. Reactions of enaminothiones with substituted b-nitrostyrenes proceeded in region- and stereoselective manner. In contrast to enaminothiones, α,β-unsaturated thioanilides were found to be less reactive in hetero-Diels-Alder reactions. Two molecules of thiocinnamamide underwent cycloaddition in acetyl chloride yielding N-acylated derivative of 3,4-dihydro-2H-thiopyran. N-acylated α,β-unsaturated thioanilides was successfully used in heterodiene reactions with N-phenylmaleimide, cyclopentene and enol ether. A recent applications of N-acylated α,β-unsaturated thioanilides deal with intramolecular hetero-Diels-Alder reactions leading to tricyclic thiopyran derivatives. The compounds containing thiocarbonyl group conjugated with carbon-carbon double bond of furan or thiophene skeleton undergo cycloaddition with maleic anhydride, cyclopentene and norbornene yielding fused 2H-thiopyran derivatives in good yield. Some of the cycloadducts are unstable and spontaneously rearrange to aromatized compounds. An application of some organometallic reagents e.g. (Me2Al)2 S in thionation of aromatic ketones extended their use to synthesis of various thiopyrans via hetero-Diels-Alder reactions.