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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.

Synteza sześcioczłonowych azotowych związków heterocyklicznych w reakcjach cykloaddycji heterodienowej

Bogdanowicz-Szwed K., Krasodomska M.

Wiadomości Chemiczne

1998

[Z] 52, 11-12

821--842

In this article we describe new developments in the synthesisof nitrogen heterocycles by hetero-Diels-Alder reaction, covering the literature since 1990. A main point of the discussion is stereoselectivity of the reactions and the preparation of enantiopure compounds either by using dienes and dienophiles carrying a chiral auxiliary or by using Lewis acids. Six-member nitrogen-heterocyclic compounds can be obtained by cycloaddition of aza-1,3-butadienes with carbon dienophiles or by cycloaddition of electron-rich dienes with imines, iminiun ions and nitriles acting as dienophiles. 1-Aza-1,3-butadienes may react as electron-rich as well as electron-deficient dienes in normal cycloaddition or in the cycloaddition with inverse electron demand. E.g. N,N-dimethyl hydrazones prepared from a,b-unsaturated aldehydes 1 reactsmoothly in normal cycloaddition with electron deficient dienophile. However, N-acyl- and N-sulfonyl-imines of a,b-unsaturated aldehydes, e.g. 10, react as electron-deficient dienes in reaction with an inverse electrondermand. Similarly to 1-aza-, 2-aza-1,3-butadienes may react as electron-rich or as electron-deficient components upon appropriate substituation. A versatile approach to functionalized pyrimidines involves the reaction of 1,3-diaza-1,3-butadienes 60 with ketenes. Intramolecular cycloadditions of 1,3-diazadienes 71 efficiently vield complex polycyclic molecules. Theoretical and synthetical studies carried out by van der Plas et al., deal with intramolecular cycloaddition of 5(v-alkynyl)pyrimidines 77. The substrates initially formed bridged adducts, that upon heating release the fused pyridine derivatives and appropriate cyanides by retro Diels-Alder reaction. The second approach to construction of six-membered nitrogen-heterocyclenby Diels-Alder reaction consists in cycloaddition of electron-rich-dienes with iminesor nitriles, that act as dienophiles. Electron deficient imines, e.g. acyl- or sulfonylimines add to many 1,3-dienes in good yields affording tetrahydropyridines. The cycloadditions of nonactivated imines have been investigated with regard to the effects of electronically neutral substituents and the influence of Lewis acids, The recent work in this area deals with asymmetric induction in azadiene reaction. E.g. the utility of chiral imines derived from enantiopure amino acids 93 in cycloaddition with electron-rich diene in presence of Lewis acid lead to pyridone 95 with very good diastereoselectivity (95% de). Chiral Lewis acids were also used in order to carry out enantioselective Diels-Alder reactions starting from achiral substrates and chiral catalyst. E.g. the reaction of imine derived from 3-pyridineformaldehyde 106 and Danishefsky's diene 89 afforded the desired cyclaadduct 107 in high optical purity. Cyanides have been found to be less reactive than imines since the reactions require extremely high temperatures. However, sulfonyl nitriles 112 react with a variety of dienes under much milder conditions affording pyridine derivatives 114.