Carbon dioxide Cycloreversion

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

The three-component reaction between isatin 432a, a-aminoacids 433 (proline and thioproline) and dipolarophiles in methanol/water medium was carried out by heating at 90 °C to afford the pyrrolidine-2-spiro-3 -(2-oxindoles) 51. The first step of the reaction is the formation of oxazlidinones 448. Loss of carbon dioxide from oxazolidinone proceeds via a stereospecific 1,3-cycloreversion to produce the formation of oxazolidinones almost exclusively with /razw-stereoselectivity. This /f-azomethine ylide undergo 1,3-dipolar cycloaddition with dipolarophiles to yield the pyrrohdinc-2-r/ V -3-(2-oxindolcs) 51. (Scheme 101) <2004EJ0413>. 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

It is assumed that the reaction proceeds with attack of phosphite on sulphur leading to a carbene, which may react with a second equivalent of phosphite during the cycloreversion that splits out carbon dioxide and yields the product alkene. 

A final example of a rDA cycloreversion of a cycloadduct that acts as a protected butadiene illustrates the generation of extended conjugation in a non-aromatic cyclic system. Equation (65) shows that the addition of cyclopropene (147) to (146) can yield either (148) or (149), and both adducts undergo rDA extrusion of carbon dioxide to produce a triene. Hydrolysis of the triene ketals gives access to substituted tropones (150). 

Though a higher reaction temperature is needed (185-195°C), 3-aryl-2,2-bis(trifluoromethyl)-5-oxazolidinones also undergo a cycloreversion losing carbon dioxide (77MI1). The azomethine ylides 101, substituted with two trifluoromethyl moieties on the same carbon, either cyclize into the corresponding aziridines when no dipolarophile is present, or are captured by dimethyl acetylenedicarboxylate as cycloadducts. 

The mechanism of formation of 3 involves a concerted cycloreversion to cyclopropanone (which decomposes to ethene and carbon monoxide), carbon dioxide and cyclopropanethione (4), which suffers valence tautomerization to yield methylenethiirane (3). ... 

Photochemical (2 + 2)-cycloreversions of heterocyclic compounds are rare. The dimer of xanthotoxin, a naturally occurring furocoumarin, was reported to undergo a photochemical (2 + 2)-cycloreversion upon irradiation (A > 250 nm).137 Irradiation of 2-oxabicyclo[2.2.0]hex-5-ene-3-ones (224) at temperatures of 8° to 20° K resulted in the elimination of carbon dioxide [Eq. (12)].12,307 Similarly, pyridine was found to give... 

The Diels—Alder reaction is a reversible reaction and the direction of cycloaddition is favored because two tt-bonds are replaced by two a-bonds. The cycloreversion also occurs more readily when the diene and/or dienophile are particularly stable molecules (i.e., formation of an aromatic ring, nitrogen, carbon dioxide, ethene, ethyne, nitriles, etc.) or when one of them can be easily removed or consumed in a subsequent reaction (Scheme 4.20). 

A number of other methods have also been used to generate nitrile ylides. Access to this 1,3-dipole has been realized by (a) treatment of imidoyl halides (5) with base, (b) thermal or photochemical elimination of phosphoric acid ester from 4,5-dihydro-1,3,5-oxazaphospholes (6), (c) 1,3-dipolar cycloreversion of 1-azetidines (7), (d)loss of carbon dioxide from 3-oxazolin-5-ones (g)44-47 treatment of boron-containing isonitriles (9) with base ... 

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