Elimination—addition 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 ... 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

The thermolysis of (358) also leads to aromatisation, in this case in a process believed to involve an intermediate nitrile ylid. Evidence for this is obtained by thermolysis of a series of cyclopropenyl-substituted oxazolinones such as (359) for which cycloreversion with elimination of C02 is known to lead to a nitrile ylid. In some cases the ylid could be trapped by addition to methyl propiolate. Substituent effects suggest that the nitrile ylids undergo stepwise addition to produce a bicyclobutyl zwitterion which can either collapse to an azabenzvalene or rearrange to a cyclobutenyl cation 286>. 

The Diels-Alder reactivity of the cyclopropenones has been examined theoretically and is well established but not extensive. The initial [2 -i- 4] adducts are rarely isolated but react further by cycloreversion , elimination , solvent addition or... 

Taylor and Raw recently designed a tethered imine-enamine cascade sequence that converts 1,2,4-triazenes into substituted pyridines. In the presence of molecular sieves, A-methylethylenediamine (147) underwent condensation with excess cyclic ketone 148 (n — 1-4) to give imine-enamine 150 (04CC508). The enamine portion of the molecule then participated in an inverse-demand Diels-Alder cycloaddition reaction with 149 to provide intermediate 151. Cycloreversion of 151 with loss of N2 then gave 152 in which the tertiary amino group underwent addition to the adjacent imine functionality to afford zwiterionic 153. Finally, an intramolecular Cope elimination produced 154 in 74-100% yield. Several other triazines were also shown to participate in this novel cascade (Scheme 27). 

A domino sequence comprising a cycloaddition and subsequent cycloreversion step can often find a more general application in organic synthesis, especially in the formation of aromatic compounds such as furans or pyrroles. Oxazole moieties as electron-deficient dienes often serve as the crucial reactive centers which cycloadd to a triple bond and eliminate a nitrile upon cycloreversion. If the first step is intramolecular, the impelling enthalpy preserved in the stability of the formed CN function is additionally accompanied by a positive entropy when the nitrile, sometimes volatile, leaves the substrate. In an older example from 1984 [10], Jacobi and coworkers devised a scheme for the preparation of a highly substituted furan on their synthetic way to paniculide A. An intramolecular Diels-Alder reaction was followed by the critical extrusion of volatile acetonitrile, furnishing the bicycle 8 in 94% yield (Scheme 6.2). 

Reaction of an anti head-to-tail 4-chlorocoumarin dimer (34) with amines afforded benzopyranocoumarin derivatives (35) in high yields the reaction is different from the expected reaction of coumarin dimer derivatives with amines. On the basis of spectral information, a possible reaction mechanism (Scheme 6) was proposed involving, in sequence, a tandem iactone ring-opening reaction, elimination, cycloreversion of cyclobutene, addition-elimination, and Michael addition. " ... 

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