Heterocumulenes special

The reverse reaction, i.e., the addition of water to the heterocumulene B with formation of carbonic acid derivatives A, is also known. With very few exceptions this reaction occurs spontaneously with catalysis by an acid or base. The hydration of a heterocumulene is a special example of an entire class of heterocumulene reactions, namely the addition of heteroatom nucleophiles H-Het3 (you have already come across such a reaction briefly in Section 6.1.1 in the context of the DCC activation of carboxylic acids). Occasionally, this addition... 

In many instances, the entropic assistance provided in the intramolecular Diels-Alder reaction is sufficient to promote azadiene participation in Diels-Alder reactions.12 The incorporation of the azadiene system, or dienophile, into a reactive or sensitive system, e.g., heterocumulene or strained olefin, allows a number of specialized azadiene Diels-Alder reactions. Many such examples may represent stepwise, polar [4 + 2] cycloaddition reactions. 

The additional substitution of the heterocyclic azadiene system with electron-withdrawing groups accents the electron-deficient nature of the heterodiene and permits the use of electron-rich, strained, or even simple olefins as dienophiles.3 415 6 Substitution of the heterocyclic azadiene with strongly electron-donating substituents in many instances is sufficient to overcome the electron-deficient nature of the azadiene and permits the use of conventional electron-deficient dienophiles in normal (HOMCWne controlled) Diels-Alder reactions.4 6 The entropic assistance provided by the intramolecular Diels-Alder reaction is sufficient in most instances to override the reluctant azadiene participation in Diels-Alder reactions.7 The incorporation of the heterocyclic azadiene, or the dienophile, into a reactive system, e.g., heterocumulene, allows a number of specialized [4 + 2] cycloaddition processes which are best characterized as stepwise addition-cyclization [4 + 2] cycloadditions.8... 

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