Diels-Alder reactions of thiazole

Oxazoles represent the most widely recognized heteroaromatic azadiene capable of [4 + 2] cycloaddition reactions. The course of the oxazole Diels-Alder reaction and the facility with which it proceeds are dependent upon the dienophile structure (alkene, alkyne), the oxazole and dienophile substitution, and the reaction conditions. Alkene dienophiles provide pyridine products derived from fragmentation of the [4 + 2] cycloadducts which subsequently aromatize through a variety of reaction pathways to provide the substituted pyridines (Scheme 14). In comparison, alkyne dienophiles provide substituted fiirans that arise from the retro Diels-Alder reaction with loss of R CN from the initial [4 + 2] cycloadduct (Scheme 15,206 Representative applications of the [4 + 2] cycloaddition reactions of oxazoles are summarized in Table 14. Selected examples of additional five-membered heteroaromatic azadienes participatiitg in [4 + 2] cycloaddition reactions have been detailed and include the Diels-Alder reactions of thiazoles, - 1,3,4-oxadiazoles, isoxazoles, pyrroles and imidazoles. ... 

Alkoxythiazoles (68) give an abnormal Diels-Alder reaction with highly reactive dienophiles such as 4-phenyl-3//-l,2,4-triazole-3,5(4/0-dione (PTAD), diethyl azodicarboxylate (dead), or diethyl oxomalonate. As an example, the reaction of (68) with PTAD (69) is illustrated in Equation (17). The product of the reaction (70) is the formal [3-1-2] cycloadduct between (69) and the corresponding nitrile ylide derived by the opening of thiazole ring system. For this abnormal Diels-Alder reaction of thiazoles a stepwise mechanism has been proposed <92BCJ3315>. 

Unlike the case of oxazoles, the attempted intramolecular Diels-Alder reactions of thiazoles bearing electron-deficient alkynes have proved unsuccessful.59 ... 

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See also in sourсe #XX -- [ ]

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