Retro-Diels—Alder type transform

There are two important general types of reaction by which six-membered heterocycles containing two or more heteroatoms can be transformed into other six-membered heterocyclic systems, namely reactions which involve an ANRORC mechanism, and reactions which proceed by a Diels-Alder/retro-Diels-Alder type of mechanism. Transformation of 1,3-oxazinones and -thiazinones into pyrimidones (equations 193 and 194) has been extensively used, especially in the conversion of isatoic anhydride into quinazolinones (e.g. equation 195). 2,4-Diaryl-l,2,3,5-oxathiadiazines, which are readily accessible by reaction of sulfur trioxide with aryl isocyanates, are useful precursors to pyrimidines and 1,3,5-triazines (equation 196). 

In this section are described those domino reactions which start with a retro-pericy-clic reaction. This may be a retro-Diels-Alder reaction, a retro-l,3-dipolar cycloaddition, or a retro-ene reaction, which is then usually followed by a pericyclic reaction as the second step. However, a combination is also possible with another type of transformation as, for example, an aldol reaction. 

In antithetical analyses of carbon skeletons the synthon approach described in chapter 1 is used in the reverse order, e.g. 1,3-difunctional target molecules are "transformed" by imaginary retro-aldol type reactions, cyclohexene derivatives by imaginary retro-Diels-Alder reactions. 

---