Unsaturated Imine-Involved Cyclization Reaction

SCHEME 2.26 Phosphoric acid-catalyzed cycloaddition reaction of 1-aza-l,3-butadienes. 

---

As presented in some detail by Gingrich and Baum (see their Table 4.31 and Scheme 4.51) azlactones react with ot,p-unsaturated imines to give ot-pyridones. Sandhu and co-workers continued the studies in this area, and they now view this reaction as involving initial electrophilic attack on the azlactone (munchnone) followed by cyclization to an ot-pyridone, rather than prior ring-opening to the corresponding ketene tautomer, as was originally proposed (Table 4.1). With azlactone 52 and A-aryl cinnamaldehyde anils 53, in the presence of acetic anhydride, the 4-substituted azlactones 54 are isolated (Fig. 4.15). Dalla Croce... 

In the reaction of nitroxyl with tertiary phosphines the corresponding aza-ylide was formed along with the phosphine oxide as the by-product (HNO + 2R3P R3P = NH + R3P = O). " A novel synthesis of pyrroles involves the reaction of a,p-unsaturated imines and acid chlorides. The cyclization is mediated by triphenylphosphine that is converted to the P-oxide during the reaction. In the reaction of phosphonoallenes with 2-iodophenol in the presence of Pd(OAc)2/PPh3, in addition of the expected benzofurans, the oxidized derivative of triphenylphosphine oxide (2-hydroxyphenyl-diphenylphosphine oxide) was also formed as a minor by-product. ... 

---