Vinyl ethers, cycloadditions with 3-substituted 2-pyrones

An interesting observation was reported by Posner who showed that a-pyrone substituted by a sulfoxide or a sulfone reacts rapidly with vinyl ethers, giving a diastereoselective [2 + 4] cycloaddition (Scheme 45). Racemic sulfide, sulfoxide or sulfone was introduced on a-pyrone via 3-cuprio-... 

A series of 5-substituted-3-bromo-2-pyrones can be thus readily synthesized as summarized in Figure 3. The resultant 2-pyrones are expected to be potent ambident dienes as exemplified by the cycloadditions of 5-phenyl-3-bromo-2-pyrone with methyl acrylate and benzyl vinyl ether (Scheme 34). 

Elevated pressure was also necessary to force the reaction of pyrone 47 with substituted vinyl ethers 48E and 48Z. Interestingly, geometric isomer 48Z reacts considerably faster and with better stereoselectivity than 48E. Thus, 48Z cycloadded to give 38% of cycloadducts 49... and 49 in a ratio of 18 1. By contrast, 48E cycloadded to give a very low yield of cycloadducts 50-.,- and in a ratio of 2 1. In each reaction, the olefin stereochemistry was retained in the reaction products, indicating that these cycloadditions occur in a concerted rather than in a stepwise fashion. 

The propensity of a-pyrones to undergo the Diels-Alder reaction makes them useful for syntheses of highly substituted aromatics and biphenyls. A practical method for the regioselective synthesis of the A-benzoyl-4-(polyfluoroalkyl)ani-lines 82 by thermal Diels-Alder cycloaddition of 71 with fluorostyrenes and acetylenes was described. Free 4-(polyfluoroalkyl)anilines were smoothly formed in good yields by DBU-assisted deprotection. In the case of the reactions of pyrone 71a with isobutyl vinyl ethers and cyclic vinyl ethers, compounds 83 and 84 were obtained, respectively [36] (Scheme 26). 

---