Thermal Rearrangements of Cyclic Amine Ylides. VIII. Intramolecular Cyclization of 2-Ethynylpyridine N-Ylides into Indolizines and Cycl [3.2.2] azines

Abstract

Treatment of 6-unsubstituted 2-ethynyl-1-phenacylpyridinium bromides (10a-c) with 1, 5-diazabicyclo [5.4.0] undec-5-ene in refluxing benzene resulted in cyclization to give the 3-benzoylindolizines (11) via the N-ylide intermediates 13, whereas heating the salts 10 in refluxing acetic acid afforded the 1-benzoylindolizines (12). Upon similar treatment with the base, 2-ethynyl-6-methylpyridinium salts (20a-c) gave the 3-benzoyl-5-methylindolizines (22); in the case of the phenylacetylene compound 20c, 2, 3-diphenylcycl [3.2.2] azine (23) was also obtained. However, the reaction of 6-amino-2-ethynylpyridines (27a-c) with phenacyl bromide produced 5-ethynyl-1-azaindolizines (28), which were converted into 1-azacycl [3.2.2] azines (29). The mechanisms of these intramolecular cyclizations are discussed.