Reactions of Guaiazulene with Heterocycle-carbaldehydes in Acetic Acid

Abstract

The reaction of guaiazulene (1) with a 1.2 molar amounts of a heterocycle-carbaldehyde (i.e., thiophene-2-carbaldehyde, pyrrole-2-carbaldehyde, and imidazole-2-carbaldehyde) in acetic acid at room temperature (25°C) for 1-8 h under argon gave the corresponding 3, 3'-methylenebis (guaiazulene)having a heterocycle-substituent at C-α position (2 and 5, respectively) in good yield (96-98%)and 3 in low yield (38%). In addition to these anticipated products, two structurally very interesting guaiazulene derivatives, 8-isopropy1-6, 10a-dimethy1-10aH-azuleno[3', 3a', 4': 4, 5, 6]pyrido[1, 2-α]pyrrole (4, 35% yield) and 8-isopropy1-6, 10a-dimethy1-10aH-azuleno[3', 3a', 4': 4, 5, 6]pyrido[1, 2α]imidazole (6, 3% yield), were also obtained as minor products by the reactions of 1 respectively with pyrrole-2-carbaldehyde and imidazole-2-carbaldehyde. The structures of these products were established on the basis of their spectroscopic (UV/VIS, IR, NMR, and MS) data. Possible reaction pathways for the formation of these compounds are discussed.