ABSTRACT A stereoselective, base‐catalyzed 3 + 2‐cyclization of 2‐acylethynylpyrroles with diacetyl (DBU/benzene, room temperature) afforded bipyrrolo1,2‐ c oxazoles or 3‐acetylpyrrolo1,2‐ c oxazoles, both integrated with α,β ‐ethylenic ketones, in 51–76% and 23–74% yields, respectively. Switching between these products was controlled by the concentration of diacetyl. Replacing the catalyst with NaOH triggered an unexpected Paternò–Büchi dimerization of diacetyl, followed by a 3 + 2‐cyclization to yield pyrrolo1,2‐ c oxazole‐oxetane ensembles (21–29%) alongside the aforementioned pyrrolo‐ and bipyrrolo1,2‐ c oxazoles.
Tomilin et al. (Tue,) studied this question.