Preparation of Oxazole Acetals from N‐Propargylamides Enabled by Visible‐Light‐Promoted Selenium‐π‐Acid Catalysis

Abstract

An unprecedented catalytic system consisting of diselenides, Selectfluor and visible light has been developed for the one‐pot synthesis of oxazole acetals from N‐propargylamides and alcohols. This system shows the processes of cyclization, aromatization and acetalization, resulting in the construction of multiple carbon‐oxygen bonds. These transformations demonstrate the excellent compatibility of visible‐light promotion with organocatalytic oxidative carbon–oxygen bond formations using air as the terminal oxidant.

Visible‐Light‐Mediated Photocatalytic Synthesis of 2‐Substituted Oxazole‐5‐carbaldehydes Promoted by Benzeneseleninic Acid

“…Thus, based on the described control experiments, and on the previous results reported in the literature, [14,15,18,20] we suggest a plausible reaction mechanism, describing the transformation of the N ‐(propargyl)benzamide 1a to 2‐phenyloxazole‐5‐carbaldehyde 2a (Scheme 4). Initially, in the presence of visible light irradiation, EY is excited to the EY*, which can easily undergo a reductive quenching in the presence of benzeneseleninate 5a’ , formed by the deprotonation of 5a .…”

“…A similar approach was recently used by Liu and co‐workers to prepare 2‐substituted oxazole‐5‐acetals 3 [14] . In this case, Selectfluor® was used in catalytic amount in the presence of alcohol under visible light irradiation (blue LEDs 6 W), through the in situ formation of the aldehyde 2 .…”

“…A similar approach was recently used by Liu and co-workers to prepare 2-substituted oxazole-5-acetals 3. [14] In this case, Selectfluor® was used in catalytic amount in the presence of alcohol under visible light irradiation (blue LEDs 6 W), through the in situ formation of the aldehyde 2. Alternatively, 2substituted oxazole-5-ketones 4 were prepared by Song and co-workers, through the Au-catalyzed cyclization of internal Npropargylamides 1, in the presence of 4-MeO-TEMPO and 3 equiv.…”

Meyer–Schuster-Type Rearrangement of Propargylic Alcohols into α-Selenoenals and -enones with Diselenides

“…As a continuation of our studies on diselenide-mediated functionalization, we recently harnessed selenium-π-acid, which is electrophilic interactions between selenenium and π bond, to develop an efficient strategy for the preparation of oxazole acetals . Consequently, we speculated that in situ generated PhSeX electrophilic species should attack the carbon–carbon triple bond of propargylic alcohols, which would undergo seleno-rearrangement to deliver α-selenoenals and -enones. , Herein, we disclose the successful implementation of selenenylation/rearrangement of propargylic alcohols and present a broadly applicable protocol for the synthesis of multisubstituted α-selenoenals and -enones under neutral reaction conditions at room temperature.…”

Metal-free electrochemical oxidative intramolecular cyclization ofN-propargylbenzamides: facile access to oxazole ketals

“…In 2020, Liu et al described an elegant visible light-promoted selenium-catalyzed 5- exo-dig cyclization of N -propargylamides, providing an expedient and appealing synthetic route toward various oxazole acetals under mild conditions (Scheme 1b). 8 This protocol circumvents the requirement of Lewis acids or Brønsted acids, used to activate the carbonyl group in typical acetalization reactions.…”