Catalyst free synthesis of 2‐Aryl‐2H‐benzo[b][1,4]oxazines and 3‐Aryl‐2H‐benzo[b][1,4]thiazin‐2‐ones: An ultrasonication‐assisted strategy

Abstract

An ultrasonication-assisted synthesis of 2-Aryl-2H-benzo[b][1,4]oxazines and 3-aryl-2H-benzo[b] [1,4]thiazin-2-ones has been established by reacting phenacyl bromides with 2-aminophenol and 2-aminothiophenol, respectively. This approach fosters flexibility in generating a diverse range of 1,4-benzoxazines and 1,4-benzothiazinones under catalyst-free reaction conditions. Further scope toward the synthesis of rarely occurring bis-benzoxazine adduct has also been explored, which enabled us to propose the reaction mechanism.

Synthesis of 1,4-benzoxazine derivatives from α-aminocarbonyls under transition-metal-free conditions

“…Based on the results described above and previous literature precedents, 18–20 we proposed a plausible reaction pathway, as shown in Scheme 4. Firstly, substrate 1a might proceed via two pathways, one was direct transformation into intermediate A , and the other was conversion to intermediate B , which was the enol structure of 1a .…”

“…Similarly, the target compound 2a could also not be obtained without adding potassium carbonate, but 68% yield of compound F was generated. According to the previous literature precedents, 18 intermediate A could offer the product F through the dehydration and double bond reposition (Scheme 3b). Furthermore, target product G was not detected when intermediate F was reacted with substrate 1b in the absence of acetic acid (Scheme 3c).…”

The Multifaceted Opportunities Provided by the Pheomelanin-Inspired 1,4-Benzothiazine Chromophore: A Still-Undervalued Issue

“…Solvent-free conditions have also been optimized for the reaction of 2-aminothiophenol with 1,3-dicarbonyl compounds involving the use of graphene oxide as a recyclable and highly efficient (yield > 75%) heterogeneous catalyst (Figure 2, path j) [38], whereas ultrasonication (40 kHz, in THF, at 60 • C, for 2 h) has been proposed for the development of catalyst-free conditions for the reaction of 2-aminothiophenol with phenacyl bromides (Figure 2, path k) [39].…”