**1. Introduction**

3-(Diarylmethylene)oxindoles belong to a major oxindole family that has recently been reported to have novel biological activities, such as AMPK activation [1] and estrogen receptor-related anticancer activity against breast cancer [2]. As valuable derivatives of 3-(diarylmethylene)oxindoles in the field of medicinal chemistry, 3-(1,3-diarylallylidene) oxindoles, which have a vinyl linker at the 3-methylene position, have attracted considerable attention from synthetic chemists, and several synthetic methods have been reported (Scheme 1) [3–6]. In 2005, a 3-(1,3-diarylallylidene)oxindole was first synthesized by Takemoto et al., utilizing double Heck reactions [3]. Recently, Sekar et al., improved this approach using palladium binaphthyl nanoparticles (Pd-BNPs) as a catalyst to broaden the substrate scope and allow easy separation [4]. In 2008, Murakami et al., developed another synthetic method featuring palladium-catalyzed oxidative cyclization of 2-(alkynyl)isocyanate, followed by the Suzuki-Miyaura reaction with styrylboronic acid [5]. As part of our ongoing efforts to identify novel synthetic methods for 3-methyleneoxindole derivatives [6–10], we recently reported a palladium-catalyzed multicomponent tandem reaction, which allowed a stereoselective approach to (*E*)- and (*Z*)-isomers of 3-(1,3 diarylallylidene)oxindoles by changing phosphine ligands, reaction temperature, and time [6]. Although several synthetic methods for 3-(1,3-diarylallylidene)oxindoles have already been developed, as described above, the narrow substrate scope and/or limited accessibility of reagents when using these methods necessitate the development of a more general approach to this unique skeleton. In all previous methods palladiumcatalyzed reactions were the key reactions, which greatly limited the range of products to which these procedures could be applied. A few non-palladium-catalyzed approaches to 3-allylideneoxindoles have been reported, but they cannot be applied to the synthesis of 3-(1,3-diarylallylidene)oxindoles [11,12]. Therefore, we attempted to

**Citation:** Koo, J.; Kim, M.; Shin, K.J.; Seo, J.H. Non-Palladium-Catalyzed Approach to the Synthesis of (*E*)-3-(1,3-Diarylallylidene)Oxindoles. *Molecules* **2022**, *27*, 5304. https:// doi.org/10.3390/molecules27165304

Academic Editors: Alison Rinderspacher, Mircea Darabantu and Gloria Proni

Received: 28 July 2022 Accepted: 15 August 2022 Published: 19 August 2022

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develop a novel synthetic method for 3-(1,3-diarylallylidene)oxindoles with a wide substrate scope using commercially available or easily accessible reagents, and not involving palladium-catalyzed reactions.

**Scheme 1.** Previous synthetic approaches to 3-(1,3-diarylallylidene)oxindoles.
