# Natural Product-Oriented Photo-Induced Deintrogenative Annulations of 1-Alkenylbenzotriazoles
## 2 Results and Discussion  
At the initial of our study, two 1-alkenylbenzotrizoles **15a** and **15b**, which bear a NHBoc and OH moiety on the side chain, respectively, were prepared through a couple of steps (for details, see Supplementary Materials). With these precursors in hand, we then focused our attention on exploring the designed photo-induced denitoregative annulation reaction. Based on some relevant works [17; 18; 19; 20; 21; 22], we first chose to conduct the photoreaction of **15a** with MeCN as the solvent and 254 nm UV lamp as the light resource. To our disappointment, the reaction outcomes turned out to be complicated, and no desired product **17a** could be identified in the reaction mixtures (Table 1, entry 1). In sharp comparison, the photoreaction of **15b** appeared to be more encouraging, with the expected product **17b** obtained in 23% yield (entry 2). Besides, we also identified another unstable product (ca. 30%) in the reaction, which was assigned to be the 5/3/5 tricyclic compound **21** based on the extensive spectroscopic studies. Furtherly, we conducted a condition screening using **15b** as the substrate. However, only a slightly improved yield of **17b** (30%) was obtained when the reaction was performed in THF (entry 3). The above-mentioned results suggest that although the designed photo-induced denitrogentative annulation is feasible, the reaction outcomes largely rely on the judicious manipulation of the substrate structures. The distinct results associated with **15a** and **15b** indicate that a nucleophile with favorable steric and electronic nature should be employed to capture the in situ-generated indolenine intermediate. Apparently, the OH group appears to be a better choice than the NHBoc for the current reaction. On the other hand, the identification of byproduct **21** indicated that there exist some unexpected reaction pathways besides the desired one. The plausible mechanism of the photoreaction of **15b** is outlined in Scheme 2. Simply, upon photo irradiation, **15b** will undergo nitrogen extrusion to generate diradical species **I-15b**. Subsequently, **I-15b** will go through radical 1,3-shift to give 1,5-diradicals **II-15b** and **III-15b** as a pair of cis/trans isomers. Between them, the cis-isomer **II-15b** could advance to **17b** though 1,5-diradical combination followed by imine capture by the hydroxyl group. Comparably, for trans-isomer **III-15b**, a 1,5-hydrogen abstraction will take place preferentially, leading to a new 1,3-diradical species (**V-15b**) [40]. Finally, a 1,3-diradical combination followed by the cyclization of the hydroxyl group onto the imine moiety will give rise to the observed byproduct **21**. Having the mechanistic rationalization in mind, we sought to improve the reaction by judiciously manipulating the substrate structure. As we envisioned, introducing two substitutions on the C1 position of the substrate (e.g., **15c** and **15d** in Scheme 3) will facilitate the reaction from two aspects. First, the competitive 1,5-hydrogen abstraction would be precluded in this scenario due to the absence of suitable C-H bonds on the C1 position. Second, the equilibrium between the _cis-_ and _trans-_imine intermediates will shift towards the desired direction. Taking **15c** as an example, **II-15c** should be thermodynamically more favorable than **III-15c**, and thus the desired reaction pathway would take place preferentially. To our delight, this speculation was validated quickly in practice. As shown in Scheme 3, when 1-alkenylbenzotrizoles **15c** and **15d** were submitted to the photoreaction, the corresponding annulation products **17c** (56%) and **17d** (54%) were obtained in notably improved yields.