# Natural Product-Oriented Photo-Induced Deintrogenative Annulations of 1-Alkenylbenzotriazoles
## 2 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Entry** & **Reactant** & **Solvent** & **Light Source (nm)** & **Yield (\%)** \\ \hline
1 & **15a** & MeCN & 254 & **17a:** 0\% \\
2 & **15b** & MeCN & 254 & **17b:** 23\% \\
3 & **15b** & THF & 254 & **17b:** 30\% \\
4 & **15b** & Ethly acetate & 254 & **17b:** 22\% \\
5 & **15b** & 1,4-Dioxane & 254 & **17b:** 22\% \\
6 & **15b** & MeOH & 254 & **17b:** 14\% \\
7 & **15b** & DMF & 254 & **17b:** 16\% \\
8 & **15b** & Toluene & 254 & **17b:** 15\% \\
9 & **15b** & THF & 311 & **17b:** 27\% \\
10 & **15b** & MeCN & 311 & **17b:** 20\% \\ \hline \hline \end{tabular} Without special note, all reactions were conducted with a 40 mg scale of substrate in 5 mL solvent under nitrogen protection.  
\end{table} 
Table 1: Condition optimization of the photoreaction. Besides the C1 substitute effect, we found that the structural variation on the side chain also exerted a profound effect on the reaction outcome. Indeed, when 1-alkenylbenzotrizole **15e** bearing a carboxylic acid on side chain was attempted in the photoreaction, a different result was obtained. In this case, two products were isolated, between which the minor component turned out to be the desired product **17e** (38%) and the major one was assigned as tetrahydrocarbazole **22** (Scheme 4A). The structure of **22** was confirmed by the comparison of its spectroscopic data with those reported in literature [41]. Although the exact mechanism for the formation of **22** remains unclear at this stage, a plausible one is suggested in Scheme 4B. Naturally, the reaction also starts from the denitrogation of **15e**. However, different from the above-mentioned cases, in the current scenario the resulting  
**I-15e** more likely advances to the zwitterion species **II-15e** and **III-15e** through N-radical 1,3-shift and intramolecular proton transfer from the carboxylic acid to the imine moiety. Between them, **II-15e** could convert to **17e** following the same pathway as suggested above. Comparably, **III-15e** might go through an intramolecular 1,5-hydrogen transfer followed by radical 1,3-shift to yield N-radical cation species **IV-15e**. Subsequently, a Nazarov-type cyclization would take place, and the resulting product **V-15e** could undergo sequential tautomerization and decarboxylation to yield byproduct **22**. **Scheme 4. Photo-induced denitrogative annulation of 1-alkenylbenzotrizole bearing a carboxylic acid-containing side chain (A,B). **Taking the above outcomes into consideration, we decided to integrate the structural modifications on the C-1 position and C-4a side chain into a single molecule. To this end, 1-alkenylbenzotrizoles **15f** and **15g** were synthetized. As we expected, both **15f** and **15g** proved to be excellent substrates for the photoreaction by providing the corresponding products in good yields (**17f**: 63%; **17g**: 64%). Having the 4a,9a-heterocycle-fused tetrahydrocarbazole skeleton (**I**) secured in an acceptable efficiency, we then moved to explore the proposed chemistry leading to the tetracyclic skeleton **II** (Figure 1). According to our design, two structural modifications should be implemented on the 1-alkenylbenzotrizoles. First, the nucleophile on the side chain should be masked, and thus the in situ-generated indolenine intermediate (e.g., **16**, Scheme 1) could not be captured. Secondly, a carbonyl group should be introduced onto the C-4 position of the cyclohexene unit, which allows the proposed 1,3-acyl migration to take place. Taking these rationalizations in mind, 1-alkenylbenzotrizoles **15h** and **15i** were prepared and evaluated in the photoreactions. To our delight, the designed chemistry worked well under the conventional photo conditions (254 nm UV, CH\({}_{3}\)CN, 25 \"C), with the desired dihydropyrido[1,2-a]indolone derivatives **20h** and **20i** obtained in 42% and 35% yields, respectively.