# Natural Product-Oriented Photo-Induced Deintrogenative Annulations of 1-Alkenylbenzotriazoles
## 1 Introduction  
Monoterpene indole alkaloids comprise a large class of natural products that exhibit diverse molecular architectures and broad biological activities [1; 2]. Among them, an array of _strychinos_ and akuammiline alkaloids featuring a 4a,9a-heterocycle-fused tetrahydrocar-bazole skeleton (**I**, Figure 1), as shown in minfinesine (**1**), vincorine (**2**) and aspidophylline A (**3**), have spurred great interest of synthetic chemists in the past decades. Characterized by a unique [4.3.3]propellane core containing two adjacent quaternary stereocenters, tetracyclic skeleton **I** has been viewed as one of the major synthetic challenges associated with this group of natural products, and thus a plethora of synthetic approaches have been developed to access it and related scaffolds [3; 4; 5]. Besides the above-mentioned molecules, there also exist some monoterpene indole alkaloids that bear the same A/B/C tricyclic ring system to **I** but differ in the connectivity patterns between the A/B/C and D rings [1]. Taking alsmaphrazine D (**4**), melodinine E (**5**) and leuconoxine (**6**) as examples, all of them share a characteristic octahydropyrido[1,2-a]pyrrolol[2,3-b]indole core (**II**), in which the D ring is connected to A/B/C ring at N1 and C9a instead of C4a and C9a as shown in **I**. The structural similarity between **I** and **II** could be rationalized by their inherent biosynthetic relationship. It has been suggested that the latter should be biosynthetically derived from the former through the cleavage of the C4a-C4 bond followed by the formation of a N1-C4 bond [6; 7]. Of note, although this biosynthetic hypothesis seems to be inspiring, it has remained yet to be validated in practice [8]. 1,2,3-benzotriazoles are an important class of heterocycles that have found widespread applications in organic synthesis, medicinal chemistry, and material science [9; 10; 11; 12; 13]. Historically, it has been reported that some 1-substituted 1,2,3-benzotriazoles could undergo denitrogenative transformations to generate other valuable products. This unique reactivity has aroused considerable interest from the synthetic community in recent years, which leads to the development of a broad range of novel denitrogenative transformations of 1,2,3-benzotriazoles [14; 15; 16; 17; 18; 19]. Among them, some photochemical reactions are particularly attractive due to their appealing synthetic potential and environmentally benign nature [20; 21; 22; 23; 24; 25; 26; 27]. For example, it has been reported that 1-vinyl-1,2,3-benzotriazole 7 couldundergo sequential photo-induced nitrogen extrusion, N-radical 1,3-shift, 1,5-diradical combination and tautomerization to yield the indole product **10** (Scheme 1A). From a synthetic point of view, this unique photo transformation represents a promising synthetic tool to access indole-derived natural products. However, such potential has rarely been explored, with only sporadic cases documented [22; 23]. In 1980s, [PERSON] and co-workers reported a seminal work on this subject, in which a series of natural product-relevant indole scaffolds were prepared by this method, as exemplified by the case leading to tetrahydroacbarazole **12**[22]. Another notable example was reported by [PERSON] and co-workers, in which a spiro-oxindole derivative **14** was obtained through the photo-induced denitropenative annulation of **13** followed by hydrolysis of the in situ-generated indoleine intermediate [23]. In recent years, our group has been striving to develop conceptually novel and practically useful denitropenative transformations of 1,2,3-benzotriazoles [28; 29; 30]. During this course, we realized that the photo-induced denitropenative annulation of 1-alkenylbenzotriazoles, if combined with rational design, might serve as an enabling tool for the synthesis of monoterene indole alkaloids.