**2. Results and Discussion**

#### *2.1. Light-Induced Intramolecular Photoredox Reaction of Renieramycins*

Renieramycins **4**–**6** were isolated from the Thai blue sponge (*Xestospongia* sp.) by pretreatment with 10% potassium cyanide following a previously reported protocol [11,12]. The photoredox reactions occurring through the light-induced radical formation and intramolecular cyclization of the natural bis-tetrahydroisoquinolinequinone alkaloids (**4**–**6**) were investigated (Scheme 1 and Table 1) [23]. To determine the optimal reaction conditions, we experimented with compound **4**, along with various solvents and light sources. Through irradiation by an 18-W fluorescent lamp (white light) in dichloromethane (CH2Cl2) for 24 h, compound **4** was smoothly transformed into compound **7** in 64% yield (Table 1, entry 1). The application of chloroform (CHCl3) and tetrahydrofuran (THF) as solvents for the photoredox reaction of compound **4** furnished compound **7** in 46% and 47% yields, respectively (Table 1, entries 2 and 3). Further improvement in the yield was achieved when the reaction solution was exposed to irradiation by a 4-W light-emitting diode (LED) lamp (blue light) in CH2Cl2, affording **7** in excellent yield (81% yield, entry 4). However, using CHCl3 and THF with blue light irradiation decreased the yields of **7** to 51% and 54%, respectively (Table 1, entries 5 and 6). Thus, optimized photoredox reactions using blue light and CH2Cl2 were further performed with natural alkaloids **5** and **6** (Table 1, entries 7 and 8).

**Scheme 1.** Photoredox reaction of the natural renieramycins.

**Table 1.** Optimization of the conditions for the photoredox of natural renieramycins.


<sup>a</sup> 18 W refers to the white light fluorescent lamp, and 4 W refers to the blue LED lamp. <sup>b</sup> Ratio was determined based on the proton signal of H-3 by nuclear magnetic resonance (NMR) spectroscopy analysis. <sup>c</sup> The yield of the major product was determined after column chromatographic purification.

Unexpectedly, the photoredox reaction of compound **5** yielded a mixture of bistetrahydroisoquinolinequinones **6** and **8** (Table 1, entry 7). Compound **8** was obtained as the major product in 37% yield, two times higher than the yield of compound **6** (18% yield). Meanwhile, the photoredox reaction of compound **6** under the optimized condition yielded compound **8** in 48% yield (Table 1, entry 8). Therefore, the light-induced radical formation and intramolecular cyclization of compound **5** to yield compounds **6** and **8** were proposed to occur via stepwise transformations. The mechanism involved air oxidation to convert the unstable 1,4-hydroquinone into the 1,4-quinone moiety, followed by the photoredox reaction [24] (Scheme 2). The results of the light-mediated transformation highlight the chemical diversity of the natural renieramycins found around the blue *Xestospongia* sponge habitat.

**Scheme 2.** Proposed transformation mechanism of **5** into **6** and **8**.

#### *2.2. Semisynthesis of 4*- *-Pyridinecarbonyl-Substituted Renieramycin-Type Derivatives*

The semisynthesis of the 4- -pyridinecarbonyl-substituted renieramycin–ecteinascidin hybrid derivative (**11**) started with the photoredox transformation and esterification (Scheme 3). The irradiation of **4** under the 4-W LED light in CH2Cl2 furnished **7**. Subsequently, a 4- -pyridinecarbonyl motif of **7** was installed by Steglich esterification using isonicotinoyl chloride as an acylating agent, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDCI) as a coupling reagent, and 4-dimethylaminopyridine (DMAP) as a nucleophilic base catalyst to obtain the desired product (**11**) in acceptable yield. The semisynthesis protocol of the renieramycin-type derivative (**12**) was envisioned. Compound **3** was prepared from compound **4** by hydrogenation, hydride reduction, and air oxidation [21,24]. Ester compound **12** was obtained in excellent yield by attaching the 4- -pyridinecarbonyl substituent to the C-22 position of renieramycin by Steglich esterification, followed by the formation of the 1,3-dioxole-bridged phenolic moiety via light-induced radical formation and intramolecular cyclization.

The chemical structures of all semisynthetic renieramycin-type derivatives were characterized by spectroscopic techniques (see Supporting Information for the spectra, Figures S1–S15). The spectral data of compounds **7**–**9** were consistent with previous reports [14,22]. Regarding the 4- -pyridinecarbonyl-substituted renieramycin–ecteinascidin hybrid derivatives (**11** and **12**), the characteristic chemical shift of the methylene moiety (–CH2–) at the newly constructed 1,3-dioxole ring showed a proton signal as a pair of doublets at 5.95 ± 0.07 ppm and a carbon signal at 101.6 ± 0.3 ppm. Moreover, the C-7 and C-8 quaternary carbon signals of both compounds **11** and **12** shifted upfield, compared with the signals of the parent compound **4**. The carbonyl carbon signals of the resulting 4- -pyridinecarbonyl ester were confirmed at 167.0 and 164.2 ppm for **11** and **12**, respectively. Moreover, the chemical shifts of the carbonyl carbon at C-15 and C-18 for both **11** and **12** are located at 186.0 ± 0.2 ppm and 182.5 ± 0.1 ppm, respectively, indicating the presence of the quinone group on ring E.

**Scheme 3.** Semisynthesis of the 4- -pyridinecarbonyl-substituted renieramycin-type derivatives.

The heteronuclear multiple bond correlations (HMBCs) between the methylene proton at the 1,3-dioxole ring and the quaternary aromatic carbons at C-7 and C-8 clearly confirmed the fused ring structure at ring A in compounds **11** and **12** (Figure 3). The key HMBCs of compound **11** included the correlations between the C-5 quaternary carbon and both the C-4 methylene protons and the C-6 methyl protons, along with those of the C-1 pyridine carbon, all the protons at the C-2 and C-3 position of pyridine, and C-6 methyl motifs. Compound **12** exhibited significant HMBCs between the C-24 carbonyl carbon and both the C-22 methylene protons and the C-2pyridine proton.

**Figure 3.** HMBCs (blue arrows) for the 4- -pyridinecarbonyl-substituted derivatives (**11** and **12**).
