2023, Vol. 34 
b Laboratory for Marine Drugs and Bioproducts, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China
Contiguous quaternary carbon stereocenters (CQS) account for one of the most synthetically challenging structural motifs that are commonly found in complex natural products and in biologically important molecular entities [1-4]. Their synthetic challenge increases exponentially when CQS moieties were embedded in polyfused/bridged ring systems (Fig. 1a) [5-12], mainly owing to the rigidity as well as steric repulsion of the molecular frameworks. Several strategies have been conceived and applied to access vicinal quaternary carbon stereocenters [13-18]. However, there are only a few examples each showing single substrate that can form four CQS as demonstrated by Trauner (oxidative [5 + 2]) [19], Careirra [20] and Gaich [21] (both via [2 + 2]) in total synthesis. To the best of our knowledge, no general methodology has been documented to generate four CQS in fused ring systems through photocatalysis [22-34] under visible light, albeit three CQS-containing examples been reported by Bach (via [2 + 2]) [35], Mateos (via Paterno–Buchi) [36] and You (via [4 + 2]) [37]. The current logic to achieve multiple CQS is using linear multistep synthesis to construct the third and fourth quaternary centers based on pre-made QS-containing substrate (Fig. 1b) [38, 39]. A general catalytic and diastereoselective synthetic method remained to be explored, when multiple (three and more) CQS-bearing subunits are targeted in polyfused ring systems.
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| Fig. 1. Representative CQS-containing natural products and design of a visible light induced [2 + 2] cycloaddition approach to access CQS skeleton. | |
We hypothesized that an intramolecular catalytic [2 + 2] cycloaddition under visible light [40-53] through energy transfer (EnT) mechanism [54-58] may be a viable reaction to create multiple CQS-bearing polyfused ring systems (Fig. 1c). It was advocated that since EnT process created a triplet biradical species, which is highly reactive, that it may overcome the unfavorable entropy gaining of forming multiple CQS. The intramolecular design is synergistic to the success of constructing this challenging motif. To minimize the conformers of transition state, the linker atom A is very important and presumably may affect the diastereomeric outcome. In the meantime, accessing versatile tetrasubstituted olefins are non-trivial job [59], we turn our attention to use aromatic compounds as the cycloaddition partner. The challenges are three fold: 1) whether the proposed biradical intermediate is reactive enough to overcome the entropy gaining of forming (up to) four CQS in a fused ring system; 2) how to control the diastereoselectivity of the [2 + 2] cycloaddition; 3) the different cyclization pathway (5-exo versus 7-endo) may be competing and cause side reaction [60, 61].
We commenced with dienamide 1a as substrate [62-64], which features N as linker atom (Scheme 1). It was speculated that the sp2 hybrydization of N atom will help rigidify the transition state conformation. Under catalytic amount (2 mol%) of Ru(bpy)3(PF6)2 (PC-Ⅰ) at room temperature irradiated by blue LED bulb (λmax = 465 nm) in acetone. The reaction yielded [2 + 2] cycloaddition product 2a/a' featuring vicinal CQS with moderate dr ratio (E-2a: E-2a' = 3:2) in almost quantitative yield (99%, entry 1). The standard condition works for substrate 1b as well, yielding the desired three CQS-containing products 2b and its β-C epimer 2b' in good yield (87%) with improved dr ratio (E-2b: E-2b' = 2.2:1, entry 2). The detection of E-2a' and E-2b' indicated an alternative mechanism [65, 66], which probably is via Int-Ⅰ (Fig. 1c). The Ir-based photosensitizers ([Ir{dFCF3ppy}2(dtbbpy)]PF6, PC-Ⅱ and [Ir(dtbbpy)(ppy)2]PF6, PC-Ⅲ) were also screened, but proved inferior to Ru-counterpart (PC-Ⅰ), in respect to both diastereomeric control and overall yield (entries 3 and 4). Four diastereomers (E/Z-2a and E/Z-2a') were obtained and verified through X-ray crystallography (CCDC: 2154873 (E-2a), 2154875 (Z-2a), 2154874 (E-2a') 2154876 (Z-2a'), (some H atoms were omitted for clarity, Scheme 1). These results indicated that Ir-based catalyst with higher EnT energy may cause mixed activation (via both Int-Ⅰ/Ⅱ in Fig. 1c) and lead to poor dr outcome. When organic photosensitizers (TPT+BF4, PC-Ⅳ and Mes-Acr-Me+, PC-Ⅴ) were tested (entries 5 and 6), it was found that the diastereoselectivity of [2 + 2] cycloaddition can be slightly improved (up to 3:1). However, olefin isomerizations were observed under these conditions compared to a well conservation in Ru-based photocatalyst (PC-Ⅰ). In addition, we found both the photocatalyst and the blue LED light source were essential to the success of this transformation, as only 1a were recycled in the control experiments (entries 7 and 8).
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| Scheme 1. Selected condition optimization. a All reactions were run with 2 mol% PC on a 20 mg scale under blue LED irradiation at room temperature for 10 h unless otherwise noted. Isolated yields for 2 and its β-C epimer 2', dr = 2/2'. b Coversions were determined based on recycled starting materials. c Isolated yields. d Numbers are for 2b and 2b'. | |
With optimized conditions in hand, we set off to investigate the reaction scope (Scheme 2). A broad series of dienamide with different steric and electronic properties have been examined. Gratifyingly, good to high yields of three CQS-containing fused rings were obtained. It was found that changing the electronegativity of aryl substituents on linker N atom of dienamides (1c–1g, see Supporting information for detail) does not affect the efficacy, and 73%−96% yields were obtained and the dr ratio fell in the range between 3:1 and 4:1, with no obvious trend for para-substituted effect (2c–2g). The diene part was also investigated by attaching different electronically substitutents to the para-position of styryl group. Luckily, both of the yields and diastereoselecitivity maintained well (78%−86% for 2h–2j). It is delighted to find that the benzofuran can be dearomatizatized under such mild conditions. Hence, we subject benzothiophene (1k) and N-Me indole (1l) analogues into our standard conditions (see Supporting information for detail). As expected, the desired tetrahydrobenzothienocyclobutan 2k was isolated in 91% yield albeit with 2:1 dr ratio. Surprisingly, its indoline analogue 2l was obtained in good yield (73%) with significantly improved diastereoselectivity (dr = 16:1). This result led us to think that fine-tuning of the electronic property may suppress the β-C epimerization during the [2 + 2] cycloaddition. The coupling partner can be tetrasubstituted olefins as well, as the tetrahydroindenyl fused cyclobutapyrrolone 2m/m' and 2n/n' could be successfully isolated in 80% and 85% yield, respectively. The latter's stereochemistry was further verified through X-ray crystallography (CCDC: 2154877 (2n), 2154879 (2n')). It is worthy to point out that both of the products displayed an exo selective configuration. The reason is unclear, but the stereospecificity is undoubtedly a merit.
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| Scheme 2. Scope of the [2 + 2] cycloaddition. All reactions were run with 2 mol% of Ru(bpy)3(PF6)2 (PC-Ⅰ) under blue LED irradiation for 10 h at room temperature unless otherwise noted and only major isomer was displayed. Isolated yields for 2 and its β-C epimer 2', dr = 2/2'. a Inseparable isomers and dr was determined based on 1H NMR. b The reaction time is 48 h. c [Ir{dFCF3ppy}2(dtbbpy)]PF6 was used as photosensitizer. | |
In addition, The pH-neutral and room temperature conditions are vital to the successful isolation of product 2o (45% yield, dr = 4:1) and 2p (80% yield, dr = 2:1), which contained acid sensitive vinyl (thiol)ether units. The diastereomeric control could be greatly increased if C3-subsubstituted benzofuran is employed. 2q was yielded as the single diastereoisomer, albeit with low yield (30%). The reason will be discussed vide infra. 2r was also smoothly isolated with good yield (88%) and moderate dr (2.5:1). The substituents on different positions of diene were also investigated (1s-1u). It was gratifyingly to find that 2-furyl (1s) at δ-position, benzofuranyl (1t) as well as benzothiophenyl (1u) at β-position were all well tolerated yielding desired three CQS-containing products 2s/s'-u/u' in synthetically useful yields (54%−95%) with moderate to good stereomeric control (dr ratio from 2:1 to 11:1). It is noteworthy that diene may not be necessary, as 1t and 1u could be deemed as acrylamide-type substrate. This prompted us to subject 1v into our standard conditions. Fortunately, 2v/v' were afforded in 72% yield with good diastereomeric control (dr = 6:1), although an Ir-based photosensitizer (PC-Ⅱ) was used. Last but not least, the γ-position substituted substrate 1w was prepared and tested. 2w/w' and its epimer were isolated in 63% yield with 6:1 dr ratio.
Having accomplished more than twenty examples of three CQS-containing fused ring skeletons, we are ready to tackle the more challenging four CQS-containing framework. The α, β-tetrasubstituted dienamide 1x was prepared and subjected into our standard conditions. Delightedly, tetrahydrobenzofuran fused cyclobutapyrrolone 2x and its β-C epimer 2x' were isolated in 77% and 16% yields, respectively. The configuration of 2x and 2x' were undisputedly confirmed by single crystallography (Scheme 2, bottom, CCDC: 2154878 (2x), 2154880 (2x')), preserving the exo selectivity. The dr ratio had been improved to 5:1. To further improve the diastereoselectivity, we synthesized the substrate 1y, which could not undergo epimerization (see Supporting information for detail). The four CQS-containing complex pentacyclic ring 2y' was isolated as the single isomer in 89% yield (with side product shown in Scheme 5). These examples demonstrated the generality as well as robustness of the [2 + 2] cycloaddition in creating multiple CQS-containing polyfused ring systems.
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| Scheme 3. Control experiments. | |
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| Scheme 4. Radical clock experiment. | |
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| Scheme 5. Proposed mechanism. | |
We have established a catalytic [2 + 2] cycloaddition strategy to successfully access multiple CQS structural motif. But the epimerization at β-C position and the mechanism of the cycloaddition still needed to be elucidated. We had carried out a few control experiments to investigate this process. We could successfully detect 2z/z' under standard conditions (10 h irradiation) in good yield (80%, Scheme 3). However, if we stopped the reaction prematurely (after 4 h irradiation), only geometry isomer 1z' (42% yield) and starting material 1z (50% yield) could be isolated in good mass balance. This olefin isomerization was also found in the case of substrate 1q, which only resulted in the formation of 1q' (45%) after partial conversion (1q recycled 41%) under shortened reaction time (recall the isolation of 2q in Scheme 2 under 48 h, Scheme 3). The olefin isomerization can be fully suppressed if α, β-carbon olefin was masked. As illustrated by substrate 1aa, only starting material was recycled under standard conditions (Eq. 1, Scheme 3) [67]. These experiments indicated that a biradical Int-Ⅰ, upon EnT with the photosensitizer, may be a viable intermediate and led to the formation of both 2 and its β-C epimer 2'.
To confirm our hypothesis, we designed a radical clock experiment (Scheme 4). A phenylcyclopropyl substituted dienamide 1ab was prepared. The idea is that if the biradical Int-A formed, the vinyl cycloproyl group could delocalize and induce the β-position radical to open the cyclpropyl ring and yield Int-B. The latter (Int-B) would undergo radical collapse and afford compound 3. As expected, compound 3 was isolated in 80% yield using 2 mol% of [Ir{dFCF3ppy}2(dtbbpy)]PF6 (PC-Ⅱ) as catalyst.
Based on the above discussion, we tentatively propose a possible mechanism for this dearomative [2 + 2] cycloaddition reaction (Scheme 5). The α, β-unsaturated olefin is selectively activated upon energy transfer from the photosensitizer and forms triplet biradical species 1*. The α, β-carbon bond can undergo a bond rotation and yields β, which give rise to the geometry isomer 1'. Both of the above biradical intermediate can participate in a β-carbon radical initiated electrophilic 7-endo cyclization with π bond of the benzofuran moiety and forms intermediate Ⅲ and Ⅳ, which yield the [2 + 2] cycloaddition product 2 and its epimer 2', respectively. The diastereoselectivity originated from steric repulsion between the Me group and R2. When R2 = H (most cases of the substate scope), it results in the least repulsion, compared to that of the cinnamyl group (upper, Scheme 5). It is uncommon that we propose an uncommon 7-endo cyclization. The reason is that we successfully isolated a side product 4 (11% yield), alongside the desired product 2y' (bottom, Scheme 5). Compound 4 could only be derived from 6-endo cyclization with the β-carbon radical. Toward this end, we have successfully deciphered the mechanism for our multiple CQS generating dearomative [2 + 2] cycloaddition catalyzed by Ru(bpy)3(PF6)2 (PC-Ⅰ) under visible light.
To conclude, we have developed a Ru(bpy)3(PF6)2-catalyzed dearomative [2 + 2] cycloaddition protocol to construct three and four CQS-containing polyfused ring systems in good yields (up to 96%) and diastereoselective control (up to 20:1 dr ratio). The reaction displayed wide compatibility towards steric as well as electronic variation on the dienamides, as demonstrated by 26 successful examples of CQS-bearing [2 + 2] products. The mechanism investigation revealed a novel activation pattern, that the α, β-carbon of dienamide was selectively activated upon energy transfer with excited photosensitizer. The resulting biradical intermediate participated in a β-C radical initiated uncommon 7-endo cyclization with benzofuran and led to the formation of [2 + 2] products. This mechanism is complementary to the previous known benzoheteraryl activation pathway and may find further use in radical mediated reaction development [68-72].
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
AcknowledgmentsThe project was supported by the National Natural Science Foundation of China (Nos. 82122063, 81991522 and 81973232), Shandong Science Fund for Distinguished Young Scholars (No. ZR2020JQ32) and the Fundamental Research Funds for the Central Universities (No. 202041003).
Supplementary materialsSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.047.
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