Divergent total syntheses of ITHQ-type bis-β-carboline alkaloids by regio-selective formal aza-[4 + 2] cycloaddition and late-stage C–H functionalization

We herein report the first total syntheses of several bis-β-carboline alkaloids, picrasidines G, S, R, and T, and natural product-like derivatives in a divergent manner. Picrasidines G, S, and T feature an indolotetrahydroquinolizinium (ITHQ) skeleton, while picrasidine R possesses a 1,4-diketone linker between two β-carboline fragments. The synthesis of ITHQ-type bis-β-carboline alkaloids could be directly achieved by a late-stage regio-selective aza-[4 + 2] cycloaddition of vinyl β-carboline alkaloids, suggesting that this remarkable aza-[4 + 2] cycloaddition might be involved in the biosynthetic pathway. Computational studies revealed that such aza-[4 + 2] cycloaddition is a stepwise process and explained the unique regioselectivity (ΔΔG = 3.77 kcal mol−1). Moreover, the successful application of iridium-catalyzed C–H borylation on β-carboline substrates enabled the site-selective C-8 functionalization for efficient synthesis and structural diversification of this family of natural products. Finally, concise synthesis of picrasidine R by the thiazolium-catalyzed Stetter reaction was also accomplished.

No synthetic studies toward ITHQ-type bis-b-carboline alkaloids have been reported to date.However, several studies on the syntheses of 1-vinyl substituted monomeric b-carboline alkaloids have been reported (Scheme S2 †).In 1982, Cook and coworkers reported the total syntheses of crenatine with the C-4 methoxy substituted b-carboline skeleton by a Pictet-Spengler reaction and DDQ oxidation strategy. 27In 2005, Ihara and coworkers reported the total synthesis of dehydrocrenatine (5a) via the application of Cook's strategy. 28In 1999, Murakami and coworkers reported the syntheses C-4 and C-8 disubstituted bcarboline alkaloids picrasidines I (5d), J and P, as well as crenatidine and dehydrocrenatidine (5c) by Fischer indole synthesis. 29In 2005, Murakami and coworkers reported a more concise Fischer indole synthesis strategy for the syntheses of C-4 and C-6 disubstituted b-carboline alkaloids. 30a Wittig reaction.Substituent groups at the C-8 position could be installed by the iridium-catalyzed C-H bond borylation followed by C-B bond functionalization.The synthesis of precursor 7a should be achieved via a Pictet-Spengler reaction and DDQ oxidation inspired by Cook and Ihara's studies. 27,28oreover, another bis-b-carboline alkaloid picrasidine R (4) could be synthesized via a Stetter reaction from vinyl ketone 10 and aldehyde 6c.The vinyl ketone 10 could also be obtained from aldehyde 6c by vinyl addition and a subsequent oxidation.

Results and discussion
Total syntheses of bis-b-carboline alkaloids and derivatives Our synthesis commenced with the preparation of the monomeric C-1 vinyl substituted b-carboline dehydrocrenatine (5a) (Scheme 2a).The tetrahydro-b-carboline skeleton of compound 8 was constructed by a Pictet-Spengler reaction between tryptamine hydrochloride and 2,2-dimethoxyacetaldehyde, 31 followed by tosyl protection of the secondary amine.Then benzyl oxidation of compound 8 by DDQ afforded ketone 9. 27,28 Acetalmethylation of ketone 9 by the treatment with trimethyl orthoformate in acidic methanol solution led to in situ elimination of the tosyl group and aromatization to afford the key synthetic building block 7a, bearing the b-carboline structure with a methoxy group at the C-4 position.Compared with the acetyl group, usage of the tosyl group could avoid extra addition of oxidant for aromatization. 28,30Aer hydrolysis of the dimethyl acetal to form the aldehyde group and installation of the vinyl group by the Wittig reaction, we successfully prepared the desired C-1 vinyl substituted b-carboline dehydrocrenatine (5a).
To explore the feasibility of the proposed regio-selective aza-[4 + 2] cycloaddition, dehydrocrenatine (5a) was chosen as a model substrate.We found that reactions performed in water solution or dioxane solution could only lead to trace formation of the dimerization product (Table 1, entries 1-5).However, upon heating an equimolar mixture of free alkaloid and its hydrochloride salt to 100 °C for 4 h under neat conditions, 40% of the reactant could be converted to the dimerization product (Table 1, entry 6).Raising the temperature to 130 °C could give almost full conversion of the reactant, with 55% isolated yield (Table 1, entry 9).The product was conrmed as the ITHQ-type bis-b-carboline alkaloid picrasidine G (1), demonstrating the feasibility of the aza-[4 + 2] cycloaddition.Although using free alkaloid as the reactant alone could also form a small amount of the dimerization product, the conversion was relatively low (Table 1, entry 11).On the other hand, using hydrochloride salt as the reactant alone could only lead to trace amounts of the dimerization product (Table 1, entry 10).This result showed that such aza-[4 + 2] cycloadditions were prone to occur between one molecule of free alkaloid and another molecule of hydrochloride salt.
Compared to the simplest ITHQ-type bis-b-carboline alkaloid picrasidine G (1), picrasidines S (2) and T (3) bear an additional oxidation state at the C-8 position.8][39][40] Previously, Smith and coworkers reported the iridium-catalyzed C-H borylation at C-2 and C-7 positions of the indole ring. 41They found that if the C-2 position was blocked, the C-H borylation could only happen at the C-7 position.To our delight, such a methodology worked well for C-8 site-selective C-H borylation of 7a.Both HBpin and B 2 pin 2 could be used as a boron source to prepare the boranate 7b, while B 2 pin 2 showed higher yield compared to HBpin.Combined with C-B bond functionalization methodologies such as Chan-Lam coupling (7c), 42 oxidation by hydrogen peroxide (7d), 43 halogenation (7e), 43,44 Suzuki coupling (7f) 44 and trioromethylation (7g), 45 we could easily install diverse functional groups at the C-8 position (Scheme 2b).By the same route, all monomeric alkaloids 7c-g could be successfully transformed into the corresponding ITHQ-type bisb-carboline alkaloids (Scheme 2c).Moreover, aldehyde 6c could be used for the concise synthesis of a bis-b-carboline alkaloid picrasidine R (4).Vinyl addition to the aldehyde 6c followed by Dess-Martin oxidation afforded the vinyl ketone 10, which could undergo a thiazolium-catalyzed Stetter reaction with the aldehyde 6c to afford picrasidine R (4) in 90% yield (Scheme 2c). 46

Computational studies of the aza-[4 + 2] cycloaddition
It should be noticed that theoretically there may be another possible pathway of the aza-[4 + 2] cycloaddition, leading to isopicrasidine G as the product, as is shown in Fig. 1.However, isopicrasidine G was neither identied in the aza-[4 + 2] cycloaddition, nor from natural product isolation.Generally, for the type-III aza-[4 + 2] cycloaddition (Scheme S3 †), the terminal alkene position of the vinyl imine structure is prone to act as an electrophile (Scheme S3, † Ihara's work as an example [47][48][49] ).However, our observation of the reaction outcome was different.It is noteworthy that such a transformation is the rst reported umpolung type-III aza-[4 + 2] cycloaddition, whose regioselectivity is different from those of all previously reported ones.To explain the unique regioselectivity and provide us mechanistic insights, the reaction prole was further investigated by DFT calculations (Scheme 3).Interestingly, if we select two molecules of free alkaloid (dehydrocrenatine, 5a) as the reactant, they will undergo a concerted aza- According to the computational result, the stepwise pathway A is the most favored mechanism under the experimental conditions, with just 26.80 kcal mol −1 activation Gibbs energies to overcome.For the free alkaloid, the electron-donating effect of the N-9 nitrogen atom and the C-4 methoxy group inverts the intrinsic polarity of the "pyridine ring", making the terminal alkene nucleophilic rather than electrophilic.However for the protonated alkaloids, the "pyridine ring" maintained its intrinsic polarity due to the protonation, making the terminal alkene a Michael acceptor.First, the terminal alkene of the free alkaloid acts as a nucleophile to attack the terminal alkene of the protonated alkaloid, forming INT A -1 via TS A -1 (DG = 22.72 kcal mol −1 ).Then the succeeding annulation process of the unstable INT A -1 can easily take place via TS A -2 to form INT A -2, requiring only 4.25 kcal mol −1 energy expense.Finally, the intramolecular proton transfer decreases the total energy of the system, making the

Conclusions
We have successfully accomplished the rst total syntheses of three ITHQ-type bis-b-carboline alkaloids picrasidines G, S and T and several derivatives by the regio-selective aza-[4 + 2] cycloaddition.][52] Computational studies revealed a stepwise mechanism of the aza-[4 + 2] cycloaddition and explained the origin of excellent regioselectivity.Another bis-b-carboline alkaloid picrasidine R was also efficiently synthesized via the thiazolium-catalyzed Stetter reaction.This work provides synthetic evidence for the proposed biosynthetic pathway of ITHQ-type bis-b-carboline alkaloids.
[4 + 2] cycloaddition (Scheme 3, pathways A ′ and B ′ ) with the activation Gibbs energies (DG) up to 40.45 kcal mol −1 for pathway A ′ to form picrasidine G (1, P A ) and 47.88 kcal mol −1 for pathway B ′ to form isopicrasidine G (P B ).However, if we select one molecule of free alkaloid and another molecule of the protonated alkaloid as reactants, they will undergo a stepwise, formal aza-[4 + 2] cycloaddition (Scheme 3, pathways A and B), and the activation Gibbs free energies decrease to 26.80 kcal mol −1 for pathway A and 30.57kcal mol −1 for pathway B. Kinetically, the difference of activation Gibbs free energy (DDG) between pathways A and B is up to 3.77 kcal mol −1 , which explains such excellent regioselectivity.

Table 1
Reaction screening of the aza-[4 + 2] cycloaddition a Reactions were performed using dehydrocrenatine (5a, represented by A) and its corresponding salt (represented by [HA] + X − ) as reactants.b Unless otherwise stated, reactions were performed at 10 mM concentration.c In 25 mM KPi buffer (pH = 6).d In 25 mM Tris buffer (pH = 7).e In 25 mM HEPES buffer (pH = 8).f Reaction was performed in the solid state without any solvent.g Unless otherwise stated, yields were based on 1 H-NMR.h Isolated yields.