Utilizing the pK_a Concept to Address Unfavorable Equilibrium Reactions in the Total Synthesis of Palau’amine

Abstract

Herein, we introduce the pK_a concept as a strategy for proceeding with unfavorable reactions in the total synthesis of palau’amine. The cascade reaction aimed at constructing palau’amine’s ABDE tetracyclic ring core initially encountered poor reproducibility due to an unfavorable equilibrium reaction. However, the addition of 1.0 equivalent of AcOH enabled the progression of the unfavorable equilibrium reaction. In the second-generation synthesis, the improved cascade reaction to construct CDE ring core also initially did not proceed due to an unfavorable equilibrium reaction, but using Ph₂NLi as a base was later found to enable the reaction to proceed smoothly. The conjugate acid of Ph₂NLi proved to be a suitable acid in the unfavorable equilibrium mixture. These investigations of the cascade reactions revealed that the coexistence of an appropriate acid played an important role in allowing the unfavorable equilibrium reaction to proceed. The authors propose a general equation for proceeding with an unfavorable equilibrium reaction.

23.1 Introduction

Palau’amine (1) belongs to the class of pyrrole-imidazole alkaloids, originally isolated by Scheuer in 1993 [1, 2], with revisions of its stereochemistry reported in 2007 [3,4,5]. Since its initial disclosure, 1 has received a great deal of attention as an attractive synthetic target due to its complex structure and potent biological activities, including antifungal, antitumor, and immunosuppressive properties. In particular, the potent immunosuppressive activity of 1 has piqued the interest of researchers, leading to investigations into its mode of action [6, 7]. The distinctive structural attributes of 1 include the following: two guanidine moieties; a complex polycyclic system characterized by spiro and fused rings; eight consecutive stereogenic centers, including one nitrogen-containing tetrasubstituted carbon center; a fully substituted cyclopentane ring; and the highly strained trans-azabicyclo[3.3.0]octane skeleton located at the D/E ring junction [8,9,10,11,12,13]. However, 1 is well known as one of the most difficult natural products to synthesize; despite many attempts, only two examples of its total synthesis have been reported. The first total synthesis of palau’amine was achieved by Baran’s group in 2010 [14], and an asymmetric version was developed in 2011 [15]. In 2015 we successfully achieved the total synthesis [16], and in 2021 we developed an efficient method for constructing the hexacyclic ring system of palau’amine as part of our second-generation synthesis [17]. Not surprisingly, we encountered several difficulties during these synthetic studies. This chapter focuses on how we addressed the challenge of allowing an unfavorable equilibrium reaction to proceed.

23.2 The Key Reaction and Its Unfavorable Equilibrium in the Total Synthesis of Palau’amine

Our total synthesis of palau’amine (1) is summarized as follows. The synthesis began with commercially available cyclopentenone 2, and the precursor 3 for the key cascade reaction was obtained from 2 in 25 steps. The treatment of 3 with a strong base, followed by the addition of acetic acid, yielded the ABDE tetracyclic ring core of palau’amine in a single step. The C ring was constructed in the next 5 steps, resulting in the formation of 5. The F ring was also formed, yielding 6 within 8 additional steps from 5. Finally, the primary alcohol and the methylthio group on the C ring were each converted to an amino group, and subsequent hydrogenation afforded 1, which achieved the total synthesis of palau’amine (Scheme 23.1). Throughout the total synthesis, 1 was obtained with an overall yield of 0.039% in 45 steps starting from 2. The conversion of 3 to 4 was the key reaction in this total synthesis. The details are discussed below.

Scheme 23.1

Overview of total synthesis of palau’amine (1) by our group

Our total synthesis first targeted the construction of a nitrogen-containing tetrasubstituted carbon center at the C16 position of palau’amine. To achieve this, we employed an Hg(OTf)₂-catalyzed olefin cyclization reaction that was originally developed within our research group. After various examinations, we achieved the catalytic construction of the tetrasubstituted carbon center at the C16 position by employing hydrazine as a nitrogen nucleophile. Subsequent ring contraction led to the formation of the pylazolidine ring 7, which was further oxidatively modified at the C10 position. After considering the structure of the intermediate 7, the author came up with the following cascade reaction. Intermediate 7 was transformed into 8 through the introduction of a pyrrole amide and a strong electron-withdrawing group into the primary amine side chain and the nitrogen on the pyrazolidine ring, respectively. The subsequent treatment of 8 with more than 3.0 equivalents of a strong base resulted in the deprotonation of active NH protons in both the pyrrole and amide groups, with the first two equivalents of the base involved in this process. If the third base can abstract the hydrogen at the α-position of the methyl ester at the C10 position, it could result in the simultaneous cleavage of the N–N bond along with the oxidation to form the imine at the C10 position due to E1cB elimination of the nitrogen carrying a strong electron-withdrawing group. This would lead to the formation of 8B. An amide anion of 8B promptly attacked the highly reactive acylimine moiety to generate the D ring, resulting in the formation of 8C. The cascade reaction did not end at this stage because the pyrrole anion still remained. The pyrrole anion also continuously attacked the methyl ester, resulting in the construction of the B ring and yielding 8D, which, upon acid quenching, afforded 9. Thus, treatment of 8 with more than 3.0 equivalents of the strong base would induce the cascade reaction described above, resulting in the formation of the ABDE tetracyclic ring core of palau’amine in a single step (Scheme 23.2).

Scheme 23.2

Plan for the cascade reaction to construct ABDE tetracyclic ring core of 1

After converting the Fmoc group of 7 to the pyrrole amide group, we attempted to introduce a highly electron-withdrawing group to the pyrazolidine ring nitrogen, but only the trifluoroacetyl group was introduced. With the precursor 3 in hand, we examined the cascade reaction. Surprisingly, in the first experiment of the cascade reaction, where 3 was treated with 3.0 equivalents of LiHMDS, the desired product 4 was obtained, albeit with a yield of only 30%. Clearly remembering that we had obtained the desired 4 in the first experiment, we were very excited by this result and immediately attempted to scale up this cascade reaction to achieve the total synthesis of palau’amine. However, we encountered a problem at this point: subsequent attempts to repeat the reaction did not yield reproducible results. In some cases, the yield of 4 was around 50%, but in many cases it was less than 30%, and in some cases was hardly obtained (Scheme 23.3). As long as the yield reproducibility is consistent, it may be possible to address this by increasing the scale of synthesis even if the yield is not high. However, in cases of poor reproducibility, it was deemed too risky to use even a slightly larger quantity of the painstakingly synthesized precursor 3, given that it required 25 steps to obtain.

Scheme 23.3

Unfavorable equilibrium between 3C and 3D in the cascade reaction forming ABDE tetracyclic ring core of 1

We carefully reconsidered the cause of the poor reproducibility based on the reaction mechanism. It became apparent to us that, in a sense, reproducibility could not be ensured. Upon treatment with a base, the D ring was formed via 3B, giving 3C. Up to this stage, the reaction had proceeded without any problems. Indeed, the conversion from 3 to 3C could be monitored by TLC. However, subsequent conversion from 3C to 3D was extremely slow. When we considered this carefully, we realized this was a matter of course. That was because a methoxide, which is less stable than the pyrrole anion of 3C, was formed during the nucleophilic addition reaction of the pyrrole anion to the methyl ester. Thus, even as the reaction proceeded, the generated methoxide attacked the amide site of 3D, bringing it back to 3C. In other words, this reaction was in an equilibrium between 3C and 3D, and 3C was much favored in this equilibrium. As we observed the equilibrium reaction with TLC and waited for 3C to disappear, 3C gradually decomposed, causing the loss of reproducibility (Scheme 23.3). Consequently, it was hypothesized that selectively quenching the minute amount of methoxide generated in this equilibrium would permit the reaction from 3C to 3D to advance, thus overcoming the unfavorable equilibrium reaction. With this premise, we attempted to eliminate only the methoxide while preserving the pyrrole anion.

As a method to selectively remove methoxide, we added exactly 1.0 equivalent of acid when the cascade reaction had reached the 3C stage (Scheme 23.4). Treatment with 3.0 equivalents of LiHMDS at − 78 °C and warming to 0 °C resulted in the progress of the cascade reaction to form the D ring, leading to 3C. After TLC confirmed the formation of 3C, exactly one equivalent of AcOH was added to the reaction mixture at − 78 °C. As the most basic of the three nitrogen anions of 3C was the Boc amide anion, it was protonated by acetic acid to afford 3E. The pyrrole anion of 3E still remained, and then the addition reaction of pyrrole anion to methyl ester continued. As the addition reaction proceeded to form 3F, methoxide was also produced as before. In this case, however, 3F had an active NH proton, and the methoxide showed a preference for abstracting the active proton as a base rather than adding to the amide site as a nucleophile. This quenched only the generating methoxide while retaining the pyrrole anion. This acid addition inserted new intermediates 3E and 3F between the previous equilibrium of 3C and 3D. This adjustment effectively shifted the equilibrium from favoring 3C to favoring 3D. Among various acids, acetic acid was found to be the best protonating reagent. In addition, precise amounts of LHMDS and acetic acid are very important for this “protonation-state switching by pK_a game” reaction. In this way, the yield of ABDE tetracyclic ring core 4 was increased to 74% with good reproducibility achieved on scales acceptable for total synthesis (Scheme 23.4). By this method, it became possible to supply 4 in quantity to support the subsequent 19 steps and to achieve the total synthesis of palau’amine.

Scheme 23.4

Overcoming unfavorable equilibrium between 3C and 3D

23.3 The Unfavorable Equilibrium Reaction in the Key Reaction of Second-Generation Total Synthesis of Palau’amine

We achieved the total synthesis of palau’amine based on the key reaction described above, but our total synthesis required a large number of steps (45 steps), as shown in Scheme 23.1. To advance both mechanistic studies of the potent immunosuppressive activity of palau’amine and structure‒activity relationship (SAR) studies, a more efficient method of constructing the hexacyclic ring system of 1 was needed. Therefore, we tried to develop a more efficient synthetic method as a second-generation synthesis. In the first-generation total synthesis, there were several issues that lengthened the number of steps, including 25 steps that were required to obtain the cascade reaction precursor 3, while the construction of additional C and F rings also required many steps (5 and 8, respectively) after the construction of the ABDE tetracyclic ring core 4. Therefore, as a new synthetic strategy based on the key reaction in the first-generation total synthesis, we devised a plan to pre-introduce the sources for the C and F rings into the precursor of the cascade reaction. Specifically, the Boc group in 3 was converted into an isothiourea group to serve as the source of the guanidino group of the C ring. Additionally, the vinyl group was transformed into a nitrile group, which served as a foothold for the construction of the F ring. Furthermore, with the prospect of future expansion into SAR studies and probe development, we designed 10, lacking both the aminomethyl side chain and the chloro group, as a precursor for the cascade reaction. This enabled us to investigate whether these functional groups affect immunosuppressive activity. As was the case in the first-generation synthesis, treatment of 10 with 3.0 equivalents of a strong base was expected to initiate a cascade reaction for constructing the CD ring in a single step. This cascade encompassed the abstraction of hydrogens from NH groups and the C10 position, cleavage of the N–N bond accompanied by oxidation at the C10 position, addition of the amide anion to the imine moiety, and subsequent addition of thioisourea to the ethyl ester (Scheme 23.6). If this cascade reaction proceeded, the construction of the C ring, which required 5 steps in the first-generation synthesis, could be accomplished in a single step simultaneously with the construction of the D ring.

Scheme 23.5

Plan for the efficient construction of the CDE ring core of 1

In the first-generation synthesis, we employed an Hg(OTf)₂-catalyzed olefin cyclization reaction to construct the nitrogen-containing tetrasubstituted carbon center at the C16 position. In the second-generation synthesis, the tetrasubstituted carbon center was constructed by an unprecedented Strecker reaction to pyrazolines; this reaction shortened the synthesis of the cascade reaction precursor 10 from commercially available cyclopentenone to just 9 steps. Since this synthetic route could supply a sufficient amount of 10, we tried the cascade reaction.

First, treatment of 10 with 3.0 equivalents of LiHMDS resulted in the formation of a trace amount (< 5%) of the CDE ring core 11. Despite this, the intended reaction did progress to some extent, albeit with significant decomposition of the starting material (Scheme 23.6). We repeated the reaction under various conditions by changing the base equivalent, temperature, solvent, and so on, but we were unable to obtain more than a trace amount of 11 and often obtained none at all. Although most of the starting material decomposed, isothiourea 12 was obtained as the major byproduct. From the formation of byproduct 12, we considered that the reaction did not proceed and decomposed for the following reason. As in the first-generation synthesis, the third base abstracted the hydrogen at the C10 position to give 10B, and the amide anion subsequently added to the reactive imine moiety to form the D ring, leading to 10C. We considered the cascade reaction to have proceeded smoothly until this stage, similar to the first-generation synthesis. Although the subsequent introduction of the carbamate anion from the isothiourea to the ethyl ester could potentially yield the desired CDE ring core 11, this transformation was regarded as difficult to achieve.

Scheme 23.6

Unfavorable equilibrium between 10C and 10D in the cascade reaction forming the CDE tetracyclic ring core in the second-generation synthesis of palau’amine

Among the three anions present in the second-generation substrate 10C, the carbamate anion derived from isothiourea exhibited the highest stability. Consequently, the conversion to 10D, which produced the least stable ethoxide, was highly unfavorable. Therefore, the equilibrium between 10C and 10D strongly favored the former, and by subjecting this equilibrium to an acidic quench, 10D, which was minimal quantity in the equilibrium, was transformed into 11 and isolated. The abundant 10C was protonated to give 10E, and the highly strained D ring was immediately cleaved due to the electron-donating effect originating from the isothiourea group, resulting in the formation of 10F. The isothiourea moiety was removed from 10F by hydrolysis, leading to 12 as a major byproduct, along with the decomposition of the DE ring moiety (Scheme 23.6). From this result, we found that while 10D can be isolated as 11 after quenching with acid, 10C cannot be isolated as an intermediate because it decomposes during quenching. This, in turn, suggested that the reaction would give only decomposed products unless the favorable intermediate in this equilibrium reaction was reversed from 10C to 10D.

Also, the stereochemistry at the C10 position of 11, a trace amount of which was obtained, was the opposite of that of the cyclization product 4 obtained in the first-generation synthesis (Scheme 23.6). The cascade reaction in the second-generation synthesis unveiled that the CDE ring core was formed with a C10-position stereochemistry that was opposite that of palau’amine. The stereochemical differences between the first- and second-generation syntheses arose from the difference in the Boc and isothiourea groups. In the first-generation synthesis, DFT calculations revealed that the configuration at the C10 position was determined by the coordination of the carbonyl oxygen of the Boc group to lithium salt (see Ref. [16] for details). Although the stereochemistry at the C10 position of 11 was undesired, we planned to increase the yield of 11 first and then attempt its stereoinversion because the intermediate 10C could not be isolated. Therefore, to improve the yield of 11, we had to determine how to drive the reaction to 10D. We had already faced and solved a similar problem in the first-generation total synthesis. We considered that the unfavorable equilibrium between 10C and 10D could be resolved by applying the same pK_a concept that selectively quenches the ethoxide generated as the reaction proceeds from 10C to 10D.

The pK_a concept in the case of 10 was as follows. At the stage where intermediate 10C was formed by treating 10 with 3.0 equivalents of LiHMDS, three nitrogen anions were generated: the pyrrole anion (pK_a of conjugate acid: ~ 23), the carbamate anion of isothiourea (~ 15), and the trifluoroacetoamide anion (~ 17). Given the basicity of these anions, adding 1.0 equivalent of AcOH at this point would protonate the pyrrole anion, resulting in the formation of 10Eʹ. The remaining carbamate anion of isothiourea then attacked the ethyl ester to afford 10H, and the simultaneously generated ethoxide abstracted the proton from the NH of pyrrole. This prevented the ethoxide ion from attacking the imide moiety of 10H, effectively avoiding the reverse reaction leading to 10Eʹ and facilitating the progression of the reaction toward the dianion 10D (Scheme 23.7). However, the addition of 1.0 equivalent of AcOH after the conversion to 10C induced quick decomposition and gave only a similar byproduct, isothiourea 12 (Scheme 23.7). This outcome was likely a result of the protonation of the pyrrole anion to form 10Eʹ, which diminished the electron-donating effect from the pyrrole anion. Consequently, the electron-withdrawing nature of the amide carbonyl group was heightened, leading to the cleavage of the highly strained D ring through the electron donation from the isothiourea carbamate anion, rather than facilitating the nucleophilic addition to the ethyl ester. This result indicated that the findings from the previous studies (the first-generation synthesis) were not applicable to the second-generation cascade reaction with the isothiourea. Thus a new solution, other than the addition of acetic acid, had to be discovered.

Scheme 23.7

Application of acid addition method in the first-generation synthesis to the cascade reaction in the second-generation synthesis

To find a solution, we revisited the reaction mechanism in detail and thereby realized that there was another intermediate within the equilibrium between 10C and 10D: the alkoxide intermediate 10I (Scheme 23.8). Although this alkoxide intermediate is often omitted when considering nucleophilic addition reactions to ester carbonyls, we realized that it was the key to allowing an unfavorable equilibrium reaction proceed. The pK_a of the conjugate acid of the alkoxide of 10I is estimated to be less than 32 (based on the pK_a of ^tBuOH in DMSO). Notably, the alkoxide of 10I stands out as the most unstable among the anions found in 10C and 10I. Additionally, its basicity significantly exceeds that of both the pyrrole anion and the amide anion. Instead of reverting to 10C by releasing the more basic ethoxide (pK_a of conjugate acid: 28.9) from this unstable intermediate, the primary focus was on preserving the most stable (less basic) isothiourea carbamate anion (pK_a of conjugate acid: ~ 15). Therefore, it was considered that an equilibrium mainly existed between 10C and 10I in this reaction system, with 10D barely involved in the equilibrium. The equilibrium between 10C and 10I significantly favored 10C, and the trace amount of 11 obtained by acid quenching was believed to originate from the scarcely present 10I through the formation of protonated 10I. Thus, we expected that if only 10I could be selectively protonated and converted to 10J, the equilibrium mixture would eventually converge to 10J, and subsequent removal of ethanol would afford 11 in good yield. There were six anions in the equilibrium mixture of 10C and 10I, with alkoxide and pyrrole being the most basic and second most basic anions, respectively. When acid was used to quench this equilibrium mixture, it protonated 10C, which was present in large excess within the equilibrium. Consequently, the reaction yielded only a mixture of decomposition products. However, if only the alkoxide of 10I could be selectively protonated and converted to 10J without protonating the pyrrole anions, 10I could be removed from the equilibrium mixture, and the mixture should eventually converge to 10J (Scheme 23.8).

Scheme 23.8

Plan to proceed with unfavorable equilibrium reaction

To facilitate the coexistence of such acids, we came up with the idea of using Ph₂NLi as the base. Although there have been few examples of the use of Ph₂NH as a base, treatment of 10 with 3.2 equivalents of Ph₂NH allowed the reaction to proceed smoothly, and the generation of 10J was suggested by ¹H and ¹⁹F NMR. The subsequent addition of 3.2 equivalents of AcOH in one pot induced the elimination of ethanol, successfully affording the desired 11 in a good yield of 72%. This reaction was considered to proceed through the following mechanism. First, 3.0 equivalents of Ph₂NLi abstracted three protons of 10, leading to 10C. Then, as in the case of LiHMDS, 10C formed the equilibrium mixture of 10C and 10I, with a strong preference for the former. In this case, 3.0 equivalents of Ph₂NH were generated as a conjugate acid in the reaction system after hydrogens were abstracted by Ph₂NLi. Since the pK_a of Ph₂NH was 25, it could protonate a slightly generated alkoxide of 10I. On the other hand, Ph₂NH was unable to protonate the pyrrole anion, as the pK_a of the conjugate acid was estimated to be less than 23. In other words, Ph₂NH functioned as a suitable acid that could selectively protonate the alkoxide formed in small quantities without interacting with the pyrrole anions, which were present in large excess in the reaction system (Scheme 23.9). The above results revealed that Ph₂NH is an interesting base that transitions into an acid after initially functioning as a base.

Scheme 23.9

Overcoming unfavorable equilibrium between 10C and 10I

Next, to validate whether the effectiveness of Ph₂NH in the cascade reaction stemmed from the generation of a conjugate acid with the suitable pK_a range (23–32) as we previously proposed, we conducted comparable experiments employing different bases (Scheme 23.9). The use of Et₂NLi and _iPr₂NLi as bases did not afford the desired cyclization product, and only decomposition occurred. The respective pK_as of the conjugate acids of Et₂NLi and ⁱPh₂NH were 40 and 36, significantly higher than the pK_a (< 32) of the conjugate acid of alkoxide of 10I. Hence, the alkoxide of 10I remained unprotonated, and a substantial quantity of 10C persisted as the dominant intermediate in the equilibrium. Following quenching, it decomposed, as depicted in Scheme 23.6. The conjugate acid of LiHMDS has a pK_a of 30, which is close to the acidity of the conjugate acid of the alkoxide of 10I (< 32). Thus, even if the alkoxide could be protonated by HMDS (TMS₂NH) and converted into 10J, the resultant LiHMDS (TMS₂NLi) had the capacity to extract the alcohol hydrogen from 10J and revert it back to 10I. This reverse reaction also generated the equilibrium between 10I and 10J, and the three intermediates 10C, 10I, and 10J were in equilibrium. Due to the large abundance of favorable 10C in the equilibrium, quenching this equilibrium reaction afforded only a trace amount of 11. To increase the abundance of 10J in the equilibrium, the addition of 10 equivalents of HMDS improved the yield of 11 to 36%. This result reinforced the idea that the pK_as of the conjugate acids of LiHMDS and alkoxide were similar, leading to an equilibrium between 10C and 10J.

Next, the use of (p-Br-C₆H₄)₂NLi, which was anticipated to have a lower pK_a of the conjugate acid compared to pyrrole, resulted in the formation of only a decomposed mixture. It was considered that the pyrrole anion of 10C, which presented in large excess, became protonated by the generated conjugate acid, (p-Br-C₆H₄)₂NH. As a consequence, 10C decomposed through the same route as shown in Scheme 23.6.

Only the use of a base with a conjugate acid pK_a range of 23–32 afforded the CDE ring core 11. Conversely, when other bases possessed conjugate acid pK_as either higher or lower than this range, only decomposed products were obtained (Scheme 23.9). This observation supports our hypothesis that the coexistence of suitable acids is essential for advancing an unfavorable equilibrium reaction.

In these synthetic studies of plau’amine, we have demonstrated that the presence of an appropriate acid can promote the successful progression of an unfavorable equilibrium reaction involving anions. Since this method is applicable to various other equilibrium reactions, we propose this concept of a coexisting acid in the following general equation (Fig. 23.1). When compound AH is subjected to a base, it produces an equilibrium mixture consisting of B⁻ and C⁻ anions. The anion whose conjugate acid has a lower pK_a is the more stable one, leading to an equilibrium mixture in which this particular anion is predominantly present. In other words, in a case where the highest pK_as of the conjugate acids of B⁻ and C⁻ are a¹ and a², respectively, and the values are a¹ < a², the equilibrium favors B⁻. The greater the difference between a¹ and a², the greater the difference in the abundance of B⁻ and C⁻. In this equilibrium mixture, if CH, which is protonated unfavorable C⁻, is the desired product, a simple acidic quench also protonates the B⁻ anion, which is present in large excess. This results in the formation of almost no CH. To obtain CH, a protonated form of the unfavorable anion, as the major product, the presence of an appropriate acid that can protonate C⁻ but cannot protonate B⁻ is necessary. In other words, the coexistence of an acid with a pK_a higher than that of a¹ and lower than that of a² is required. Therefore, it is necessary to have coexisting acids that selectively protonate the unfavorable anion C⁻ in the equilibrium, satisfying the following relationship: a¹ < a³ < a², where a³ represents the pK_a of the coexisting acids (Fig. 23.1).

Fig. 23.1

General equation for proceeding with an unfavorable equilibrium reaction

Here, we attempt to apply this general equation to the aforementioned cascade reactions. In the first-generation cascade reaction (Scheme 23.4), when the precursor 3 was treated with 3.0 equivalents of LiHMDS, it produced an equilibrium mixture of 3C and 3D. Obtaining 3D as the major product was challenging due to the predominance of 3C in the equilibrium. On the other hand, if this reaction is considered as an equilibrium reaction between anions as shown in the general equation, it should also be viewed as an equilibrium reaction between 3C and the methoxide. The addition of 1.0 equivalent of acetic acid to this equilibrium mixture mainly protonated the Boc amide anion of 3C to convert to NHBoc 3E due to the much greater abundance of 3C compared to the methoxide. The equilibrium is then between the pyrrole anion of 3E and the methoxide, and the pK_a of the generated NHBoc is less than 24 (a³), which is higher than that of the pyrrole anion of 3E (pK_a: < 23) (a¹) and lower than that of the methoxide (pK_a: 28) (a²). Therefore, the pK_a of generated NHBoc satisfied the inequality “a¹ < a³ < a²” as the coexisting acid, so the reaction proceeded by quenching only methoxide as an unfavorable anion. In addition, in the second-generation cascade reaction, the pK_a 25 (a³) of Ph₂NH generated from Ph₂NLi is higher than 23 (highest pK_a of conjugate acid: a¹) of the pyrrole anion of 10C and lower than 32 (a³) of the alkoxide of 10I, and the general inequality “a¹ < a³ < a²” is satisfied. Hence, to selectively protonate only the unfavorable anion in the equilibrium of anions, it is effective to have an appropriate acid coexist, which adheres to the suggested general inequality “a¹ < a³ < a²” (Fig. 23.1).

Of course, while the pK_a concept is well known among synthetic chemists, it has often been applied based on individual chemists’ knowledge and experience rather than as a formalized general equation. Therefore, we proposed this general equation based on the results of the key cascade reactions in the synthetic studies of palau’amine. We plan to investigate the scope of this general equation in the future.

As described above, we overcame the unfavorable equilibrium reaction to achieve the total synthesis of palau’amine. In addition, by establishing a concept for proceeding with an unfavorable equilibrium reaction, we successfully developed a second-generation synthesis that can more efficiently construct the hexacyclic ring core of palau’amine. Although the first-generation total synthesis required 45 steps, the second-generation synthesis required only 20 steps to synthesize 13, which has all the ring structures of palau’amine (Scheme 23.10). By evaluating the activity of 13, we found that the immunosuppressive activity of palau’amine is retained even without the aminomethyl side chain and the chloro group, although the activity somewhat decreased. This suggested that these functional groups can be utilized to design probes. With the aim of supplying palau’amine for probe development, we are currently applying this concept to substrates with side chains.

Scheme 23.10

Comparison of the first-generation synthesis and second-generation synthesis

23.4 Conclusion

In this chapter, we introduced the problems we encountered in the cascade cyclization reactions as the key steps of the total synthesis of palau’amine, and we developed solutions to them. In both the first- and second-generation syntheses, it took a long time for the cascade cyclization reactions to proceed with good yields and good reproducibility. In both cases, it was essential to conduct numerous experiments and engage in thorough deliberations before recognizing the presence of an unfavorable equilibrium reaction in the reaction system. Even after this realization, we performed many failed experiments before arriving at the idea of adding a coexisting acid. For example, to prevent the generation of highly basic methoxide and ethoxide, we first attempted the use of esters with acidic alcohols such as phenol and fluorinated alcohols, but the formation of active esters induced unexpected side reactions. In the end, the addition of an appropriate coexisting acid was the only solution that enabled the cascade cyclization reaction to proceed. Once we understood this, we realized it was a very simple solution. Although this research phenomenon may be common in any field, this study makes us realize once again that it can be difficult to notice the obvious. There is no doubt that repeated deep consideration and hard work are sometimes important to realize the obvious.