First Total Synthesis of Tanzawaic Acid B

The first total synthesis of (+)-tanzawaic acid B, a natural polyketide bearing a pentadienoic ester and octalin moiety, has been accomplished. The synthetic improvement from previous synthetic conditions facilitated our gram-scale synthesis of the chiral octalin that possesses seven stereogenic centers and that is the core skeleton of almost all of the tanzawaic acid family.

I n 1995, Kyowa Hakko Kogyo Co., Ltd., reported the isolation of tanzawaic acid A (GS-1302-3) (1) and tanzawaic acid B (GS-1302-1) (2) from Penicillium sp. GS1302 1 (Figure 1). Two years later of the first isolation report, Uemura and co-workers reported the isolation of tanzawaic acids A and B (1 and 2) with other analogues of tanzawaic acids, tanzawaic acids C and D (3 and 4), from the different fungus, Penicillium citrinum, 2 and they named the four compounds "tanzawaic" acids after a name of the Japanese toponym, "tanzawa". After Uemura had reported the isolation of tanzawaic acids, more than 25 tanzawaic acid analogues have been isolated. 3 The biological activity of 2 had also been researched. Kyowa Hakko Kogyo Co., Ltd., reported its antibacterial activity 1 in 1995, and Uemura and co-workers reported that 2 has an inhibition activity of superoxide production in human neutrophils induced by TPA (12-O-tetradecanoylphorbol-13acetate). 2 Since two groups had reported the biological activity, five groups have independently reported the antifungal activity, 1,3g,n PTP1B inhibitory activity, 3c and antimalarial activity. 3k The structure and relative configuration of 2 were first suggested by Kyowa Hakko Kogyo Co., Ltd. However, they had not shown the information on the absolute configuration of 1 and 2. Arimoto and Uemura and co-workers accomplished the total synthesis of 1 and revealed the absolute configuration of 1. 4 The contribution of their work expected us that the absolute configuration of 2 was corresponding stereochemistry to the one of 1. In 2018, Guo and Li groups confirmed the structure and the absolute configuration of 2 from X-ray crystallography. 3i While there have been a number of isolation and biological researches, there has never been a synthetic study of tanzawaic acid B (2). Therefore, we initiated the synthetic study of 2 with the aim of establishing a method to supply 2, and the details are presented in this paper ( Figure 2).

■ RESULTS AND DISCUSSION
Tanzawaic acids are structurally composed of a pentadienoic acid moiety and a multi-substituted octalin skeleton bearing six stereogenic centers. We had previously reported the total synthesis of coprophilin 5 that contains a similar skeleton with tanzawaic acids. So, we planned to start this study with a strategy for the utilization of the chiral skeleton.
Tanzawaic acid B would be obtained from the corresponding ester 5, following the Horner−Wadsworth−Emmons (HWE) reaction of aldehyde 6. The aldehyde could be prepared from xanthate 8 through the Barton−McCombie deoxygenation and several functional transformations. We assumed that the radical reduction reaction should be conducted before elongation to prevent from geometric isomerization. The xanthate would be given by the protection/deprotection sequence of two hydroxyl groups from the key intermediate 9. Octalin 9 could be obtained from 2,4-hexadien-1-ol, 11, ent-11, and 12 by reference to our previous report. 5,6 As discussed above, we had already prepared the chiral octalin in less than gram-scale. However, the gram-scale synthesis of key intermediate 9 was needed for the synthetic study of tanzawaic acid, which is a reason for considering a number of steps after the construction of the octalin unit. Therefore, we began to start the gram-scale synthesis of octalin 9, which bears the seven stereogenic centers, and improve our synthetic route at first.
At the beginning of the synthesis, we improved the synthetic conditions to Weinreb amide 19 (Scheme 1). Commercially available 2,4-hexadien-1-ol was brominated to dienyl bromide 13 with simple purification, 7 and it was used for the Evans alkylation reaction with chiral auxiliary 11 to afford alkylation product 14. After the reductive removal of the chiral auxiliary of 14 by lithium aluminum hydride, alcohol 15 was subjected to the conditions of the Parikh−Doering oxidation to give the crude solution of the chiral and α-epimerizable aldehyde 16. The reaction mixture of 16 was directly sent by a cannula into the other enolate system, which was prepared from ent-11 with dibutylboron trifluoromethanesulfonate and triethylamine, to give all-syn-adduct 17 in over decagram-scale. This twosequence reaction system was necessary for the Mukaiyama boron-mediated aldol reaction to afford all-syn-adduct 17. In the case of using isolated aldehyde for the aldol reaction, the epimerized aldol product 4′-epi-17 was detected, which is not non-Evans-syn adduct 2′-epi-3′-epi-17. 8 All-syn-adduct 17 was led to Weinreb amide 18, and then its free hydroxyl group was protected by the tert-butyldimethylsilyl (TBS) group. The reductive removal of the Weinreb amide group was delivered by diisobutylaluminum hydride (DIBAL) to afford aldehyde 20.
Since we could obtain aldehyde 20 with enough amounts, we conducted the gram-scale synthesis of enantiomeric octalin 9 with more improvement than the small amount synthesis in the previous report. 5 Aldehyde 20 was subjected to the Wittig reaction conditions or HWE reaction conditions (Scheme 2 and Table 1), but the thermal intramolecular Diels−Alder (thermal IMDA) reaction proceeded in a gram-scale with a low trans-octalin 21 selectivity (entry 1). Therefore, we had to investigate more effective conditions from 20 to 9. 9 To suppress the thermal IMDA reaction, we performed the corresponding HWE reaction, which is a more kinetic Wittig reaction and can be realized at a lower temperature than the Wittig reaction. As a result, the yield of 10 was increased and the yields of IMDA adducts were suppressed, but the geometric isomer of 10 was also given in an 18% yield (entry 2). Thus, we investigated the solvent effect on the Wittig/ thermal IMDA reactions, but no solvent effect against selectivity in the Diels−Alder reaction step was observed (entries 3−5 and 7). Hence, we analyzed that this thermal IMDA reaction was dependent only on the reaction temperature. As a result, 35°C was the highest temperature at which we could get the Wittig product efficiently without proceeding with the thermal IMDA reaction (entry 6). The temperature could also be explained using the Arrhenius equation by calculating the activation energy from the yield of the Wittig product.
According to the above investigation, we compared the yields of 9 from 20 over four steps in the Wittig conditions between Table 1 entries 1 and 6 (Scheme 3). In the case that condition A at high temperatures in toluene was employed for the Wittig reaction, octalin 9 was given in 28% yields over four steps and the undesired cis-fused octalin 24 was given in 13% yields over four steps. On the other hand, in the case that condition B at moderate temperatures in dichloromethane was employed, we could get octalin 9 in 42% yield over four steps and the undesired cis-fused octalin 24 was given in 6% over four steps, which shows a high selectivity of trans-fused octalin. Therefore, we determined that condition B was the best condition for the large-scale synthesis of trans-octalin 9, which gives the most amount of trans-octalin 9 without proceeding with the thermal IMDA reaction in the former Wittig reaction. The enantiomeric purity of the octalin skeleton was confirmed to be >99.99% ee at the later step, and it is demonstrated that gram-scale synthesis conditions do not affect the enantiomeric purity of the octalin skeleton.
After the establishment of the gram-scale synthesis of transoctalin 9, we pursued the total synthesis of tanzawaic acid B (Scheme 4). The acylation of the hydroxyl group following  11 After reductive deprotection of the benzoyl group, alcohol 27 was oxidized to 6 and aldehyde 6 was immediately used for the next HWE reaction to afford pentadienoic ester 5.
The hydrolysis of methyl ester 5 to accomplish the total synthesis of tanzawaic acid B was carried out.
Since we could obtain the synthetic sample of tanzawaic acid B, we compared the 1 H and 13 C NMR spectrum of the synthetic sample with the ones of the natural tanzawaic acid B (see details in the Supporting Information). The 1 H and 13 C NMR spectrum of the synthetic sample was in accordance to the ones of the natural tanzawaic acid B. 3n,12 In addition, the optical rotation of synthetic 2 matches the one of naturally occurring tanzawaic acid B. Therefore, we demonstrated that the absolute In conclusion, the improvement in the gram-scale synthesis of trans-octalin 9 facilitates us to achieve the first total synthesis of (+)-tanzawaic acid B (2). The comparison of the 1 H NMR spectrum, the 13 C NMR spectrum, and the optical rotation between the synthetic tanzawaic acid B and the reported naturally occurring tanzawaic acid B demonstrated that the chemical structure and stereochemistry of our synthetic tanzawaic acid B match the naturally occurring tanzawaic acid B. The further improvement of synthesis of 2 and investigation of the biological activity of tanzawaic acid B and the synthetic analogues are currently underway.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c03634. 1 H NMR spectra, 13 C NMR spectra, and the other physical properties of all new compounds and the improvement of the synthetic procedure and the comparison of the NMR spectra between the synthetic sample and natural tanzawaic acid B (PDF) ■ REFERENCES (7) Since dienyl bromide 13 was unstable under both acidic aqueous and basic aqueous conditions, it needs to purify the crude product of 13 carefully. In addition, it needs to use as a crude product for the next reaction after the simple purification. See the detailed procedure in the Supporting Information.
(8) We considered that the epimerized aldol product 4′-epi-17 was produced from epimerized aldehyde ent-16, which has an epimerizable carbonyl α-position. In addition, in the case of employing the smaller amount of the sulfur trioxide pyridine complex, triethylamine, and dimethyl sulfoxide, the starting material of alcohol was remained, and the two-step yield was decreased.
(9) The selectivity between 21 and 22 in the thermal intramolecular Diels−Alder reaction was lower than the Lewis acid-assisted intramolecular Diels−Alder reaction. We had already reported each experimental data in the previous report. (12) Although there are several reports for the isolation of tanzawaic acid B, the report including the assignment of the carbon number was rare. From our HMQC spectrum analysis of tanzawaic acid B, the group that isolated tanzawaic acid B misassigned the hydrogen and carbon numbers. See the details in the Supporting Information.