Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Total Synthesis of an Isatis indigotica-Derived Alkaloid Using a Biomimetic Thio-Diels−Alder Reaction Emma K. Davison, Paul A. Hume, and Jonathan Sperry* School of Chemical Sciences, University of Auckland, 23 Symonds Street, Auckland, New Zealand S Supporting Information *
ABSTRACT: A biomimetic thio-Diels−Alder reaction between a dienylthiadiazole and 3-thioisatin leads to the Isatis indigotica-derived alkaloid (1), along with its diastereomer 2. This synthetic study, supported by molecular modeling, establishes the viability of the proposed biosynthesis by thioDiels−Alder cycloaddition, a very rare reaction in nature. Moreover, the results described infer that the diastereomer 2 is an as-yet undiscovered natural product present in Isatis indigotica.
T
Scheme 1. Proposed Biosynthesis of 1 and the Putative Diastereomer 2
he herbaceous plant Isatis indigotica is cultivated throughout China for its medicinal properties. The dried roots and leaves, named “Ban Lan Gen” and “Da Qing Ye” respectively, have been used in Chinese traditional medicine for hundreds of years to treat a variety of viral diseases, infections, and fevers.1 Owing to the medicinal properties of these plant preparations, several of the natural products present in Isatis indigotica have been isolated and tested for biological activity. The compounds isolated include lignans,2 flavonoids3 and a remarkable array of alkaloids,4 including the structurally remarkable natural product 1.5 This unnamed alkaloid, isolated as a scalemic mixture (2:1) of 1a and 1b from the roots of Isatis indigotica in 2012, comprises a unique heterocyclic framework containing spirodihydrothiopyran-oxindole and 1,2,4-thiadiazole moieties (Figure 1).6 Interestingly, the diastereomer of the natural product, compound 2, was not isolated in the original report.
Interestingly, the putative biosynthetic intermediate 7 was recently isolated9 from Isatis indigotica and is subsequently shown to comprise four stereoisomers 7a−d in a 2:4:1:2 ratio, individually named as insatindigothiadiazoles A−D. The ratio of 7a−d strongly supports their proposed biosynthetic assembly from the union of 5a−b and 6a−b, and, hence, the 2:1 mixture of epiprogoitrin (3a) and progoitrin (3b). The insatindigothiadiazoles 7a−d are then thought to undergo a selective monodehydration to generate diene 8. The 3-thioisatin 9,
Figure 1. Alkaloid 1 and its diastereomer 2.
A detailed biosynthesis of the alkaloid 1 was proposed in the original report (Scheme 1).5 The putative biosynthetic precursors, the glucosinolates epiprogoitrin (3a) and progoitrin (3b), are present in I. indigotica in a 2:1 ratio.7 Myrosinasecatalyzed hydrolysis of 3a and 3b is known to liberate the thiohydroximate-O-sulfonate 4,7 which in turn is reported to breakdown into nitrile 5 and imidothioate 6.8 Heterocyclization between 5a−b and 6a−b would form the 1,2,4-thiadiazole 7. © XXXX American Chemical Society
Received: April 26, 2018
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DOI: 10.1021/acs.orglett.8b01321 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters proposed to be derived from glucobrassicin (10),4 then undergoes thio-Diels−Alder reaction with diene 8 followed by double bond isomerization to give 1a and 1b. The ratio of the naturally occurring enantiomers 1a and 1b (2:1) strongly suggests that the 2‴ stereocenter is derived from the 2:1 mixture of glucosinolates 3a and 3b, but some uncertainty surrounds the diastereoselectivity of the thio-Diels−Alder cycloaddition. It was proposed in the original report that the stereocenter in diene 8 is too distant from the spirocenter to exert any diastereocontrol in this cycloaddition, and as a result, the diastereomer 2 must also exist in Isatis indigotica (presumably as a 2:1 ratio of 2a:2b). An ongoing interest in thiadiazole natural products10 and biomimetic synthesis11 led us to instigate a synthetic program to test the biosynthetic hypothesis outlined in Scheme 1, focusing on the thio-Diels−Alder reaction. To the best of our knowledge, examples of thio-Diels−Alder reactions in nature are limited to the assembly of vinyldithiins from thioacrolein in Allium sativum.12,13 Our preliminary investigations focused on using a model diene in the biomimetic thio-Diels−Alder cycloaddition as a means to examine the viability of this proposed reaction (Scheme 2).14,15 Commercially available 3-bromo-5-chloro-
Figure 2. DFT computed Gibbs free energies and transition state structures for thio-Diels−Alder reactions of 9 and 13.
of these products were investigated using density functional theory (DFT) calculations. The DFT calculations employed the M06-2X exchange-correlation functional and polarized triple-ζ 6-311+G (d,p) basis set. Solvent effects were taken into account by employing a polarized continuum model. Further details regarding computational methodology can be found in the Supporting Information (SI). The DFT calculations support regioselective formation of 15 via intermediates (±)-16a and (±)-16b due to a free energy difference of ∼6−7 kcal/mol between transition states TS1a/ TS1b and TS2a/TS2b. The difference in energy was attributed to favorable orbital overlap in TS1a/TS1b, including significant secondary orbital interactions between the π-systems of the reactants. In contrast, these interactions are not present in transition states TS2a/TS2b. The frontier orbital energies of 9 (HOMO = −8.0 eV, LUMO = −2.8 eV) and 13 (HOMO = −8.1 eV, LUMO = −1.7 eV) indicate that the reaction proceeds via a normal Diels−Alder cycloaddition, as opposed to an inverse electron-demand pathway.19 However, it should be noted that the difference between the LUMOdienophile− HOMOdiene and LUMOdiene−HOMOdienophile gaps is ∼1 eV, so both interactions are likely to play a role in the reaction.20 This assessment is supported by the fact that the transition state frontier molecular orbitals do not resemble a simple linear combination of the HOMO of 13 and the LUMO of 9. With the model study successful, we turned our attention to the natural product itself. Our strategy was modeled on the proposed biosynthesis outlined in Scheme 1, and as such, we targeted the selective dehydration of the insatindigothiadiazole natural products (7) as a means to access the key diene 8 (Scheme 3). Silylation of known hydroxyamide (±)-1821 gave (±)-19 which was converted to the thioamide (±)-20 using Lawesson’s reagent (LR). Oxidative dimerization of (±)-20 using iodosobenzene diacetate22 followed by desilylation gave an inseperable mixture of insatindigothiadiazoles A−D (7a−d, 1:1:1:1), the NMR spectroscopic data of which was in excellent agreement with that in the isolation report.9 Next, the selective dehydration was attempted; upon treatment of diol mixture 7a−d with 1 equiv of methanesulfonyl chloride and 2 equiv of base, a single product was formed, which was identified as the mesylate (±)-21. As only 1 equiv of mesyl chloride was used in this process, we posit that the diols 7a−d undergo selective mesylation−elimination to give the desired product (±)-8, which itself is mesylated by the liberated methanesulfonic acid23
Scheme 2. Model Thio-Diels−Alder Reaction
1,2,4-thiadiazole (11) underwent C5-selective Suzuki coupling16 with the dienyl boronate ester 1217 to give the somewhat unstable diene 13 as the model substrate for the thio-Diels−Alder cycloaddition.18 The dienophile 9 was also very unstable and was prepared in situ. Thus, 2-oxindole (14) was treated with N-chlorosulfenylsuccinimide (succNSCl)15 and triethylamine to form an intermediate sulfenamide, at which point the diene 13 was added. The dienophile 9 was generated upon addition of pyridine, and the reaction mixture was stirred for 15 h at room temperature. A single product was formed, identified as the spirodihydrothiopyran-oxindole (±)-15 by X-ray crystallography. This model study was considered a success; the cycloaddition had proceeded with the desired regioselectivity and the initial cycloadduct (i.e., 16) underwent isomerization in situ, as would be required for the eventual synthesis of the natural product. In principle, the cycloaddition between 9 and 13 could lead to four compounds that correspond to the endo- and exoadducts of each “ortho” (16a/b) and “meta” regiosiomer (17a/ b), respectively (Figure 2). In order to gain further insight into this key reaction, the thio-Diels−Alder reactions leading to each B
DOI: 10.1021/acs.orglett.8b01321 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3. Synthesis of Diene (±)-8 via Insatindigothiadiazoles (7a−d)
and hence explain why it was not detected in the original report. It is possible that the thio-Diels−Alder cycloaddition of 8 and 9 is under enzymatic control, thus favoring the formation of 1 over 2. The reaction could also follow a stepwise, conjugate addition pathway (perhaps facilitated by a metal ion) that favors the diastereomer 1. To conclude, the biomimetic synthesis of a structurally unprecedented alkaloid isolated from Isatis indigotica is described. A thio-Diels−Alder reaction between insatindigothiadiazole-derived diene (±)-8 and the 3-thioisatin 9 gave the natural product (±)-1, along with its diastereomer (±)-2, in a 1:1 ratio. The regiochemical outcome and lack of diastereocontrol in this cycloaddition is supported by molecular modeling, and as such, it is probable that diastereomer 2 is also present in Isatis indigotica, as a scalemic mixture of 2a and 2b (2:1). Moreover, the realization of the original biosynthesis proposal represents a very rare example of a thio-Diels−Alder reaction occurring in nature.
(from the elimination) to give (±)-21. This side reaction was of little consequence, as the mesylate (±)-21 was readily converted to the desired alcohol (±)-8 upon stirring in aqueous tetrahydrofuran. Distinctive HMBC correlations from both protons of the methylene group to the thiadiazole C3 confirmed the diene component had been successfully installed at the desired C5 position.24 With the diene (±)-8 in hand, attention was focused toward the key biomimetic thio-Diels−Alder reaction (Scheme 4). In
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01321. Full experimental procedures and NMR spectra of all novel compounds; full details of the computational methodology employed (PDF)
Scheme 4. Biomimetic Thio-Diels−Alder Reaction
Accession Codes
CCDC 1825337 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jonathan Sperry: 0000-0001-7288-3939
line with the model study, the diene (±)-8 and the dienophile 9 were stirred together at room temperature. After ∼3 days, a 1:1 mixture of the natural product (±)-1 and its diastereomer (±)-2 was formed; the absence of diastereocontrol in this biomimetic thio-Diels−Alder reaction corroborates the postulate5 that the diastereomer 2 is also likely to be present in Isatis indigotica. This assessment is further supported by DFT modeling of the thio-Diels−Alder reaction between diene 8 and dienophile 9 (SI). In line with the model study (Scheme 2), the regioselectivity of the cycloaddition between 8 and 9 is controlled by stabilizing π−π interactions in the transition state. However, the free energy difference between the transition states leading to 1 and 2 was calculated to be
