Synthetic efforts towards the stereoselective synthesis of NF00659B₁

Abstract

NF00659B₁ is a novel α-pyrone diterpenoid natural product with potent anti-colon cancer activity. A stereoselective approach to the 2,2-dimethyl oxepanol core of NF00659B₁ is described enlisting a sequence of olefinic ester ring-closing metathesis, epoxidation, and Grignard addition. This strategy paves the way to a total synthesis of NF00659B₁ for further biological studies.

TOC

Natural products represent a unique and privileged chemical space,^{1, 2} and they have been extensively used as chemical probes to systematically explore important cellular components, molecular events, and signaling.³ As a consequence, there is continued interest in searching for novel chemotypes suitable for chemical probe development. In this regard, α- and γ-pyrone diterpenoids are an attractive class of natural products since they possess unique structural features and important biological activities such as antitumor and immunosuppressive activity (Figure 1).^4–14

Figure 1.

Examples of naturally occurring α- and γ-pyrone diterpenoids.

Among α- and γ-pyrone diterpenoids, NF00659A₁, A₂, A₃, B₁, and B₂ (5–9, NF00659s, Figure 2) are novel anticancer α-pyrone diterpenoids isolated from Aspergillus sp. NF00659.^15–17 They possess a 2,2-dimethyl oxepanol group that is a unique structural feature among pyrone diterpenoids. NF00659s potently inhibit the growth of SW480 (colon cancer cells, IC₅₀: 4–560 nM) and possess excellent selectivity (90–820 fold) for SW480 over A2780 (ovarian cancer cells, IC₅₀: 1–50 µM).¹⁶ Intrigued by the promising activity and selectivity for colon cancer as well as the scarcity from natural sources of NF00659s, we embarked on a stereoselective synthesis of NF00659B₁ (8). NF00659B₁ (8) is one of the most potent and selective among NF00659s. Herein, we report the stereoselective construction of the 2,2-dimethyl oxepanol core of NF00659B₁ (8).

Figure 2.

The structure of NF00659s and retrosynthesis of NF00659B₁ (8).

Suzuki and co-workers used an extensive 1D- and 2D-NMR spectroscopic data analysis to determine the structure of 8, but they did not establish the absolute and relative stereochemistry of 8.¹⁷ After extensive analyses of NMR spectral data and literature precedents, we tentatively assigned the absolute and relative stereochemistry of 8 as shown in Figure 2. The absolute stereochemistry of C8, C9, C10, and C14 was assigned with confidence as 8R, 9R, 10S, 14R because the naturally occurring α- and γ-pyrone diterpenoids with known stereochemistry possess the identical absolute stereochemistry without any exception.^{5, 11–13, 18–22} Since both the C5(S)- and C5(R)-configurations exist in natural products (e.g., subglutinol A (1) and candelalide A (2), respectively, Figure 1), we analyzed the coupling constants and splitting patterns of naturally occurring α- and γ-pyrone diterpenoids. For example, subglutinol A (1) with 5S shows δ 3.17 (dd, J = 4.0, 11.5 Hz) for axial C5-H. On the other hand, the equatorial C5-H of candelalide A (2) with 5R appears at δ 3.71 (dd, J = 2.0, 4.0 Hz). Their ¹H NMR spectral data correlates well with the dihedral angle of axial and equatorial hydrogens. Since the C5-H of NF00659B₁ (8) appears at δ 3.59 (br d, J = 10.8 Hz), we confidently assigned the C5-configuration as (S). Since the 2,2-dimethyl oxepanol subunit is unique to 8, we envisioned that preparation of both C3(R) and C3(S) diastereomers and comparison of their spectral data with the authentic data¹⁷ would establish the C3-configuration. Since there is no other α- or γ-pyrone diterpenoids that possess the acyl side chain as in NF00659B₁ (8) and the C6"-CH₃ group is remote from the core of 8, we expected that the determination of the C6"-configuration would be challenging.

As illustrated in Figure 2, the key strategy underlying our synthetic plan for 8 was to apply the sequence of olefinic ester ring-closing metathesis reaction, DMDO epoxidation, and CH₃MgCl addition in stereoselectively constructing the 2,2-dimethyl oxepanol subunit of 8. Then, the late-stage installation of exo-methylene, α-pyrone, and acyl side chain following the procedures we have successfully applied in the synthesis of structurally related subglutinol A (1)^{18, 23} would complete the synthesis of 8.

As shown in Scheme 1, the synthesis of the 2,2-dimethyl oxepanol subunit of 8 began with the preparation of the substrates 16 and 17 for the key olefinic ester cross-metathesis reaction. Starting from the known alcohol 14,^{18, 23, 24} which is readily available from the enantiomerically pure (S)-(+)-5-methyl Wieland–Miescher ketone (13),²⁵ the Ac protection, hydroboration, Parikh–Doering oxidation, and Wittig reaction successfully provided the olefinic esters 16 and 17. To prepare the cyclic enol ether 11, we attempted the olefinic ester cyclization of 16 using the Tebbe reagent.^{26, 27} We observed the consumption of 16 and formation of the intermediate diene 18, but 18 was not consumed to give the desired cyclic enol ether 11 under refluxing conditions. The ring-closing metathesis of 18 using Hoveyda–Grubbs II catalyst²⁸ was not successful as well. After an extensive search for reaction conditions, we were delighted to find that subjection of 16 or 17 to the olefinic ester ring-closing metathesis reaction conditions reported by Rainier and co-workers^29–40 successfully provided the hydrolysis-sensitive cyclic enol ether 11 in 55–60%.

Scheme 1.

The synthesis of the cyclic enol ether 11 via the olefinic ester ring-closing metathesis.

To install both C3-hydroxy and C4-dimethyl groups, we subjected 11 to the DMDO epoxidation followed by CH₃MgCl addition (Scheme 2).⁴¹ Treatment of 11 with DMDO successfully afforded the intermediate epoxide 19. Then, the hydrolysis-sensitive epoxide 19 was treated with CH₃MgCl without further purification. The Grignard addition successfully afforded the desired 2,2-dimethyl oxepanol 10 as a single diastereomer in 62% overall yield for 2 steps. The configuration of the newly formed C3 stereocenter of 10 was determined to be (S) by a single crystal X-ray diffraction analysis.⁴²

Scheme 2.

The synthesis of the 4,4-dimethyl-3-oxepanol subunit 10 and determination of the C3-configuration.

Since the 2,2-dimethyl oxepanol subunit of 8 is unique among pyrone diterpenoids and there are no literature precedents, we decided to prepare both C3(R) and C3(S) diastereomers to unambiguously establish the C3-configuration. An oxidation/reduction approach provided (3R)-10 for the determination of C3-configuration. Esterification of (3R)-10 and (3S)-10 with the commercially available (2E,4E)-hexa-2,4-dienoic acid (20) provided the diastereomeric esters (3R)-21 and (3S)-21, respectively. Analysis of the chemical shifts and coupling constants of (3R)-21 and (3S)-21 established the C3-configuration as (R).

The completion of the stereoselective synthesis of NF00659B₁ (8) requires the installation of exo-methylene, α-pyrone, and acyl side chain. We envision that the conversion of (3R)-10 to 8 could be achieved following the procedures established by our group for the stereoselective synthesis of subglutinol A (1).^{18, 23} To install the α-pyrone of 8, we could use either the [2,3]-Wittig rearrangement/pyrone addition/deoxygenation route or the Cu(I)-mediated S_N2′/aldol reaction route. These two complementary approaches were successfully applied in the stereoselective synthesis of subglutinol A (1). The alcohol 25 could be coupled with the readily available C6"(R)- and C6"(S)-carboxylic acids⁴³ to complete the synthesis of C6"(R)- and C6"(S)-NF00659B₁ isomers.

In conclusion, we successfully explored the sequence of olefinic ester metathesis, DMDO epoxidation, and CH₃MgCl addition in the stereoselective construction of the 2,2-dimethyl oxepanol subunit of NF00659B₁ (8). Then, the extensive analysis of NMR spectroscopic and literature data tentatively established the configuration of C3 and C5 as (R) and (S). We expect that our synthetic approach will allow access to NF00659B₁ (8) and its analogs for further biological studies.

Figure 3.

The potential end-game strategy for NF00659B₁.^{18, 23}