Syntheses of 7-O-Methylnigrosporolide, Pestalotioprolide D, Nigrosporolide, Mutolide and (4S,7S,13S)-4,7-Dihydroxy-13-tetradeca-2,5,8-trienolide

Abstract

7-O-Methylnigrosporolide (1), pestalotioprolide D (2), nigrosporolide (3), (4S,7S,13S)-4,7-dihydroxy-13-tetradeca-2,5,8-trienolide (4) and mutolide (5) are 14-membered macrolides isolated from various strains of fungi. The core structure of compounds 1 and 3–5 consists of a 14-membered macrolactone with E-olefin at C2–C3 and Z- or E-double bond at C5–C6 and C8–C9 as well as three alcohol stereogenic centers at the 4, 7 and 13 positions. The structure of 2 is nearly identical to 1 but 2 contains one ketone functional group at the C4-position and saturation at C2–C3. Due to the promising biological activities of this class of natural products and unprecedented total syntheses of 1–5, this work involves the syntheses of these natural products to confirm the absolute configurations as well as to further evaluation of biological activities. The key strategy of our synthesis relied on Shiina macrolactonization to assemble 14-membered macrolactone of targets 1–5. The C2–C3 E-olefin moiety of macrolides 1 and 3–5 was constructed via Wittig olefination. The C8–C9 Z- and E-alkenes of 10 and 11 for the synthesis of 1–4 were generated from Lindlar and Red-Al reduction of propargylic alcohol 9, respectively. Propargylic alcohol intermediate 9 was synthesized by addition of the corresponding acetylide of (S)-tert-butyl(hept-6-yn-2-yloxy)dimethylsilane (7) to (Z)-enal 8 to form the C7–C8 bond and install the C7 alcohol stereogenic center. The synthesis of (S)-alkyne 7 was accomplished in 5 steps from (S)-propylene oxide (5), while (Z)-enal 8 was prepared from (S)-benzyl glycidyl ether (6) in 7 steps using Jacobsen hydrolytic kinetic resolution (HKR) to install the stereogenic center at the C4-position and Still–Gennari olefination to generate C5–C6 Z-olefin as the key strategies. The synthesis of mutolide (5) was accomplished via the similar synthetic procedure as that of 4 but the E-double bond at C8–C9 was constructed via cross metathesis between (R)-hept-6-en-2-yl benzoate (13) and chiral allylic alcohol 14. Alkene 13 was synthesized in 2 steps from (R)-propylene oxide (12). Chiral allylic alcohol 14 was prepared in 8 steps via the similar synthetic sequence as (Z)-enal 8 except for the formation of E-alkene at C5–C6, which was achieved via Wittig olefination. Our research group found that the tert-butyldiphenylsilyl (TBDPS) group was a suitable protecting group for the C4 hydroxy group of the macrocyclic intermediates, which allowed for smooth final deprotection. In addition, our synthesis led to the hypothesis that macrolide pestalotioprolide D (2) might be an artifact from a facile 1,2-hydride shift of 7-O-methylnigrosporolide (1). The syntheses of macrolides 1–4 were accomplished in a longest linear sequence of 17 steps and total of 22 steps and 1.7%, 2.6%, 1.8%, 1.1% overall yields, respectively. The synthetic 5 was obtained in a longest linear sequence of 16 steps and a total of 18 steps in 1.5% overall yield. The 1H and 13C NMR spectroscopic data, HRMS data as well as specific rotation of synthetic compounds 1–5 were in excellent agreement with those reported, which confirmed the assigned absolute configurations of the natural products. Cytotoxic activities of synthetic 7-O-methylnigrosporolide (1) and pestalotioprolide D (2) against six human cancer cell lines consisting of two breast adenocarcinoma (MDA-MB-231 and MCF-7), three cervical carcinoma (C33A, HeLa and SiHa) and one colorectal carcinoma (HCT116) cell lines were evaluated. Compound 2 showed more potent cytotoxic activities against these cancer cell lines than 1. Moreover, the SiHa cervical cancer cell line was the most sensitive cell line to synthetic 1 and 2 with IC50 values of 35.17  10.77 M and 8.90  2.51 M, respectively. Synthetic compounds 3–5 were tested for their cytotoxic activities against three cancer cell lines, including HCT116, MCF-7 and Calu-3 lung adenocarcinoma cell lines as well as their inhibitory effect on cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride secretion in human intestinal epithelial (T84) cells. Mutolide (5) showed significant cytotoxic activity against HCT116 colon cancer cells with an IC50 value of ~12 M. Moreover, 5 also exhibited potent CFTR inhibitory effect with an IC50 value of ~1 μM. However, macrolides 3 and 4 displayed no cytotoxic effects on three cancer cell lines and showed inhibitory effects on the CFTR channels with less efficacy compared to 5.