Synthesis of Deoxyradicinin, an Inhibitor of Xylella fastidiosa and Liberibacter crescens, a Culturable Surrogate for Candidatus Liberibacter asiaticus

Abstract

Pierce's disease of grapevine and citrus huanglongbing are caused by the bacterial pathogens Xylella fastidiosa and Candidatus Liberibacter asiaticus (CLas), respectively. Both pathogens reside within the plant vascular system, occluding water and nutrient transport, leading to a decrease in productivity and fruit marketability and ultimately death of their hosts. Field observations of apparently healthy plants in disease-affected vineyards and groves led to the hypothesis that natural products from endophytes may inhibit these bacterial pathogens. Previously, we showed that the natural product radicinin from Cochliobolus sp. inhibits X. fastidiosa. Herein we describe a chemical synthesis of deoxyradicinin and establish it as an inhibitor of both X. fastidiosa and Liberibacter crescens, a culturable surrogate for CLas. The key to this three-step route is a zinc-mediated enolate C-acylation, which allows for direct introduction of the propenyl side chain without extraneous redox manipulations. P ierce's disease of grapevine and citrus huanglongbing (also known as citrus greening) are devastating plant diseases threatening important US agricultural industries. Both are caused by Gram-negative bacterial pathogens, spread by way of insect vectors. Pierce's disease, known in California since the 1800s, is caused by the bacterium Xylella fastidiosa, which inhabits the xylem of the plant. 1−3 As bacteria levels increase, the xylem becomes blocked and the flow of water and nutrients from the roots to the leaves is impeded, resulting in discolored and dried leaves, shriveled fruit, and stunted vine growth, ultimately leading to vine death. 4−6 X. fastidiosa is transmitted by xylem-feeding insects commonly known as leafhoppers. Native leafhoppers are limited in their flight range and transmission efficiency, and until recently, Pierce's disease outbreaks remained mostly isolated to small areas at a time. However, in 1989, an especially effective insect vector, the glassy-winged sharpshooter, was introduced to Southern California. The glassy-winged sharpshooter has infested most of Southern California, including the Malibu, Temecula Valley, and San Diego wine regions, and has made its way north into the Central Valley, posing an imminent threat to the California wine, raisin, and table grape industries.Huanglongbing is caused by the unculturable bacterium Candidatus Liberibacter asiaticus (CLas). 7−9 Unlike X. fastidiosa, CLas colonizes the phloem (food transport tissue) of citrus trees. Huanglongbing-infected trees experience root loss, dieback, starch buildup, and mottled yellow leaves. 10,11 Affected fruit are often green, stunted, and bitter and may drop prematurely. 12,13 Huanglongbing can drastically impact the yield productivity of a tree and can ultimately prove fatal. CLas

Gene-Directed Generation of Unprecedented Bioactive Compounds

“…Furthermore, a set of proton resonances at δ H 1.83 (3H, dd, J = 6.8, 1.2 Hz), 6.49 (1H, dq, J = 14.0, 6.8 Hz), and 6.04 (1H, dq, J = 14.0, 1.2 Hz) highlighted the presence of a ( E )-4-hydroxy-6-(prop-1-en-1-yl)-2 H -pyran-2-one moiety (Table S4 and Figures S29–S34). The singlet signified at 6.06 (1H, s) and the HMBC correlations of H-10 with C-5, C-6 and C-4′a indicated the connection of indole vestige with the ( E )-4-hydroxy-6-(prop-1-en-1-yl)-2 H -pyran-2-one moiety (Figure and Scheme ). …”

“… The singlet signified at 6.06 (1H, s) and the HMBC correlations of H-10 with C-5, C-6 and C-4′a indicated the connection of indole vestige with the ( E )-4-hydroxy-6-(prop-1-en-1-yl)-2 H -pyran-2-one moiety (Figure and Scheme ). …”

“…Aochrabine G, a light yellow powder, was conjectured to have a molecular formula of C 26 H 18 N 2 O 3 according to the [M + H] + peak at m / z 407.1392 in its HR-ESI-MS spectrum (407.1390 calcd for C 26 H 19 N 2 O 3 ). In its 1 H NMR spectrum (Tables S4), a set of proton resonances at δ H 1.99 (3H, dd, J = 6.8, 1.2 Hz), 6.74 (1H, dq, J = 14.0, 6.8 Hz), 6.35 (1H, dd, J = 14.0, 1.2 Hz), and 9.82 (1H, s) highlighted the presence of a 5-substituted-( E )-4-hydroxy-6-(prop-1-en-1-yl)-2 H -pyran-2-one moiety . However, in contrast to those of aochrabine F, the 5,7-dihydroindolo­[2,3- b ]­carbazole vestige resonated at δ H 10.11 (2H, brs), 8.81 (1H, s), 8.20 (2H, d, J = 7.6 Hz), 7.45 (2H, d, J = 7.6 Hz), 7.32 (2H, t, J = 7.6 Hz), and 7.17 (2H, t, J = 7.6 Hz) (Figure and Figures S35–S39).…”

“…Aochrabine F, a light yellow powder, was evidenced to have a molecular formula of C 17 S4 and Figures S29−S34). 24 The singlet signified at 6.06 (1H, s) and the HMBC correlations of H-10 with C-5, C-6 and C-4′a indicated the connection of indole vestige with the (E)-4-hydroxy-6-(prop-1-en-1-yl)-2H-pyran-2-one moiety (Figure 3 and Scheme 2). 24 Aochrabine G, a light yellow powder, was conjectured to have a molecular formula of C 26 H 18 N 2 O 3 according to the [M + H] + peak at m/z 407.1392 in its HR-ESI-MS spectrum (407.1390 calcd for C 26 H 19 N 2 O 3 ).…”

“…In its 1 H NMR spectrum (Tables S4), a set of proton resonances at δ H 1.99 (3H, dd, J = 6.8, 1.2 Hz), 6.74 (1H, dq, J = 14.0, 6.8 Hz), 6.35 (1H, dd, J = 14.0, 1.2 Hz), and 9.82 (1H, s) highlighted the presence of a 5substituted-(E)-4-hydroxy-6-(prop-1-en-1-yl)-2H-pyran-2-one moiety. 24 However, in contrast to those of aochrabine F, the Proposed Biosynthetic Pathways of Aochrabines A− E. As a partially reducing polyketide synthase (PR-PKS), ChrA consists of the multiple functional domains including βketoacylsynthase (KS), acyltransferase (AT), dehydratase (DH), C-methyltransferase (MT 0 , inactive), ketoreductase (KR, active or inactive), and acyl carrier protein (ACP). For unknown reasons, the MT domain is silent explaining why no methylated polyketide was characterized from D. eschscholzii 14 and ChrA/ChrB-AO co-transformant; 19 KR can be active or inactive to diversify ChrA-synthesized polyketides.…”