Key structural features of cis-cinnamic acid as an allelochemical

Structure-Activity Relationship (SAR) Study of trans-Cinnamic Acid and Derivatives on the Parasitic Weed Cuscuta campestris

Cuscuta campestris Yunck. is a parasitic weed responsible for severe yield losses in crops worldwide. The selective control of this weed is scarce due to the difficult application of methods that kill the parasite without negatively affecting the infected crop. trans-Cinnamic acid is secreted by plant roots naturally into the rhizosphere, playing allelopathic roles in plant-plant communities, although its activity in C. campestris has never been investigated. In the search for natural molecules with phytotoxic activity against parasitic weeds, this work hypothesized that trans-cinnamic acid could be active in inhibiting C. campestris growth and that a study of a series of analogs could reveal key structural features for its growth inhibition activity. In the present structure-activity relationship (SAR) study, we determined in vitro the inhibitory activity of trans-cinnamic acid and 24 analogs. The results showed that trans-cinnamic acid's growth inhibition of C. campestris seedlings is enhanced in eight of its derivatives, namely hydrocinnamic acid, 3-phenylpropionaldehyde, trans-cinnamaldehyde, trans-4-(trifluoromethyl)cinnamic acid, trans-3-chlorocinnamic acid, trans-4-chlorocinnamic acid, trans-4-bromocinnamic acid, and methyl trans-cinnamate. Among the derivatives studied, the methyl ester derivative of trans-cinnamic acid was the most active compound. The findings of this SAR study provide knowledge for the design of herbicidal treatments with enhanced activity against parasitic weeds.

Structure-Activity Relationship Study of Xanthoxyline and Related Small Methyl Ketone Herbicides

Xanthoxyline (1), a small natural methyl ketone, was previously reported as a plant growth inhibitor. In this research, related methyl ketones bearing electron-donating and electron-withdrawing groups, together with heteroaromatics, were investigated against seed germination and seedling growth of Chinese amaranth (Amaranthus tricolor L.) and barnyard grass [Echinochloa crus-galli (L.) Beauv]. The structure-activity relationships (SARs) of methyl ketone herbicides were clarified, and which types and positions of substituents were crucially important for activity were also clarified. Indole derivatives, namely, 3-acetylindole (43) and 3-acetyl-7-azaindole (44) were found to be the most active methyl ketones that highly suppressed plant growth at low concentrations. The molecular docking on the 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzyme indicated that carbonyl, aromatic, and azaindole were key interactions of HPPD inhibitors. This finding would be useful for the development of small ketone herbicides.

Functionalization of 1N-Protected Tetrazoles by Deprotonation with the Turbo Grignard Reagent

1N-PMB-protected tetrazole undergoes C-H deprotonation with the turbo Grignard reagent, providing a metalated intermediate with increased stability. This can be used for the reaction with electrophiles such as aldehydes, ketones, Weinreb amides, and iodine. C-H deprotonation with the turbo Grignard reagent is compatible with the PMB-protecting group at the tetrazole, which can be cleaved using oxidative hydrogenolysis and acidic conditions. The method enables the tetrazole functionalization at the fifth position by overcoming the difficulties associated with retro [2 + 3] cycloaddition of the metalated intermediates.

1,3-Diphenyldisiloxane Enables Additive-Free Redox Recycling Reactions and Catalysis with Triphenylphosphine

The recently reported chemoselective reduction of phosphine oxides with 1,3-diphenyldisiloxane (DPDS) has opened up the possibility of additive-free phosphine oxide reductions in catalytic systems. Herein we disclose the use of this new reducing agent as an enabler of phosphorus redox recycling in Wittig, Staudinger, and alcohol substitution reactions. DPDS was successfully utilized in ambient-temperature additive-free redox recycling variants of the Wittig olefination, Appel halogenation, and Staudinger reduction. Triphenylphosphine-promoted catalytic recycling reactions were also facilitated by DPDS. Additive-free triphenylphosphine-promoted catalytic Staudinger reductions could even be performed at ambient temperature due to the rapid nature of phosphinimine reduction, for which we characterized kinetic and thermodynamic parameters. These results demonstrate the utility of DPDS as an excellent reducing agent for the development of phosphorus redox recycling reactions.

Design and chemical synthesis of root gravitropism inhibitors: Bridged analogues of ku-76 have more potent activity

Previously, we found (2Z,4E)-5-phenylpenta-2,4-dienoic acid (ku-76) to be a selective inhibitor of root gravitropic bending of lettuce radicles at 5 μM, with no concomitant growth inhibition, and revealed the structure-activity relationship in this inhibitory activity. The conformation of ku-76 is flexible owing to the open-chain structure of pentan-2,4-dienoic acid with freely rotating single bonds, and the (2Z)-alkene moiety may be isomerized by external factors. To develop more potent inhibitors and obtain insight into the target biomolecules, various analogues of ku-76, fixed through conformation and/or configuration, were synthesized and evaluated. Stereochemical fixation was effective in improving the potency of gravitropic bending inhibition. Finally, we found highly potent conformational and/or configurational analogues (ku-257, ku-294 and ku-308), that did not inhibit root growth. The inhibition of root curvature by these analogues was comparable to that of naptalam.

Understanding the role played by protic ionic liquids (PILs) and the substituent effect for enhancing the generation of Z-cinnamic acid derivatives†

Photoisomerization of a series of substituted E-cinnamic acids in MeCN in their acid forms and as their corresponding protic ionic liquids (PILs) with light of 300 nm is studied. The nature, strength, number, and position effects of substituents on the photochemical behavior of E-cinnamic derivatives are investigated. The photosensitization of the reaction in the presence of Michler's ketone is also studied at 366 nm and it demonstrates that the triplet-excited state is involved in the reaction. As the presence of n-butylamine needed to form the PILs significantly increases the photoproduct yields in all cases, the role of the PILs is also discussed. Thus, understanding of these fundamental aspects has allowed us to establish an excellent and practical synthetic protocol for successfully synthesizing Z-cinnamic acids.

Essential structural features of (2Z,4E)-5-phenylpenta-2,4-dienoic acid for inhibition of root gravitropism

Previously, we found (2Z,4E)-5-phenylpenta-2,4-dienoic acid (ku-76) to be a selective inhibitor of root gravitropic bending of lettuce radicles at 5 μM, with no concomitant growth inhibition. Here, we describe a structure-activity relationship study of ku-76 to determine the essential structural features for the inhibitory activity. A series of ku-76 analogues was synthesized and the key features of ku-76 that are necessary for inhibition of lettuce root gravitropic bending were determined. The (2E,4E)-, (2Z,4Z)- (2E,4Z)- analogues were inactive, and 4,5-saturated and 4,5-alkynyl analogues also did not show inhibitory activity, demonstrating the importance of the (2Z,4E) diene unit. The aromatic ring was also crucial and could not be replaced with an alkyl chain. Derivatives in which the carboxylic acid was replaced with amides, alcohols, or esters were much less potent. These results suggest that the (2Z,4E)-diene, the carboxylic acid moiety, and the aromatic ring are essential for potent inhibitory activity against gravitropic bending.

Herbicidal Activity of Flavokawains and Related trans-Chalcones against Amaranthus tricolor L. and Echinochloa crus-galli (L.) Beauv

Flavokawains have a broad spectrum of biological activities; however, the herbicidal activity of these naturally occurring chalcones has been less investigated. Flavokawains and their analogues were prepared by the Claisen-Schmidt condensation reaction between xanthoxyline (or aromatic ketones) and a variety of aromatic and heteroaromatic aldehydes. These compounds were then evaluated for their inhibitory effect against representative dicot and monocot plants. Among 45 synthetic chalcones, derivatives containing phenoxyacetic acid, 4-(N,N-dimethylamino)phenyl, N-methylpyrrole, or thiophenyl groups inhibited the germination and growth of Chinese amaranth (Amaranthus tricolor L.) with moderate to high degrees compared to commercial butachlor. For barnyardgrass (Echinochloa crus-galli (L.) Beauv.), most of the thiophenyl chalcones interrupted shoot and root emergence. This finding highlighted the importance of functional groups on the herbicidal activity of chalcones. The level of inhibition also depended on the applied concentrations, plant species, and plant organs. (E)-2-(2-(3-Oxo-3-(thiophen-2-yl)prop-1-enyl)phenoxy)acetic acid (14f) was the most active compound among 45 derivatives. This chalcone could be a promising structure for controlling the germination and growth of weeds. The structure-activity relationship results provide useful information about the development of active chalconoids as novel natural product-like herbicides.

cis-Cinnamic acid is a natural plant growth-promoting compound

Agrochemicals provide vast potential to improve plant productivity, because they are easy to implement at low cost while not being restricted by species barriers as compared with breeding strategies. Despite the general interest, only a few compounds with growth-promoting activity have been described so far. Here, we add cis-cinnamic acid (c-CA) to the small portfolio of existing plant growth stimulators. When applied at low micromolar concentrations to Arabidopsis roots, c-CA stimulates both cell division and cell expansion in leaves. Our data support a model explaining the increase in shoot biomass as the consequence of a larger root system, which allows the plant to explore larger areas for resources. The requirement of the cis-configuration for the growth-promoting activity of CA was validated by implementing stable structural analogs of both cis- and trans-CA in this study. In a complementary approach, we used specific light conditions to prevent cis/trans-isomerization of CA during the experiment. In both cases, the cis-form stimulated plant growth, whereas the trans-form was inactive. Based on these data, we conclude that c-CA is an appealing lead compound representing a novel class of growth-promoting agrochemicals. Unraveling the underlying molecular mechanism could lead to the development of innovative strategies for boosting plant biomass.

Divergent Synthetic Access to E- and Z-Stereodefined All-Carbon-Substituted Olefin Scaffolds: Application to Parallel Synthesis of (E)- and (Z)-Tamoxifens

A highly substrate-general synthesis of all-carbon-substituted E- and Z-stereodefined olefins is performed. The method comprises two sets of parallel and stereocomplementary preparations of (E)- and (Z)-α,β-unsaturated esters involving two robust and distinctive reactions: 1) stereocomplementary enol tosylations using readily available TsCl/diamine/(LiCl) base reagents, and 2) stereoretentive Negishi cross-coupling using the catalysts [Pd(dppe)Cl2] (for E) and [Pd(dppb)Cl2] (for Z). The present parallel approach is categorized as both type I (convergent approach: 16 examples, 56-87 % yield) and type II (divergent approach: 18 examples, 70-95 % yield). The obtained (E)- and (Z)-α,β-unsaturated ester scaffolds are successfully transformed into various E- and Z-stereodefined known and novel olefins (8×2 derivatization arrays). As a demonstration, application to the parallel synthesis of both (E)- and (Z)-tamoxifens, a representative motif of all-carbon-substituted olefins, is accomplished in a total of eight steps with an overall yield of 58 % (average 93 %) and 57 % (average 93 %), respectively.

cis-Cinnamic acid selective suppressors distinct from auxin inhibitors

The activity of cis-cinnamic acid (cis-CA), one of the allelochemicals, in plants is very similar to that of indole-3-acetic acid (IAA), a natural auxin, and thus cis-CA has long been believed to be an analog of auxin. We have reported some structure-activity relationships studies by synthesizing over 250 cis-CA derivatives and estimating their inhibitory activities on root growth inhibition in lettuce. In this study, the compounds that showed low- or no-activity on root growth inhibition were recruited as candidates suppressors against cis-CA and/or auxin and tested for their activity. In the presence of cis-CA, lettuce root growth was inhibited; however, the addition of some cis-CA derivatives restored control-level root growth. Four compounds, (Z)-3-(4-isopropylphenyl)acrylic acid, (Z)-3-(3-butoxyphenyl)acrylic acid, (Z)-3-[3-(pentyloxy)phenyl]acrylic acid, and (Z)-3-(naphthalen-1-yl)acrylic acid were selected as candidates for a cis-CA selective suppressor they allowed the recovery of root growth from inhibition by cis-CA treatment without any effects on the IAA-induced effect or elongating activity by themselves. Three candidates significantly ameliorated the root shortening by the potent inhibitor derived from cis-CA. In brief, we have found some cis-CA selective suppressors which have never been reported from inactive cis-CA derivatives for root growth inhibition. cis-CA selective suppressors will play an important role in elucidating the mechanism of plant growth regulation.

Substituent effects of cis-cinnamic acid analogues as plant growh inhibitors

1-O-cis-Cinnamoyl-β-D-glucopyranose is one of the most potent allelochemicals that has been isolated from Spiraea thunbergii Sieb by Hiradate et al. It derives its strong inhibitory activity from cis-cinnamic acid (cis-CA), which is crucial for phytotoxicity. By preparing and assaying a series of cis-CA analogues, it was previously found that the key features of cis-CA for lettuce root growth inhibition are a phenyl ring, cis-configuration of the alkene moiety, and carboxylic acid. On the basis of a structure-activity relationship study, the substituent effects on the aromatic ring of cis-CA were examined by systematic synthesis and the lettuce root growth inhibition assay of a series of cis-CA analogues having substituents on the aromatic ring. While ortho- and para-substituted analogues exhibited low potency in most cases, meta-substitution was not critical for potency, and analogues having a hydrophobic and sterically small substituent were more likely to be potent. Finally, several cis-CA analogues were found to be more potent root growth inhibitors than cis-CA.

Design and synthesis of conformationally constrained analogues of cis-cinnamic acid and evaluation of their plant growth inhibitory activity

1-O-cis-Cinnamoyl-β-D-glucopyranose is known to be one of the most potent allelochemical candidates and was isolated from Spiraea thunbergii Sieb by Hiradate et al. (2004), who suggested that it derived its strong inhibitory activity from cis-cinnamic acid, which is crucial for phytotoxicity. In this study, key structural features and substituent effects of cis-cinnamic acid (cis-CA) on lettuce root growth inhibition was investigated. These structure-activity relationship studies indicated the importance of the spatial relationship of the aromatic ring and carboxylic acid moieties. In this context, conformationally constrained cis-CA analogues, in which the aromatic ring and cis-olefin were connected by a carbon bridge, were designed, synthesized, and evaluated as plant growth inhibitors. The results of the present study demonstrated that the inhibitory activities of the five-membered and six-membered bridged compounds were enhanced, up to 0.27 μM, and were ten times higher than cis-CA, while the potency of the other compounds was reduced.