Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants.

The glucosinolates (GSLs) is a well-defined group of plant metabolites characterized by having an S-ß-d-glucopyrano unit anomerically connected to an O-sulfated (Z)-thiohydroximate function. After enzymatic hydrolysis, the sulfated aglucone can undergo rearrangement to an isothiocyanate, or form a nitrile or other products. The number of GSLs known from plants, satisfactorily characterized by modern spectroscopic methods (NMR and MS) by mid-2018, is 88. In addition, a group of partially characterized structures with highly variable evidence counts for approximately a further 49. This means that the total number of characterized GSLs from plants is somewhere between 88 and 137. The diversity of GSLs in plants is critically reviewed here, resulting in significant discrepancies with previous reviews. In general, the well-characterized GSLs show resemblance to C-skeletons of the amino acids Ala, Val, Leu, Trp, Ile, Phe/Tyr and Met, or to homologs of Ile, Phe/Tyr or Met. Insufficiently characterized, still hypothetic GSLs include straight-chain alkyl GSLs and chain-elongated GSLs derived from Leu. Additional reports (since 2011) of insufficiently characterized GSLs are reviewed. Usually the crucial missing information is correctly interpreted NMR, which is the most effective tool for GSL identification. Hence, modern use of NMR for GSL identification is also reviewed and exemplified. Apart from isolation, GSLs may be obtained by organic synthesis, allowing isotopically labeled GSLs and any kind of side chain. Enzymatic turnover of GSLs in plants depends on a considerable number of enzymes and other protein factors and furthermore depends on GSL structure. Identification of GSLs must be presented transparently and live up to standard requirements in natural product chemistry. Unfortunately, many recent reports fail in these respects, including reports based on chromatography hyphenated to MS. In particular, the possibility of isomers and isobaric structures is frequently ignored. Recent reports are re-evaluated and interpreted as evidence of the existence of "isoGSLs", i.e. non-GSL isomers of GSLs in plants. For GSL analysis, also with MS-detection, we stress the importance of using authentic standards.

Synthesis and Biochemical Evaluation of an Artificial, Fluorescent Glucosinolate (GSL).

The synthesis of the first example of a fluorescent glucosinolate (GSL)-BODIPY conjugate based on an azide-containing artificial GSL precursor (GSL-N3 ) is reported. Biochemical evaluation of the artificial GSLs revealed that the compounds are converted to the corresponding isothiocyanates in the presence of myrosinase. Furthermore, myrosinase-catalyzed hydrolysis in the presence of plant specifier proteins yielded the expected alternative products, namely nitriles. The easy assembly of the fluorescent GSL-BODIPY conjugate by click chemistry from GSL-N3 holds potential for application as a fluorescence labeling tool to investigate GSL-associated processes.

ω-Methylsulfanylalkyl Glucosinolates: A General Synthetic Pathway.

A general pathway was devised to synthesize ω-methylsulfanylalkyl glucosinolates, which represent an important class of structurally homogeneous plant secondary metabolites. The required thiofunctionalized hydroximoyl chlorides were obtained from the corresponding α,ω-nitroalkyl methylsulfide precursors, involving as the key-step, a nitronate chlorination strategy. A coupling reaction with 1-thio-beta-d-glucopyranose, followed by O-sulfation of the intermediate thiohydroximate and final deprotection of the sugar moiety afforded the target compounds.

Synthesis of aromatic and indole alpha-glucosinolates.

Aromatic and indole glucosinolates are important members of the glucosinolate family of compounds du to their potential medicinal properties. They are known to exert antioxidant and anti-carcinogenic activity either by the natural products themselves, or their metabolic products including indole-3-carbinol and isothiocyanates. Natural glucosinolates are all ß-glucosinolates; however, α-glucosinolates are also promising compounds for medicinal applications and hence have to be produced synthetically for any bio-activity studies. Here we report on the successful synthesis of a series of α-glucosinolates: α-neoglucobrassicin, α-4-methoxyglucobrassicin, 2,3-dichlorophenyl-α-glucosinolate for the first time. Testing for anti-inflammatory properties of these synthetic GLs, however, did not yield the expected activity.

Synthesis and anti-inflammatory activity of indole glucosinolates.

The nitronate and nitrovinyl methods to synthesize indole glucosinolates (GLs) have been investigated. The results were applied to generally the most prevalent natural indole glucosinolates to synthesize 4-methoxyglucobrassicin (MGB) and neo-glucobrassicin (NGB) in moderate overall yield for the first time. The anti-inflammatory activity of the synthetic indole GLs was determined by inhibition of TNF-α secretion in LPS-stimulated THP-1 cells. The data showed that glucobrassicin (GB) exhibited higher activity than other synthetic indolyl GLs.

Synthesis and anti-inflammatory activity of aromatic glucosinolates.

Aromatic GLs are important members of the glucosinolate family of compounds because of their potential biological activity and medicinal properties. This study has shown success in the high yielding synthesis of some important aromatic GLs as well as the results of testing for anti-inflammatory properties of the synthetic GLs. 3,4-Dimethoxyphenylglucosinolate was found to be the most active anti-inflammatory of the seven glucosinolates assayed.

Unveiling the phytoalexin biosynthetic puzzle in salt cress: unprecedented incorporation of glucobrassicin into wasalexins A and B.

Salt cress (Thellungiella salsuginea also known as T. halophila) is a wild cruciferous extremophile highly resistant to salt, drought, and cold. The recent discovery that salt cress produces the phytoalexins wasalexins A and B, and the phytoanticipins 1-methoxyglucobrassicin and 4-methoxyglucobrassicin in relatively higher amounts than other cruciferous species, prompted investigation of their biosynthetic relationships. Toward this end, perdeuterated 1-methoxybrassinin, l-Trp, glucobrassicin, 1-methoxyindolyl-3-acetaldoxime, brassinin, and methionine, as well as the corresponding natural abundance compounds, were administered to salt cress plants previously irradiated with UV-light (λ(max) 254 nm). Remarkably, administration of hexadeuterated glucobrassicin led to incorporation of several deuterium atoms into wasalexins A and B, 1-methoxyglucobrassicin and 4-methoxyglucobrassicin. This unprecedented discovery suggests that glucobrassicin is a biosynthetic precursor of wasalexins and methoxylated glucosinolates in salt cress.

First total synthesis of 4-methylthio-3-butenyl glucosinolate.

The first total synthesis of 4-methylthio-3-butenyl glucosinolate (MTBG), a natural bioactive compound and a precursor of radish phototropism-regulating substances, was achieved from commercially available 1,4-butanediol. The glucosinolate framework was prepared by coupling of an oximyl chloride derivative and tetraacetyl thioglucose. A methylthio group was introduced to the framework by a Wittig reaction between triphenylphosphonium thiomethylmethylide and an aldehyde intermediate of glucosinolate. The synthetic route should facilitate preparation of various derivatives needed for probe synthesis based on MTBG.

Identification, synthesis, and enzymology of non-natural glucosinolate chemopreventive candidates.

Isothiocyanates (ITCs) are one of the many classes of breakdown products of glucosinolates found in crucifers such as broccoli and are thought to be partially responsible for the reduced risk of degenerative diseases associated with the consumption of vegetables. The production of ITCs such as L-sulforaphane is dependent on the hydrolytic bioactivities of myrosinase, localized both within vegetable tissues and within flora of the human GI tract, and is associated with important cancer chemopreventive activities. We hypothesized that novel isothiocyanates with enhanced chemopreventive properties relative to L-sulforaphane could be identified and that their glucosinolate precursors could be synthesized. From a library of 30 synthetic ITCs, we identified several with bioactivities equal or superior to those of L-sulforaphane. The corresponding non-natural glucosinolate precursors to these novel ITCs were constructed and found to be substrates for myrosinase. By utilizing a novel RP-HPLC assay to monitor myrosinase-dependent hydrolysis reactions, 2,2-diphenylethyl glucosinolate and (biphenyl-2-yl)methyl glucosinolate were shown to exhibit 26.5 and 2.8 %, respectively, of the relative activity of sinigrin and produced their corresponding ITCs in varying yields. These data support the notion that non-natural glucosinolates can act as prodrugs for novel ITCs, with a mechanism of action reliant on their hydrolytic cleavage by myrosinase. Such non-natural glucosinolates may serve as very economical chemopreventive agents for individuals at risk for cancers of and around the GI tract.

The synthesis and enzymic hydrolysis of (E)-2-[2,3-2H2]propenyl glucosinolate: confirmation of the rearrangement of the thiohydroximate moiety.

(E)-2-[2,3-2H2]propenyl glucosinolate was synthesised starting from (E)-[3,4-2H2]but-3-en-1-ol, which was produced by reduction of but-3-yn-1-ol with deuterium gas in the presence of Lindlar's catalyst. The synthesis of (E)-2-[2,3-2H2]propenyl glucosinolate was completed via the nitro intermediate to form the basic desulphoglucosinolate skeleton. The (E)-2-[2,3-2H2]propenyl glucosinolate was fully characterised and deuterium NMR spectroscopy used to examine the rearrangement of the thiohydroximate to the isothiocyanate and thiocyanate.

Synthesis, analysis and rearrangement of novel unnatural glucosinolates.

As part of a structure activity study to examine the interaction of glucosinolates with leaf surfaces, a number of glucosinolates were synthesised bearing novel side chain functionalities. These included 7-carboxyheptyl, heptyl, and naphthyl side chains. For the carboxyheptyl glucosinolate, a novel intramolecular rearrangement reaction was observed during the final deprotection step, which generated an ester attached to the C-3 of glucose. Studies by 1H NMR spectroscopy showed that the hydrophobic side chain associated with one face of the glucose ring and it was proposed that this was the driving force for the rearrangement. Similar hydrophobic interactions were also observed between the heptyl and naphthyl side chains and the glucose.

Thio sugars: biological relevance as potential new therapeutics.

The biological relevance of sulfur containing carbohydrates is gaining substantial attention. Thus the new developments, especially in the synthetic and medicinal chemistry of thio-sugars are critically important for carbohydrate drug design. New studies of biological processes including biosynthetic reactions and enzyme control mechanisms, discovered during the last few years clearly contributed to an understanding of their biological roles. These roles of carbohydrates and thio-sugars in particular through biological processes and diseases are becoming better understood now. These new trends will provide tremendous opportunities for the development of carbohydrates as new potential drugs. The main objective of this article is to address these new promising advances