Stereochemistry of casbene biosynthesis

Collective Total Synthesis of Casbane Diterpenes: One Strategy, Multiple Targets

Of the more than 100 casbane diterpenes known to date, only the eponymous parent hydrocarbon casbene itself has ever been targeted by chemical synthesis. Outlined herein is a conceptually new approach that brings not a single but a variety of casbane derivatives into reach, especially the more highly oxygenated and arguably more relevant members of this family. The key design elements are a catalyst‐controlled intramolecular cyclopropanation with or without subsequent equilibration, chain extension of the resulting stereoisomeric cyclopropane building blocks by chemoselective hydroboration/cross‐coupling, and the efficient closure of the strained macrobicyclic framework by ring‐closing alkyne metathesis. A hydroxy‐directed catalytic trans‐hydrostannation allows for late‐stage diversity. These virtues are manifested in the concise total syntheses of depressin, yuexiandajisu A, and ent‐pekinenin C. The last compound turned out to be identical to euphorhylonal A, the structure of which had clearly been misassigned.

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

Long-Range Reactivity Modulations in Geranyl Chloride Derivatives

Derivatives of geraniol are versatile synthetic intermediates that are useful for synthesizing a variety of terpenoid natural products; however, the results presented herein show that subtle differences in the structures of functionalized geranyl chlorides can significantly impact their abilities to function as effective electrophiles in synthetic reactions. A series of focused kinetics experiments identify specific structure-activity relationships that illustrate the importance not only of steric bulk, but also of electronic effects from distant regions of the molecules that contribute to their overall levels of reactivity. Computational modeling suggests that destabilization of the reactant by filled-filled orbital mixing events in some, but not all, conformations may be a critical contributor to these important electronic effects.

Engineering low phorbol ester Jatropha curcas seed by intercepting casbene biosynthesis

Casbene is a precursor to phorbol esters and down-regulating casbene synthase effectively reduces phorbol ester biosynthesis. Seed-specific reduction of phorbol ester (PE) helps develop Jatropha seed cake for animal nutrition. Phorbol esters (PEs) are diterpenoids present in some Euphorbiaceae family members like Jatropha curcas L. (Jatropha), a tropical shrub yielding high-quality oil suitable as feedstock for biodiesel and bio jet fuel. Jatropha seed contains up to 40 % of oil and can produce oil together with cake containing high-quality proteins. However, skin-irritating and cancer-promoting PEs make Jatropha cake meal unsuitable for animal nutrition and also raise some safety and environmental concerns on its planting and processing. Two casbene synthase gene (JcCASA163 and JcCASD168) homologues were cloned from Jatropha genome and both genes were highly expressed during seed development. In vitro functional analysis proved casbene synthase activity of JcCASA163 in converting geranylgeranyl diphosphate into casbene which has been speculated to be the precursor to PEs. A seed-specific promoter driving inverted repeats for RNAi interference targeting at either JcCASA163 or both genes could effectively down-regulate casbene synthase gene expression with concurrent marked reduction of PE level (by as much as 85 %) in seeds with no pleiotropic effects observed. Such engineered low PE in seed was heritable and co-segregated with the transgene. Our work implicated casbene synthase in Jatropha PE biosynthesis and provided evidence for casbene being the precursor for PEs. The success in reducing seed PE content through down-regulation of casbene synthase demonstrates the feasibility of intercepting PE biosynthesis in Jatropha seed to help address safety concerns on Jatropha plantation and seed processing and facilitate use of its seed protein for animal nutrition.

Stereochemistry of the Macrocyclization and Elimination Steps in Taxadiene Biosynthesis through Deuterium Labeling

Taxadiene synthase catalyzes the cyclization of (E,E,E)-geranylgeranyl diphosphate (GGPP) to taxa-4(5),11(12)-diene (Scheme 1, 5 --> 2) as the first committed step of Taxol biosynthesis. Deuterated GGPPs labeled stereospecifically at C-1, C-4, and C-16 were synthesized and incubated with recombinant taxadiene synthase from Taxus brevifolia to elucidate the stereochemistry of the cyclization reaction at these positions. The deuterium-labeled taxadienes obtained from (R)-[1-(2H1)]-, (S)-[1-(2H1)]-, and [16,16,16-(2H3)]GGPPs (9, 10, and 23b) were established to have deuterium in the 2alpha and 2beta CH2 and 16CH3 positions, respectively, by high-field 1H NMR spectroscopy (eqs 1-3). Incubation of (R)-[4-(2H1)]GGPP (17) with the recombinant enzyme gave a 10:10:80 mixture of [5beta-(2H1)]taxa-3(4),11(12)-diene, [5beta-(2H1)]taxa-4(20),11(12)-diene, and unlabeled taxa-4(5),11(12)-diene according to GC/MS analyses of the products (eq 4). It follows that C-1 of GGPP underwent inversion of configuration, that the A ring cyclization occurs on the si face of C15, and that the terminating proton abstraction removes H5beta from the final taxenyl carbocation intermediate. Thus, the C1-C14 and C15-C10 bonds are formed on the opposite faces of the 14,15 double bond of the substrate, i.e., overall anti electrophilic addition. The implications of these findings for the mechanism of the cyclization and rearrangement are discussed.

Mono and diterpene production inEscherichia coli

Mono- and diterpenoids are of great industrial and medical value as specialty chemicals and pharmaceuticals. Production of these compounds in microbial hosts, such as Escherichia coli, can be limited by intracellular levels of the polyprenyl diphosphate precursors, geranyl diphosphate (GPP), and geranylgeranyl diphosphate (GGPP). To alleviate this limitation, we constructed synthetic operons that express three key enzymes for biosynthesis of these precursors: (1). DXS,1-deoxy-d-xylulose-5-phosphate synthase; (2). IPPHp, IPP isomerase from Haematococcus pluvialis; and (3). one of two variants of IspA, FPP synthase that produces either GPP or GGPP. The reporter plasmids pAC-LYC and pACYC-IB, which encode enzymes that convert either FPP or GGPP, respectively, to the pigment lycopene, were used to demonstrate that at full induction, the operon encoding the wild-type FPP synthase and mutant GGPP synthase produced similar levels of lycopene. To synthesize di- or monoterpenes in E. coli using the GGPP and GPP encoding operons either a diterpene cyclase [casbene cyclase (Ricinus communis L) and ent-kaurene cyclase (Phaeosphaeria sp. L487)] or a monoterpene cyclase [3-carene cyclase (Picea abies)] was coexpressed with their respective precursor production operon. Analysis of culture extracts or headspace by gas chromatography-mass spectrometry confirmed the in vivo production of the diterpenes casbene, kaur-15-ene, and kaur-16-ene and the monoterpenes alpha-pinene, myrcene, sabinene, 3-carene, alpha-terpinene, limonene, beta-phellandrene, alpha-terpinene, and terpinolene. Construction and functional expression of GGPP and GPP operons provides an in vivo precursor platform host for the future engineering of di- and monoterpene cyclases and the overproduction of terpenes in bacteria.

Cationic cyclopropanation by antibody catalysis

Reactions involving highly reactive carbocations play a central role in many important chemical processes, such as cyclization reactions. However, the potential for controlling the pathways of such reactions to obtain energetically disfavoured (but desirable) products has been hard to realize because of the difficulties inherent in controlling the conformation and chemical environment of the carbocation intermediates. Antibody catalysts, with their high specificity and binding energies, can provide the degree of conformational and chemical control necessary for directing such reactions. Here we show how antibody catalysis can guide cationic cyclization reactions selectively to form products (in high yield) that would otherwise be highly disfavoured. Most notable is the formation of a strained bicyclic compound containing a rare cyclopropane group. To explain our results, we propose a common reaction scheme in which the key step is the formation of a highly reactive protonated cyclopropane intermediate; subtle structural modifications to the substrate (the compound on which the catalytic antibody acts) lead to dramatic differences in the structure of the final product.