The spinosyns, spinosad, spinetoram, and synthetic spinosyn mimics - discovery, exploration, and evolution of a natural product chemistry and the impact of computational tools.

Natural products (NPs) have long been a source of insecticidal crop protection products. Like many macrolide NPs, the spinosyns originated from a soil inhibiting microorganism (Saccharopolyspora spinosa). More than 20 years after initial registration, the spinosyns remain a unique class of NP-based insect control products that presently encompass two insecticidal active ingredients, spinosad, a naturally occurring mixture of spinosyns, and spinetoram, a semi-synthetic spinosyn product. The exploration and exploitation of the spinosyns has, unusually, been tied to an array of computational tools including artificial intelligence (AI)-based quantitative structure activity relationship (QSAR) and most recently computer-aided modeling and design (CAMD). The AI-based QSAR directly lead to the discovery of spinetoram, while the CAMD studies have recently resulted in the discovery and building of a series of synthetic spinosyn mimics. The most recent of these synthetic spinosyn mimics show promise as insecticides targeting lepidopteran insect pests as demonstrated by field studies wherein the efficacy has been shown to be comparable to spinosad and spinetoram. These and a range of other aspects related to the exploration of the spinosyns over the past 30 years are reviewed herein. © 2020 Society of Chemical Industry.

Discovery of highly insecticidal synthetic spinosyn mimics - CAMD enabled de novo design simplifying a complex natural product.

Simplifying complex natural products: Computer modeling-based design leads to highly insecticidal, chemically simpler synthetic mimics of the spinosyn natural products that are active in the field. © 2018 Society of Chemical Industry.

De Novo Design of Potent, Insecticidal Synthetic Mimics of the Spinosyn Macrolide Natural Products.

New insect pest control agents are needed to meet the demands to feed an expanding global population, to address the desire for more environmentally-friendly insecticide tools, and to fill the loss of control options in some crop-pest complexes due to development of insecticide resistance. The spinosyns are a highly effective class of naturally occurring, fermentation derived insecticides, possessing a very favorable environmental profile. Chemically, the spinosyns are composed of a large complex macrolide tetracycle coupled to two sugars. As a means to further exploit this novel class of natural product-based insecticides, molecular modeling studies coupled with bioactivity-directed chemical modifications were used to define a less complex, synthetically accessible replacement for the spinosyn tetracycle. These studies lead to the discovery of highly insecticidal analogs, possessing a simple tri-aryl ring system as a replacement for the complex macrolide tetracycle.

Fifty years of herbicide research: comparing the discovery of trifluralin and halauxifen-methyl.

Fifty years separate the commercialization of the herbicides trifluralin and halauxifen-methyl. Despite the vast degree of technological change that occurred over that time frame, some aspects of their discovery stories are remarkably similar. For example, both herbicides were prepared very early in the iterative discovery process and both were developed from known lead compound structures by hypothesis-driven research efforts without the use of in vitro assays or computer-aided molecular design. However, there are aspects of the halauxifen-methyl and trifluralin discovery stories that are substantially different. For example, the chemical technology required for the cost-effective production of halauxifen-methyl simply did not exist just two decades prior to its commercial launch. By contrast, the chemical technology required for the cost-effective production of trifluralin was reported in the chemical literature more than two decades prior to its commercial launch. In addition, changes in regulatory environment since the early 1960s ensured that their respective discovery to commercial launch stories would also differ in substantial ways. Ultimately, the time and cost required to develop and register halauxifen-methyl demanded a global initial business case while the lower registration hurdles that trifluralin cleared enabled a narrow initial business case mainly focused on the USA. © 2017 Society of Chemical Industry.

Synthesis and Insecticidal Activity of Spinosyns with C9-O-Benzyl Bioisosteres in Place of the 2',3',4'-Tri-O-methyl Rhamnose.

The spinosyns are fermentation-derived natural products active against a wide range of insect pests. They are structurally complex, consisting of two sugars (forosamine and rhamnose) coupled to a macrocyclic tetracycle. Removal of the rhamnose sugar results in a >100-fold reduction in insecticidal activity. C9-O-benzyl analogues of spinosyn D were synthesized to determine if the 2',3',4'-tri-O-methyl rhamnose moiety could be replaced with a simpler, synthetic bioisostere. Insecticidal activity was evaluated against larvae of Spodoptera exigua (beet armyworm) and Helicoverpa zea (corn earworm). Whereas most analogues were far less active than spinosyn D, a few of the C9-O-benzyl analogues, such as 4-CN, 4-Cl, 2-isopropyl, and 3,5-diOMe, were within 3-15 times the activity of spinosyn D for larvae of S. exigua and H. zea. Thus, although not yet quite as effective, synthetic bioisosteres can substitute for the naturally occurring 2',3',4'-tri-O-methyl rhamnose moiety.

Discovery of the butenyl-spinosyn insecticides: novel macrolides from the new bacterial strain Saccharopolyspora pogona.

A new bacterium, Saccharopolyspora pogona (NRRL30141) was discovered which produced a series of very potent insecticidal compounds structurally related to the 'classical' (i.e., C-21-ethyl) spinosyns. A series of fermentations gave sufficient extract to allow the isolation and characterization of a total of 31 new metabolites. The majority of these compounds contained a but-1-enyl group at C-21 of the macrolide in place of the ethyl group in the 'classical' spinosyn series, corresponding to an additional acetate group incorporated during their biosynthesis. Additionally a variety of other new functionality was seen including hydroxylations, several novel forosamine sugar replacements, and a novel 14-membered macrolide ring analog.

Neural network-based QSAR and insecticide discovery: spinetoram.

Improvements in the efficacy and spectrum of the spinosyns, novel fermentation derived insecticide, has long been a goal within Dow AgroSciences. As large and complex fermentation products identifying specific modifications to the spinosyns likely to result in improved activity was a difficult process, since most modifications decreased the activity. A variety of approaches were investigated to identify new synthetic directions for the spinosyn chemistry including several explorations of the quantitative structure activity relationships (QSAR) of spinosyns, which initially were unsuccessful. However, application of artificial neural networks (ANN) to the spinosyn QSAR problem identified new directions for improved activity in the chemistry, which subsequent synthesis and testing confirmed. The ANN-based analogs coupled with other information on substitution effects resulting from spinosyn structure activity relationships lead to the discovery of spinetoram (XDE-175). Launched in late 2007, spinetoram provides both improved efficacy and an expanded spectrum while maintaining the exceptional environmental and toxicological profile already established for the spinosyn chemistry.

Engineering of the spinosyn PKS: directing starter unit incorporation.

The spinosyns are a family of potent and highly selective insect control agents that display a favorable environmental profile. As some regions of the spinosyn molecule are recalcitrant to chemical modification, a targeted genetic approach was carried out to generate new analogues. The polyketide synthase (PKS) loading modules from the avermectin PKS of Streptomyces avermitilis and the erythromcyin PKS of Saccharopolyspora erythraea were each used to replace the spinosyn PKS loading module. Both of the resulting strains containing hybrid PKS pathways produced the anticipated spinosyn analogues. Supplementation of the culture media with a range of exogenous carboxylic acids led to the successful incorporation of these novel elements to yield further novel spinosyn molecules, some of which demonstrated potent and new insecticidal activities. Furthermore, it has been demonstrated that semisynthesis of such novel metabolites can then be used to generate active analogues, demonstrating the effectiveness of utilizing these complementary methods to search the chemical space around this template.

Engineered biosynthesis of novel spinosyns bearing altered deoxyhexose substituents.

Novel spinosyns have been prepared by biotransformation, using a genetically engineered strain of Saccharopolyspora erythraea, in which the beta-D-forosamine moiety in glycosidic linkage to the hydroxy group at C17 is replaced by alpha-L-mycarose.