Natural products syntheses and chiral recognition methods.

Abstract This PhD thesis integrates three complementary research areas: the synthesis of bioactive metabolites of different origins and endowed with diverse biological properties; the assignment of absolute configuration using methods based on chiroptical spectroscopies; the development and implementation of chemical synthesis processes from the laboratory to the industrial scale. Chapter 1 The first chapter discusses the limitations of synthetic pesticides and herbicides, highlighting the associated risks to human health and the environment, and emphasizes the potential of bioherbicides as sustainable alternatives. In particular, fungal phytotoxins stand out due to their high specificity of action and reduced environmental impact, making them promising candidates for the control of resistant pests. Nevertheless, their application is limited by scarce natural availability and difficulties in large-scale production, which makes the development of sustainable and efficient synthetic strategies mandatory. Two phytotoxins of interest are presented: the ascaulitoxin aglycone (AscA), a bioactive metabolite produced by the fungus Ascochyta caulina for the biocontrol of the weed Chenopodium album, and 3-deoxyradicinin, the biosynthetic precursor of radicinin, first identified in the fungus Stemphylium radicinum. Synthetic racemic 3-deoxyradicinin exhibited phytotoxic activity comparable to that of natural, optically active radicinin, thus representing a valid alternative for the biocontrol of buffelgrass. The specificity of action and the favorable ecotoxicological profile confirm the potential of these metabolites as promising candidates for low-environmental-impact herbicides. Chapter 2 The second chapter is subdivided into three sections, each focusing on the total synthesis of a natural compound (Figure I). The first section describes the synthesis of the fragment (2R,3R)-AHMHA of the marine cyclic octapeptide perthamide C. AHMHA is an asymmetric 3-amino-2-hydroxy acid whose synthesis relies on Sharpless asymmetric syn-dihydroxylation, followed by a Mitsunobu azidation reaction for the enantioselective introduction of the two stereocenters of the target molecule. The developed approach provides a general protocol for the synthesis of chiral 3-amino-2-hydroxy acids and is also applicable to the synthesis of more complex natural metabolites. The second section presents two synthetic strategies for the synthesis of the anti and syn stereoisomers of AscA. To date, no synthesis of AscA has been reported in the literature; moreover, its absolute configuration remains unknown, while its relative configuration is still uncertain. Both strategies originate from the same olefinic intermediate, which is readily prepared from N-protected l-allylglycine via a self-metathesis reaction of its amino esters. The syntheses were designed assuming an l configuration for the two 𝛼-amino acid stereocenters at C2 and C7, thereby reducing the number of possible stereoisomers from 16 to 4. The presence of a 4-amino-5-hydroxy functionality at C4 and C5 of AscA justified the application of the synthetic strategy previously developed for AHMHA to the anti stereoisomers of this metabolite. Conversely, the syn stereoisomers of AscA were approached through Sharpless syn-aminohydroxylation of the olefinic precursor. Furthermore, all immediate precursors of AscA were evaluated through biological assays. The third section concerns the racemic synthesis of 3-deoxyradicinin. The syntheses reported in the literature feature several limitations, such as the use of toxic reactants and solvents, the high number of synthetic steps, the overall low yields and the unfeasible reproducibility on large scale. The synthetic methodology presented in this PhD thesis allowed to afford 3-deoxyradicinin in six steps, four of which were conducted under neat conditions, while reducing the extraction and purification operations to a minimum throughout the overall synthetic sequence. Furthermore, 3-deoxyradicinin and a short-chain analogue were tested to evaluate their antifungal activity at different concentrations against four fungal species of agronomical relevance. Chapter 3 This chapter describes a novel synthetic methodology toward (-)-illisimonin A (Figure II), a sesquiterpenoid isolated from the fruits of Illicium simonsii, an evergreen shrub used in traditional Chinese medicine. This compound features seven contiguous stereocenters and a highly strained, unprecedented illisimonane framework. The research introduces an alternative route to the numerous syntheses reported in the literature and is based on a sequence of key transformations, including an intramolecular Pauson-Khand reaction, a photochemical [2+2] cycloaddition with an allene, and a type-II semipinacol rearrangement, enabling access to the carbon skeleton of the natural product. Experimental results demonstrate the feasibility of the proposed approach and lay the foundation for the completion of the total synthesis, including the installation of the remaining oxygenated functionalities and the stereochemical control required to obtain the natural enantiomer. Chapter 4 This chapter describes the development of novel dithienyl chiroptical probes for the assignment of the absolute configuration of chiral amines and carboxylic acids, inspired by biphenyl probes previously developed and extensively validated in the laboratory where the PhD project was carried out. Dithienyl chiroptical probes are characterized by UV-Vis absorptions at longer wavelengths compared to their biphenyl analogues, allowing a bathochromic shift (320-330 nm) of the diagnostic band associated with the axial torsion of the biphenyl-type system, and thus reducing interference from substrate chromophoric groups. The electroactivity and, in some cases, fluorescence of the dithienyl probes could enable the development of complementary and more sensitive detection methods based on electrochemical techniques employing chiral electrodes and circularly polarized luminescence (CPL). To establish general rules for the assignment of absolute configuration in natural compounds – often unknown, as in the case of AscA – the probes were tested on chiral amines and carboxylic acids with known absolute configuration. Chiral primary amines were converted into their corresponding dithienylazepines. The transfer of central to axial chirality in flexible dithienyl systems induces a preferential torsion originating from the amine moiety, which can be detected through the Cotton effect in the electronic circular dichroism (ECD) spectrum at 320–330 nm, whose sign correlates with the absolute configuration of the amine. The same principle was applied to establish general, non-empirical rules for chiral carboxylic acids, which were derivatized into their corresponding dithienylamides (Figure III). Chapter 5 This chapter describes two alternative synthetic strategies developed for the preparation of an active pharmaceutical ingredient (API) used in the treatment of hypercholesterolemia, a compound of significant therapeutic and industrial relevance. The internship period focused on the study and implementation of reaction scale-up, with the aim of overcoming laboratory-scale limitations and orienting the synthetic routes toward potential large-scale production, suitable for applicative and biological studies (Figure IV). The project integrates an academic approach with the requirements of process chemistry, including scalability assessment, cost and safety optimization, reduction of synthetic steps, solvent selection, and the adoption of innovative technologies.