Structural Elucidation of Agrochemicals and Related Derivatives Using Infrared Ion Spectroscopy.

Agrochemicals frequently undergo various chemical and metabolic transformation reactions in the environment that often result in a wide range of derivates that must be comprehensively characterized to understand their toxicity profiles and their persistence and outcome in the environment. In the development phase, this typically involves a major effort in qualitatively identifying the correct chemical isomer(s) of these derivatives from the many isomers that could potentially be formed. Liquid chromatography-mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy are often used in attempts to characterize such environment transformation products. However, challenges in confidently correlating chemical structures to detected compounds in mass spectrometry data and sensitivity/selectivity limitations of NMR frequently lead to bottlenecks in identification. In this study, we use an alternative approach, infrared ion spectroscopy, to demonstrate the identification of hydroxylated derivatives of two plant protection compounds (azoxystrobin and benzovindiflupyr) contained at low levels in tomato and spinach matrices. Infrared ion spectroscopy is an orthogonal tandem mass spectrometry technique that combines the sensitivity and selectivity of mass spectrometry with structural information obtained by infrared spectroscopy. Furthermore, IR spectra can be computationally predicted for candidate molecular structures, enabling the tentative identification of agrochemical derivatives and other unknowns in the environment without using physical reference standards.

Enantioselective isothiourea-catalysed trans-dihydropyridinone synthesis using saccharin-derived ketimines: scope and limitations.

The catalytic enantioselective synthesis of a range of trans-dihydropyridinones from aryl-, heteroaryl- and alkenylacetic acids and saccharin-derived ketimines with good to excellent stereocontrol (15 examples, up to >95 : 5 dr, up to >99 : 1 er) is reported. After extensive optimisation, HyperBTM proved the optimal isothiourea catalyst for this transformation at -78 °C, giving trans-dihydropyridones with generally excellent levels of diastereo- and enantioselectivity.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies.

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.

Enantioselective Synthesis of 3,5,6-Substituted Dihydropyranones and Dihydropyridinones using Isothiourea-Mediated Catalysis.

The scope of dihydropyranone and dihydropyridinone products accessible by isothiourea-catalyzed processes has been expanded and explored through the use of 2-N-tosyliminoacrylates and 2-aroylacrylates in a Michael addition-lactonization/lactamization cascade reaction. Notably, to ensure reproducibility it is essential to use homoanhydrides as ammonium enolate precursors with 2-aroyl acrylates, while carboxylic acids can be used with 2-N-tosyliminoacrylates, delivering a range of 3,5,6-substituted dihydropyranones and dihydropyridinones with high enantioselectivity (typically >90 % ee). The derivatization of the heterocyclic core of a 3,5,6-substituted dihydropyranone through hydrogenation is also reported.

Synthesis of di-, tri-, and tetrasubstituted pyridines from (phenylthio)carboxylic acids and 2-[aryl(tosylimino)methyl]acrylates.

An isothiourea-catalyzed Michael addition-lactamization followed by the sulfide oxidation-elimination/N- to O-sulfonyl transfer sequence for the formation of 2,3,5- and 2,3-substituted pyridine 6-tosylates from (phenylthio)acetic acids and α,ß-unsaturated ketimines is described. Incorporation of the valuable 2-sulfonate group allows derivatization to a range of di-, tri-, and tetrasubstituted pyridines.

Organocatalytic Michael addition-lactonisation of carboxylic acids using α,ß-unsaturated trichloromethyl ketones as α,ß-unsaturated ester equivalents.

Isothiourea HBTM-2.1 catalyses the Michael addition-lactonisation of 2-aryl and 2-alkenylacetic acids and α,ß-unsaturated trichloromethyl ketones. Ring-opening of the resulting dihydropyranones and subsequent alcoholysis of the CCl3 ketone with an excess of methanol gives a range of diesters in high diastereo- and enantioselectivity (up to 95 : 5 dr and >99% ee). Sequential addition of two different nucleophiles to a dihydropyranone gives the corresponding differentially substituted diacid derivative.

Isothiourea-mediated one-pot synthesis of functionalized pyridines.

Acids to bases: The synthesis of 2,4,6-trisubstituted pyridines from (phenylthio)acetic acid and a range of α,ß-unsaturated ketimines is reported. This process proceeds by intermolecular Michael addition/lactamization, thiophenol elimination, and N- to O-sulfonyl migration, giving 2-sulfonate-substituted pyridines which are readily derivatized to generate structural diversity.

Asymmetric synthesis of the fully elaborated pyrrolidinone core of oxazolomycin A.

The asymmetric synthesis of the key pyrrolidinone core, including a highly elaborated exocyclic carbon chain, of the γ-lactam ß-lactone antibiotic oxazolomycin A is described. Principal features include the Birch reduction of an aromatic pyrrole nucleus, a late stage RuO(4) catalyzed pyrrolidine oxidation, and a highly diastereoselective organocerium addition to an aldehyde.

Synthesis of (+)-DGDP and (-)-7-epialexine.

The partial reduction of electron deficient pyrroles is an extremely versatile method that allows us to prepare substituted pyrrolidines and pyrrolizidines with trans-diol stereochemistry on the five membered ring.