Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm-CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

Synthesis of trifold-labeled versatile reagent [3,5-13 C2 ,4-15 N]4-phenyl-1,2,4-triazoline-3,5-dione

Triazolinediones are an important class of derivatization agents that have found application in various research disciplines. Their unique reactivity often allows precise and selective tagging of relevant molecular scaffolds to facilitate structural elucidation, tracking in biological systems, and stabilization of labile compounds. Recent research efforts mainly focused on the development of novel fluorescent and ionizable or isotopically labeled tags improving the quantification and identification of the parent molecule by suitable analytical methods. However, these concepts often lack the ability to improve properties facilitating the analysis by nuclear magnetic resonance (NMR) spectroscopy. We herein describe the first synthesis of 13 C and 15 N labeled [3,5-13 C2 ,4-15  N]4-phenyl-1,2,4-triazoline-3,5-dione utilizing the Cookson/Zinner-Deucker synthesis of urazoles. The introduced isotopic labels are ideally suited to support the structural elucidation of unknown and complex derivatization mixtures by NMR, thereby exploiting the increased sensitivity of detecting long-range JHC and additional JCC and JCN couplings within the derivatized compounds of interest.

Development of a Scalable and Sublimation-Free Route to MTAD

The cyclic azodicarbonyl 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) is a versatile and powerful reagent used mainly in cycloaddition chemistry. Though known for more than 50 years, its unsafe preparation, as well as purification by sublimation, hampered its widespread applicability on a larger scale. Herein we report a scalable and safe route to MTAD, which avoids the generation of methyl isocyanate. Moreover, we demonstrate that sublimation can be circumvented by the application of judicious oxidation conditions, followed by simple filtration. Overall, up to 25 g of MTAD was prepared in a single batch from commercial starting materials in three steps, with recrystallization serving as the only purification in the sequence. When employed in dearomative methodologies, the MTAD obtained by this protocol displayed synthetic efficiency equivalent to that of MTAD purified by sublimation.

Iron-Catalyzed Highly Enantioselective cis-Dihydroxylation of Trisubstituted Alkenes with Aqueous H2 O2

Reliable methods for enantioselective cis-dihydroxylation of trisubstituted alkenes are scarce. The iron(II) complex cis-α-[Fe^II (2-Me₂ -BQPN)(OTf)₂ ], which bears a tetradentate N₄ ligand (Me₂ -BQPN=(R,R)-N,N'-dimethyl-N,N'-bis(2-methylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine), was prepared and characterized. With this complex as the catalyst, a broad range of trisubstituted electron-deficient alkenes were efficiently oxidized to chiral cis-diols in yields of up to 98 % and up to 99.9 % ee when using hydrogen peroxide (H₂ O₂ ) as oxidant under mild conditions. Experimental studies (including ¹⁸ O-labeling, ESI-MS, NMR, EPR, and UV/Vis analyses) and DFT calculations were performed to gain mechanistic insight, which suggested possible involvement of a chiral cis-Fe^V (O)₂ reaction intermediate as an active oxidant. This cis-[Fe^II (chiral N₄ ligand)]²⁺ /H₂ O₂ method could be a viable green alternative/complement to the existing OsO₄ -based methods for asymmetric alkene dihydroxylation reactions.

Palladium-catalyzed three-component tandem cyclization of buta-2,3-dien-1-ol, aryl iodides, and imines: an efficient protocol for the synthesis of oxazolidine derivatives

An efficient three-component tandem cyclization reaction for the synthesis of highly substituted oxazolidines was achieved through the Pd⁰-catalyzed cyclization of buta-2,3-dien-1-ol with aryl iodides and imines.

Stereoselectivity in ene reactions with 1O2: matrix effects in polymer supports, photo-oxygenation of organic salts and asymmetric synthesis

The ene reaction of chiral allylic alcohols is applied as a tool for the investigation of intrapolymer effects by means of the stereoselectivity of the singlet-oxygen addition. The diastereo selectivity strongly depends on the structure of the polymer, the substrate loading degree and also on the degree of conversion demonstrating additional supramolecular effects evolving during the reaction. The efficiency and the stability of polymer-bound sensitizers were evaluated by the ene reaction of singlet oxygen with citronellol. The ene reaction with chiral ammonium salts of tiglic acid was conducted under solution phase conditions or in polystyrene beads under chiral contact ion-pair conditions. The products thus obtained precipitate during the photoreaction as ammonium salts. Moderate asymmetric induction was observed for this procedure for the first time.