Continuous-Flow retro-Diels-Alder Reaction: A Process Window for Designing Heterocyclic Scaffolds

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Synthesis and biological evaluation of the new ring system benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepine and its cycloalkane and cycloalkene condensed analogues

Derivatives of the new ring system benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepinone and its cycloalkane and cycloalkene condensed analogues have been conveniently synthesized through a three-step reaction sequence. An atom-economical, one-pot, three-step cascade process engaging five reactive centers (amide, amine, carbonyl, azide, and alkyne) has been performed for the synthesis of alicyclic derivatives of quinazolinotriazolobenzodiazepine using cyclohexane, cyclohexene, and norbornene β-amino amides. The stereochemistry and relative configurations of the synthesized compounds were determined by 1D and 2D NMR spectroscopy and X-ray crystallography. The reaction was also performed using enantiomeric starting materials leading to enantiomeric quinazolinotriazolobenzodiazepine with an ee of 95%. The synthesis of 9H-benzo[f]pyrimido[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepinone, a new heterocyclic system, was achieved in a good yield using a retro Diels-Alder (RDA) procedure. Some compounds were tested for antiproliferative activities against five human cancer cell lines of gynecological.

Retro Diels Alder protocol for regioselective synthesis of novel [1,2,4]triazolo[4,3-a]pyrimidin-7(1H)-ones

Reactions of diastereochemically varied norbornene-condensed 2-thioxopyrimidin-4-ones 6 and 10 with variously functionalized hydrazonoyl chlorides 2a-h gave regioselectively angular norbornene-based [1,2,4]triazolo[4,3-a]pyrimidin-7(1H)-ones 7a-h and 11a,c-e, respectively. Thermal retro Diels-Alder (RDA) reaction of 7a-h and 11a,c-e resulted in the target compounds 4a-h as single products. On the other hand, reactions of thiouracil 1 and hydrozonoyl chlorides 2a-e gave regioselectively [1,2,4]triazolo[4,3-a]pyrimidinone-5(1H)-ones 3a-e. The opposite regioselectivity of thiouracil 1 and norbornene-condensed 2-thioxopyrimidin-4-ones 6 and 10 was attributed to electronic factors according to DFT calculations. The angular structure of norbornene based [1,2,4]triazolo[4,3-a]pyrimidin-7(1H)-ones was confirmed by single crystal X-ray crystallography.

Synthesis of Novel N-Heterocyclic Compounds Containing 1,2,3-Triazole Ring System via Domino, "Click" and RDA Reactions

An uncomplicated, high-yielding synthetic route has been developed to constitute complicated heterocycles, applying domino, click and retro-Diels–Alder (RDA) reaction sequences. Starting from 2-aminocarboxamides, a new set of isoindolo[2,1-a]quinazolinones was synthesized with domino ring closure. A click reaction was performed to create the 1,2,3-triazole heterocyclic ring, followed by an RDA reaction resulting in dihydropyrimido[2,1-a]isoindole-2,6-diones. The absolute configuration, concluded by the norbornene structure that served as a chiral source, remained constant throughout the transformations. The structure of the synthesized compounds was examined by ¹H and ¹³C Nuclear Magnetic Resonance (NMR) methods.

Continuous flow processing as a tool for the generation of terpene-derived monomer libraries

A Diels–Alder reaction employing terpenes for rapid synthesis of monomer libraries under flow conditions is presented.

Flow-chemistry enabled efficient synthesis of β-peptides: backbone topology vs. helix formation

Enantiodiscriminative helix formation was observed for β-peptide H14 helices when enantiomers of bridged bicyclic residues were introduced.