Synthesis of Isoxazolin-5-one Glucosides by a Cascade Reaction

Synthesis of Isoxazol-5(2H)-one Derivatives via (tBuO)2Mg Promoted [3 + 2] Annulations of δ-Acetoxy Allenoates with Hydroxylamine

Herein, an efficient (tBuO)2Mg promoted [3 + 2] annulation of δ-acetoxy allenoates with N-benzylhydroxylamine has been developed. This method provides a concise and facile protocol to synthesize vinylated isoxazol-5(2H)-one derivatives stereospecifically in a broad substrate scope with high efficiency (31 examples, up to 87% yield).

Involvement of CYP347W1 in neurotoxin 3-nitropropionic acid-based chemical defense in mustard leaf beetle Phaedon cochleariae

Chrysomelina beetlesstore 3-nitropropionic acid in form of a pretoxin, isoxazolin-5-one glucoside-conjugated ester, to protect themselves against predators. Here we identified a cytochrome P450 monooxygenase, CYP347W1, to be involved in the production of the 3-nitropropionic acid moiety of the isoxazolin-5-one glucoside ester. Knocking down CYP347W1 led to a significant depletion in the concentration of the isoxazolin-5-one glucoside ester and an increase in the concentration of the isoxazolin-5-one glucoside in the larval hemolymph. Enzyme assays with the heterologously expressed CYP347W1 showed free β-alanine was not the direct substrate. Homology modeling indicated that β-alanine-CoA ester can fit into CYP347W1's active site. Furthermore, we proved that Phaedon cochleariae eggs are not able to de novo synthesize 3-NPA, although both isoxazolin-5-one glucoside and its 3-NPA-conjugated ester are present in the eggs. These results provide direct evidence for the involvement of CYP347W1 in the biosynthesis of a P. cochleariae chemical defense compound.

Highly Diastereo- and Enantioselective Synthesis of Isoxazolone-Spirooxindoles via Squaramide-Catalyzed Cascade Michael/Michael Addition Reactions

A highly enantioselective [3+2] annulation of 3-((2-oxoindolin-3-yl)oxy)acrylates with isoxazol-5(4H)-ones has been accomplished by squaramide-catalyzed cascade Michael/Michael addition reactions under mild conditions. The corresponding isoxazolone-spirooxindoles with four continuous stereocenters were obtained in moderate to high yields with excellent stereoselectivities (up to >20:1 dr, 97% ee). The synthetic practicality of this methodology was illustrated by performing the reaction on a gram scale with the same efficiency and stereoselectivity. Meanwhile, the isoxazol-5(4H)-one ring could be opened by the reaction with iron powder and ammonium chloride, and the corresponding acyl derivative can be obtained with a maintained yield and stereoselectivity.

Asymmetric Synthesis of Isoxazol-5-ones and Isoxazolidin-5-ones

Isoxazol-5-ones and isoxazolidin-5-ones represent two important classes of heterocycles, with several applications as bioactive compounds and as versatile building blocks for further transformations. Unlike the parent aromatic isoxazoles, the presence of one or two stereocenters in the ring renders their asymmetric construction particularly important. In this review, starting from the description of general features and differences between these two related compound families, we present an overview on the most important enantioselective synthesis strategies to access these heterocycles. Both chiral metal catalysts and organocatalysts have recently been successfully employed for this task and some of the most promising approaches will be discussed.

1 Introduction

2 Isoxazol-5-ones as Nucleophiles

2.1 Isoxazol-5-ones as C-Nucleophiles

2.2 Isoxazol-5-ones as N-Nucleophiles

2.3 Isoxazol-5-ones as C-Nucleophiles in Cyclization Processes

3 Asymmetric Construction of Isoxazolidin-5-ones

3.1 Enantioselective α-Functionalizations of Isoxazolidin-5-ones

4 Arylideneisoxazol-5-ones in Conjugated Addition

5 Conclusions

A tale of four kingdoms – isoxazolin-5-one- and 3-nitropropanoic acid-derived natural products

The occurrence, structural diversity, (bio-)synthesis, properties and detoxification mechanisms of isoxazolinone- and 3-nitropropanoic acid-derived natural compounds are reviewed.

Two Defensive Lines in Juvenile Leaf Beetles; Esters of 3-nitropropionic Acid in the Hemolymph and Aposematic Warning

Juveniles of the leaf beetles in subtribe Chrysomelina have efficient defense strategies against predators. When disturbed, they transiently expose volatile deterrents in large droplets from nine pairs of defensive glands on their back. Here, we report on an additional line of defense consisting of the non-volatile isoxazolin-5-one glucoside and its 3-nitropropanoyl ester in the larval hemolymph. Because isoxazolin-5-one derivatives were not detectable in related leaf beetle taxa, they serve as a diagnostic marker for the Chrysomelina subtribe. Conjugation of isotopically labelled 3-nitropropionic acid to isoxazolin-5-one glucoside in vivo demonstrates its function as a carrier for the 3-nitropropanoyl esters. The previous identification of characteristic glucosides as precursors of the volatile deterrents underlines the general importance of glucosides for sequestration from food plants, and the subsequent transport in the hemolymph to the defense system. The combination of repellent volatiles with non-volatile toxic compounds in the hemolymph has the potential to create synergistic effects since the odorant stimulus may help predators learn to avoid some foods. The combination of the two defense lines has the advantage, that the hemolymph toxins provide reliable and durable protection, while the repellents may vary after a host plant change.

Biosynthesis of isoxazolin-5-one and 3-nitropropanoic acid containing glucosides in juvenile Chrysomelina

Biosynthesis of isoxazolin-5-one glucoside and 3-nitropropanoate esters as hemolymph defenses in leaf beetle larvae.

Synthesis and photosensitivity of isoxazolin-5-one glycosides

An improved synthetic protocol is described, allowing the synthesis of novel and naturally occurring isoxazolin-5-one glycosides. The photohydrolysis efficiency and pH stability of the obtained isoxazolin-5-one glycosides was studied.