1,3-Dipolar cycloaddition reactions on carbohydrate-based templates: synthesis of spiro-isoxazolines and 1,2,4-oxadiazoles as glycogen phosphorylase inhibitors

Preparing glycosyl benzothiazoles from 2-isocyanoaryl thioethers and glycosyl radicals under thermal conditions

Herein, we report a method for preparing glycosyl benzothiazoles via radical cascade cyclization, in which glycosyl radicals are generated from readily available and bench-stable allyl glycosyl sulfones. This cascade reaction proceeds under simple conditions and tolerates a broad substrate scope in high yield with excellent stereoselectivity. Mechanistic studies support that the reactions proceed via the intermediacy of imidoyl radicals, which attack the appended sulfide unit by a SH2 process to forge the thiazole ring.

1,6-Dioxo-2-azaspiro[3.4]oct-2-enes and Related Spirocycles: Heterocycles from [3 + 2] Nitrile Oxide Cycloadditions with 2-Methyleneoxetanes, -Thietanes, and -Azetidines

Isoxazolines and 4-membered heterocycles are significant structural motifs in numerous synthetic intermediates and natural products. [3 + 2] Cycloadditions between enol ethers and nitrile oxides have been well studied; however, nitrile oxide cycloadditions with 4-membered heterocycles to give spiroisoxazolines are unreported. Here, we showcase the regio- and diastereoselective [3 + 2] nitrile oxide cycloadditions of 2-methyleneoxetanes, -azetidines, and -thietanes to give an array of 1,6-dioxo-2-azaspiro[3.4]oct-2-enes and related spirocycles. 2D NMR experiments suggested that most of the observed diastereoselectivities were dictated by steric interactions; however, dipolarophiles with H bonding donors reversed the stereochemical outcome. X-ray crystallography confirmed the structural assignments.

Pyranoid Spirosugars as Enzyme Inhibitors

Background:

Pyranoid spirofused sugar derivatives represent a class of compounds with a significant impact in the literature. From the structural point of view, the rigidity inferred by the spirofused entity has made these compounds object of interest mainly as enzymatic inhibitors, in particular, carbohydrate processing enzymes. Among them glycogen phosphorylase and sodium glucose co-transporter 2 are important target enzymes for diverse pathological states. Most of the developed compounds present the spirofused entity at the C1 position of the sugar moiety; nevertheless, spirofused entities can also be found at other sugar ring positions. The main spirofused entities encountered are spiroacetals/thioacetals, spiro-hydantoin and derivatives, spiro-isoxazolines, spiro-aminals, spiro-lactams, spiro-oxathiazole and spiro-oxazinanone, but also others are present.

Objectives:

The present review focuses on the most explored synthetic strategies for the preparation of this class of compounds, classified according to the position and structure of the spirofused moiety on the pyranoid scaffold. Moreover, the structures are correlated to their main biological activities or to their role as chiral auxiliaries.

Conclusion:

It is clear from the review that, among the different derivatives, the spirofused structures at position C1 of the pyranoid scaffold are the most represented and possess the most relevant enzymatic inhibitor activities. Nevertheless, great efforts have been devoted to the introduction of the spirofused entity also in the other positions, mainly for the preparation of biologically active compounds but also for the synthesis of chiral auxiliaries useful in asymmetric reactions; examples of such auxiliaries are the spirofused chiral 1,3-oxazolidin-2-ones and 1,3-oxazolidine-2-thiones.

2-Isoxazolines: A Synthetic and Medicinal Overview

Isoxazolines are nitrogen- and oxygen-containing five-membered heterocyclic scaffolds with extensive biological activities. This framework can be readily obtained in good to excellent yields through 1,3-dipolar cycloaddition between nitrones with alkynes or allenes, aryl/alkyl halides, alkynes, and oxaziridines under mild conditions. This scaffold has been an emerging area of interest for many researchers given their wide range of bioactivities. Herein we review synthetic strategies toward isoxazolines and the role these efforts have had in enhancing the biological activity of natural products and synthetic compounds such as antitubercular agents, COX-1 inhibitors, COX-2 inhibitors (e. g., valdecoxib), nicotinic receptor modulators, and MIF inhibitors. With a focus on efforts from 2010 onward, this review provides in-depth coverage of the design and biological evaluation of isoxazoline systems and their impact on various pathologies.

Glucose-based spiro-oxathiazoles as in vivo anti-hyperglycemic agents through glycogen phosphorylase inhibition

The design of glycogen phosphorylase (GP) inhibitors targeting the catalytic site of the enzyme is a promising strategy for a better control of hyperglycaemia in the context of type 2 diabetes. Glucopyranosylidene-spiro-heterocycles have been demonstrated as potent GP inhibitors, and more specifically spiro-oxathiazoles. A new synthetic route has now been elaborated through 1,3-dipolar cycloaddition of an aryl nitrile oxide to a glucono-thionolactone affording in one step the spiro-oxathiazole moiety. The thionolactone was obtained from the thermal rearrangement of a thiosulfinate precursor according to Fairbanks' protocols, although with a revisited outcome and also rationalised with DFT calculations. The 2-naphthyl substituted glucose-based spiro-oxathiazole 5h, identified as one of the most potent GP inhibitors (K_i = 160 nM against RMGPb) could be produced on the gram-scale from this strategy. Further evaluation in vitro using rat and human hepatocytes demonstrated that compound 5h is a anti-hyperglycaemic drug candidates performing slightly better than DAB used as a positive control. Investigation in Zucker fa/fa rat model in acute and subchronic assays further confirmed the potency of compound 5h since it lowered blood glucose levels by ∼36% at 30 mg kg^-1 and ∼43% at 60 mg kg^-1. The present study is one of the few in vivo investigations for glucose-based GP inhibitors and provides data in animal models for such drug candidates.

1,3-Dipolar Cycloadditions of Substituted Vinylphosphonate with Nitrile Oxides or Nitrones

1,3-dipolar cycloaddition reactions are among the most important synthetic manipulations allowing the construction of five-membered ring carbocycles and heterocycles. Vinylphosphonates and its analogues have found wide application in organic chemistry during the last two decades and have become very useful for the construction of functionalised organophosphorous compounds. In this report, we have developed a method for the synthesis of β-Substituted vinylphosphonates and the novel structural heterocyclic compound containing a phosphonyl group.

Base mediated green synthesis of enantiopure 2-C-spiro-glycosyl-3-nitrochromenes

A novel green synthetic methodology has been developed to obtain enantiopure (2S)-2-C-spiro-glycosyl-3-nitrochromenes following the oxa-Michael-aldol condensation reaction of sugar derived 3-C-vinyl nitro olefins with substituted salicylaldehydes using Et3N as a base under neat conditions at rt-40 °C. The stereochemistry of the product is confirmed by a single crystal X-ray study. Several advantages are associated with this protocol such as cost effectiveness, easy accessibility, short reaction time, high yields, wide substrate scope and high enantiopurity.

Synthesis of New C- and N-β-d-Glucopyranosyl Derivatives of Imidazole, 1,2,3-Triazole and Tetrazole, and Their Evaluation as Inhibitors of Glycogen Phosphorylase

The aim of the present study was to broaden the structure-activity relationships of C- and N-β-d-glucopyranosyl azole type inhibitors of glycogen phosphorylase. 1-Aryl-4-β-d-gluco-pyranosyl-1,2,3-triazoles were prepared by copper catalyzed azide-alkyne cycloadditions between O-perbenzylated or O-peracetylated β-d-glucopyranosyl ethynes and aryl azides. 1-β-d-Gluco-pyranosyl-4-phenyl imidazole was obtained in a glycosylation of 4(5)-phenylimidazole with O-peracetylated α-d-glucopyranosyl bromide. C-β-d-Glucopyranosyl-N-substituted-tetrazoles were synthesized by alkylation/arylation of O-perbenzoylated 5-β-d-glucopyranosyl-tetrazole or from a 2,6-anhydroheptose tosylhydrazone and arenediazonium salts. 5-Substituted tetrazoles were glycosylated by O-peracetylated α-d-glucopyranosyl bromide to give N-β-d-glucopyranosyl-C-substituted-tetrazoles. Standard deprotections gave test compounds which were assayed against rabbit muscle glycogen phosphorylase b. Most of the compounds proved inactive, the best inhibitor was 2-β-d-glucopyranosyl-5-phenyltetrazole (IC₅₀ 600 μM). These studies extended the structure-activity relationships of β-d-glucopyranosyl azole type inhibitors and revealed the extreme sensitivity of such type of inhibitors towards the structure of the azole moiety.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

Synthesis of Unprecedented Sulfonylated Phosphono-exo-Glycals Designed as Inhibitors of the Three Mycobacterial Galactofuranose Processing Enzymes

This study reports a new methodology to synthesize exo-glycals bearing both a sulfone and a phosphonate. This synthetic strategy provides a way to generate exo-glycals displaying two electron-withdrawing groups and was applied to eight different carbohydrates from the furanose and pyranose series. The Z/E configurations of these tetrasubstituted enol ethers could be ascertained using NMR spectroscopic techniques. Deprotection of an exo-glycal followed by an UMP (uridine monophosphate) coupling generated two new UDP (uridine diphosphate)-galactofuranose analogues. These two Z/E isomers were evaluated as inhibitors of UGM, GlfT1, and GlfT2, the three mycobacterial galactofuranose processing enzymes. Molecule 46-(E) is the first characterized inhibitor of GlfT1 reported to date and was also found to efficiently inhibit UGM in a reversible manner. Interestingly, GlfT2 showed a better affinity for the (Z) isomer. The three enzymes studied in the present work are not only interesting because, mechanistically, they are still the topic of intense investigations, but also because they constitute very important targets for the development of novel antimycobacterial agents.

Glycogen phosphorylase inhibitors: a patent review (2013 - 2015)

INTRODUCTION

Control of glycemia is crucial in the treatment of type 2 diabetes complications. Glycogen phosphorylase (GP) releases glucose from the liver into the blood stream. Design of potent GP inhibitors is a therapeutic strategy in the context of type 2 diabetes.

AREAS COVERED

Glucose-based inhibitors have found potential applications since they now reach low nanomolar Ki values. Another set of patents disclose cholic acid/7-aza-indole conjugates for targeted drug delivery to the liver. A series of benzazepinones have also been reported as potent GP inhibitors. In vitro data are reported for GP inhibition but the in vivo biological data at the cellular or animal levels are often missing, even though the literature reported for these molecules is also discussed.

EXPERT OPINION

A structural analogy between glucose-based GP inhibitors and C-glucosides targeting sodium glucose co-transporter 2 (SGLT2) is intriguing. Cholic acid/7-aza-indole conjugates are promising in vivo drug delivery systems to the liver. Benzazepinones were very recently described and no associated literature is available, making it very difficult to comment at present. While industry has slowed down on GP inhibitors design, academic groups are pursuing investigations and have provided potential drug candidates which will resuscitate the interest for GP, including its potential for targeting cancer.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

Strong deshielding in aromatic isoxazolines

Very strong proton deshielding was found in di/tri-aromatic isoxazoline regioisomers prepared from acridin-4-yl dipolarophiles and stable benzonitrile oxides (BNO). Three alkenes, (acridin-4-yl)-CH=CH-R (R = COOCH3, Ph, and CONH2), reacted with three BNO dipoles (2,4,6-trimethoxy, 2,4,6-trimethyl, 2,6-dichloro) to give pairs of target isoxazolines with acridine bound to C-4 or C-5 carbon of the isoxazoline (denoted as 4-Acr or 5-Acr). Regioselectivity was dependent on both the dipolarophile and dipole character. The ester and amide dipolarophile displayed variable regioselectivity in cycloadditions whereas the styrene one afforded prevailing 4-Acr regioisomers. 2,4,6-Trimethoxy-BNO was most prone to form 5-Acr isoxazolines while mesitonitrile oxide gave major 4-Acr isoxazolines. Basic hydrolysis of the amide cycloadduct led to an unexpected isoxazolone product. The structure of the target compounds was studied by NMR, MS, and X-ray crystallography.

3-Glucosylated 5-amino-1,2,4-oxadiazoles: synthesis and evaluation as glycogen phosphorylase inhibitors

Glycogen phosporylase (GP) is a promising target for the control of glycaemia. The design of inhibitors binding at the catalytic site has been accomplished through various families of glucose-based derivatives such as oxadiazoles. Further elaboration of the oxadiazole aromatic aglycon moiety is now reported with 3-glucosyl-5-amino-1,2,4-oxadiazoles synthesized by condensation of a C-glucosyl amidoxime with N,N’-dialkylcarbodiimides or Vilsmeier salts. The 5-amino group introduced on the oxadiazole scaffold was expected to provide better inhibition of GP through potential additional interactions with the enzyme’s catalytic site; however, no inhibition was observed at 625 µM.

Binding evaluation of fragment-based scaffolds for probing allosteric enzymes

Fragment-based drug discovery has become a powerful method for the generation of drug leads against therapeutic targets. Beyond the identification of novel and effective starting points for drug design, fragments have emerged as reliable tools for assessing protein druggability and identifying protein hot spots. Here, we have examined fragments resulting from the deconstruction of known inhibitors from the glycogen phosphorylase enzyme, a therapeutic target against type 2 diabetes, with two motivations. First, we have analyzed the fragment binding to the multiple binding sites of the glycogen phosphorylase, and then we have investigated the use of fragments to study allosteric enzymes. The work we report illustrates the power of fragmentlike ligands not only for probing the various binding pockets of proteins, but also for uncovering cooperativity between these various binding sites.

A THEORETICAL STUDY ON THE MECHANISM OF 2:1 1, 3 DIPOLAR CYCLOADDITION REACTIONS

The reactions between nitrile oxides and alkenes are of considerable interest in organic synthesis as the resulting heterocycles are versatile intermediates for the synthesis of natural products and biologically active compounds. In this paper, we design a series of reactions of phosphonyl nitrile oxides with acrylonnitrile, which can give 2:1 cycloaddition products with no crystal structure released so far, and present a detailed theoretical study on the mechanism of the 2:1 1, 3-dipolar cycloaddition reaction, which has been explored with density functional theory calculations at B3LYP/6-31G* level. The results reveal that the following mechanism is quite possible. Firstly, it starts as a normal 1,3-dipolar cycloaddition reaction to produce a regiospecific 1:1 product. Subsequently, highly reactive diisopropanyl phosphonyl nitrile oxide sequentially reacts with the aforementioned regiospecific 1:1 product and gives the corresponding cycloadduct. Further study is underway to expand the scope of this methodology, as well as to ascertain mechanistic details of the cycloaddition process.