Design And Synthesis Of Aldose Reductase Inhibitors Based On Quinoxalinone And Benzothiazine

Diabetes mellitus（DM）is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion,insulin action,or both.All forms of diabetes including type I and type II are vulnerable to chronic diabetic complications,such as neuropathy,nephropathy,retinopathy and cataracts,and those diseases are the major menace to diabetic patients.Increasing evidence established that the abnormal polyol pathway flux is obviously related to pathogenesis of diabetes complications.Aldose reductase（ALR2,EC1.1.1.21）,the first enzyme in the polyol pathway,is a monomeric oxidoreductase that catalyses the NADPH-dependent reduction of glucose.At high glucose concentration,particularly in diabetics,the increased polyol pathway flux directly leads to the accumulation of sorbitol in the cells,which is hardly to penetrate through cellular membranes,eventually resulting in osmotic imbalance,cell swelling and membrane permeability changes,mainly in the lens.In addition,the dramatic reduction of NADPH and NAD⁺gives rise to changes in cellular redox potentials,and deteriorates the activity of enzymes,further exacerbating intracellular oxidative stress.Also,several radical precursor molecules generated downstream of the polyol pathway,which contribute to free radicals and lead to oxidative stress.Accordingly,all these ALR2-mediated oxidative stress reactions along with the polyol pathway represent important pathogenesis of diabetic complications.Aldose reductase inhibitors（ARIs）,which can restrain the abnormal accumulation of sorbitol and indirectly inhibit oxidative stress,have potential as therapeutic drugs.To date,numerous structurally different ARIs have been developed,but most of them that appeared to be promising have not yet succeeded in the clinical trials mainly due to low in vivo efficacies or pharmacokinetic drawbacks.As the pathogenesis of diabetic complications and oxidative stress always promote to each other,the design and synthesis of multifunctional ARIs to directly suppress oxidative stress and simultaneously inhibit the accumulation of sorbitol may be a feasible therapy strategy for increasing efficacy.Thus,we designed and synthesized several groups of quinoxalinone and benzothiazine derivatives for the development of multifunctional ARIs,combining both the inhibition of aldose reductase and antioxidation into an organic whole.Firstly,a series of quinoxalin-2（1H）-one based derivatives having a 1-hydroxypyrazole head group as the bioisosteric replacement of the highly ionized acetic acid moiety,were synthesized and evaluated for their aldose reductase inhibitory activity and antioxidant potency.Most of the derivatives showed potent and selective aldose reductase inhibition,and 7-fluoro-1-（1-hydroxy-1H-pyrazol-4-yl）-3-（4-hydroxystyryl）quinoxalin-2（1H）-one 69d(IC₅₀=0.107μM)was proved to be the most active,which possesses a C3 p-hydroxystyryl side chain and a C7-fluoro substitution on the quinoxalinone core,suggesting the successful bioisosteric strategy.In addition,compounds containing phenolic structure revealed not only good inhibitory activity against aldose reductase but also potent antioxidant activity,and among them phenolic 3,4-dihydroxyl compound 13f showed strong antioxidant ability even comparable to that of the well-known antioxidant Trolox at the concentration of 100μM,both in scavenging the DPPH radical and suppressing lipid peroxidation.In addition to the importance of the 1-hydroxypyrazole head,impact of vinyl spacer and phenolic hydroxyl in the C3 side chain on constructing multifunctional ARIs were highlighted by structure-activity relationship and molecular docking studies.Then a novel series of（1,2-benzothiazin-4-yl）acetic acid enantiomers was prepared by chiral resolution and their absolute configurations were determined using the PGME method.Two key amines were employed throughout the chemistry in the present study.First,racemic acid（±）-82 was coupled with（L）-（-）-alpha-methylbenzylamine to produce diastereomericamides,whichresulted inthechiralresolution ofthe1,2-benzothiazine-1,1-dioxide based racemic ARIs with excellent enantiomeric purities.Then,the separated single enantiomers（-）-82 and（+）-82 were coupled again with both of the enantiomeric（R）-and（S）-phenylglycine methyl esters used as chiral anisotropic reagents,to yield four diastereomeric amides;NMR analysis of these coupling products enabled the assignment of the absolute configurations（R）-（-）-82 and（S）-（+）-82 to the enantiomers.The biological evaluation of the racemate and single enantiomers has shown a remarkable difference for the aldose reductase inhibitory activity and selectivity.The（R）-（-）-enantiomer exhibited the strongest aldose reductase activity with an IC₅₀ value of0.120μM,which was 35 times more active than the S-（+）-enantiomer.In agreement with the biological results,docking studies indicated a more favored interaction for binding of（R）-（-）-82 with the active site of the enzyme than that for binding of（S）-（+）-82.Consequently,the stereocenter at the C4 position of these 1,2-benzothiazine-1,1-dioxide based ARIs has a major impact on both the activity and selectivity of the ALR2 inhibition.Lastly,another series of carboxylic acid derivatives based on quinoxalinone scaffold were designed and synthesized as multifunctional aldose reductase inhibitors.All the obtained compounds were investigated for their inhibitory activity against ALR2 and antioxidant activity.Most of the derivatives were found to be potent and selective inhibitors,and 2-（3-（4-hydroxystyryl）-7-methoxy-2-oxoquinoxalin-1（2H）-yl）acetic acid（93b）was the most active with an IC₅₀ value of 0.059μM.Particularly,the phenolic hydroxyl structure was introduced for the combination of antioxidant ability as well as enhancing ALR2inhibitory activity.Thus,most of the compounds endowed with antioxidant activity and among them the phenolic 3,4-dihydroxyl compound 93f with 7-hydroxyl in the quinoxalinone core showed the most potent activity in scavenging the DPPH radical,even comparable with the well-known antioxidant Trolox.Further,structure-activity relationship and docking studies highlighted the importance of C3 vinyl spacer and phenolic hydroxyl both in C3 side chain and the core structure for constructing more powerful multifunctional ALR2 inhibitors.