Effect of heteroatom insertion at the side chain of 5-alkyl-1H-tetrazoles on their properties as catalysts for ester hydrolysis at neutral pH

Herein we introduce tetrazole and its suitably designed derivatives as powerful ester-cleaving reagents. By first performing a detailed ab initio computational study, we found that, in the side chain of 5-alkyl-1H-tetrazoles, introduction of a heteroatom (e.g., N, O, or S at the a-position of the tetrazole ring) raises the charge on the tetrazole nucleus significantly. All calculations have been performed using restricted Hartree-Fock (RHF) and hybrid ab initio/DFT (B3LYP) methods employing 6-31G∗ and 6-31+G∗ basis sets. To estimate the nucleophilicity of these reagents, the charges on conjugate bases of various tetrazole derivatives have been calculated using natural population (NBO) analysis in gas phase and in water. Free energy of protonation (fep) of the 1H-tetrazole derivatives (1-7), free energy of solvation, ΔGaq, and the corresponding pKa values have been calculated by self-consistent reaction field (SCRF) methods applying the polarized continuum model (PCM). Since the calculation indicates that incorporation of heteroatom leads to enhanced nucleophilicity in their deprotonated anionic tetrazole forms, a series of 5-substituted 1H-tetrazole derivatives have been synthesized. These compounds indeed catalyze the hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) and p-nitrophenyl hexanoate (PNPH) efficiently in cationic cetyl trimethylammonium bromide (CTABr) micelles at pH 7.0 and 25 °C. The pseudo-first-order rate constants (kobs) were determined for each catalyst against both substrates. The experimental and theoretical results show that, to achieve better kobs values for the cleavage of PNPDPP and PNPH under micellar conditions, charge on the N- atom (nucleophile) of conjugate base is important. Replacing the α-CH2 in alkyl substituent with S (3), NH (4), or O (5) enhances the accumulation of charge on N- in conjugate bases of tetrazoles and subsequently increases their intrinsic nucleophilic reactivity toward hydrolytic reactions. Significantly large rate enhancements were observed for the cleavage of PNPDPP and PNPH at pH 7.0 in the presence of catalytic system 5/CTABr over background (only CTABr). Tetrazole 4 (α-isomer) showed 4-5-fold superior reactivity over 6 (β-isomer) under identical conditions. Natural charges obtained from NBO analysis (B3LYP/6-31+G∗) are -0.94 and -0.852 on N- in the conjugate bases of 4 and 6, respectively. This also predicts that 4 is a better nucleophile than 6. All the newly synthesized tetrazole derivatives in micellar media display true catalytic properties by cleaving several fold excess of substrates

Effect of Heteroatom Insertion at the Side Chain of 5-Alkyl-1H-tetrazoles on Their Properties as Catalysts for Ester Hydrolysis at Neutral pH

Herein we introduce tetrazole and its suitably designed derivatives as powerful ester-cleaving reagents. By first performing a detailed ab initio computational study, we found that, in the side chain of 5-alkyl-1H-tetrazoles, introduction of a heteroatom (e.g., N, O, or S at the \alpha position of the tetrazole ring) raises the charge on the tetrazole nucleus significantly. All calculations have been performed using restricted Hartree-Fock (RHF) and hybrid ab initio/DFT (B3LYP) methods employing 6-31G* and 6-31+G* basis sets. To estimate the nucleophilicity of these reagents, the charges on conjugate bases of various tetrazole derivatives have been calculated using natural population (NBO) analysis in gas phase and in water. Free energy of protonation (fep) of the 1Htetrazole derivatives (1-7), free energy of solvation, \delta G_a_q, and the corresponding $pK_a$ values have been calculated by self-consistent reaction field (SCRF) methods applying the polarized continuum model (PCM). Since the calculation indicates that incorporation of heteroatom leads to enhanced nucleophilicity in their deprotonated anionic tetrazole forms, a series of 5-substituted 1H-tetrazole derivatives have been synthesized. These compounds indeed catalyze the hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) and p-nitrophenyl hexanoate (PNPH) efficiently in cationic cetyl trimethylammonium bromide (CTABr) micelles at pH 7.0 and $25^\circ C$. The pseudo-first-order rate constants (k_o_b_s) were determined for each catalyst against both substrates. The experimental and theoretical results show that, to achieve better kobs values for the cleavage of PNPDPP and PNPH under micellar conditions, charge on the $N^-$ atom (nucleophile) of conjugate base is important. Replacing the $\alpha CH_2$ in alkyl substituent with S (3), NH (4), or O (5) enhances the accumulation of charge on $N^-$ in conjugate bases of tetrazoles and subsequently increases their intrinsic nucleophilic reactivity toward hydrolytic reactions. Significantly large rate enhancements were observed for the cleavage of PNPDPP and PNPH at pH 7.0 in the presence of catalytic system 5/CTABr over background (only CTABr). Tetrazole 4 (\alpha-isomer) showed 4-5-fold superior reactivity over 6 (\beta-isomer) under identical conditions. Natural charges obtained from NBO analysis (B3LYP/ 6-31+G*) are -0.94 and -0.852 on $N^-$ in the conjugate bases of 4 and 6, respectively. This also predicts that 4 is a better nucleophile than 6. All the newly synthesized tetrazole deriatives in micellar media display true catalytic properties by cleaving several fold excess of substrates

Enzyme Catalysis: Tool to Make and Break Amygdalin Hydrogelators From Renewable Resources: A Delivery Model for Hydrophobic Drugs

We report a novel approach for the controlled delivery of an antiinflammatory, chemopreventive drug by an enzyme-triggered drug release mechanism via the degradation of encapsulated hydrogels. The hydro- and organogelators are synthesized in high yields from renewable resources by using regioselective enzyme catalysis, and a known chemopreventive and antiinflammatory drug, i.e., curcumin, is used for the model study. The release of the drug occurred at physiological temperature, and control of the drug release rate is achieved by manipulating the enzyme concentration and/or temperature. The byproducts formed after the gel degradation were characterized and clearly demonstrated the site specificity of degradation of the gelator by enzyme catalysis. The present approach could have applications in developing cost-effective controlled drug delivery vehicles from renewable resources, with a potential impact on pharmaceutical research and molecular design and delivery strategies

Enantioselective Synthesis of Pladienolide B and Truncated Analogues as New Anticancer Agents

An enantioselective synthesis of natural anticancer macrolide pladienolide B is described. The synthetic highlights include Sharpless asymmetric epoxidation, ring closing metathesis (RCM), Ireland–Claisen rearrangement, Shi epoxidation, and Pd-catalyzed Stille coupling as key steps. The synthetic route also allowed the synthesis of the truncated analogues (<b>41a</b>–<b>d</b>) of pladienolide B

Enantioselective Synthesis of Pladienolide B and Truncated Analogues as New Anticancer Agents

An enantioselective synthesis of natural anticancer macrolide pladienolide B is described. The synthetic highlights include Sharpless asymmetric epoxidation, ring closing metathesis (RCM), Ireland–Claisen rearrangement, Shi epoxidation, and Pd-catalyzed Stille coupling as key steps. The synthetic route also allowed the synthesis of the truncated analogues (<b>41a</b>–<b>d</b>) of pladienolide B

Microbial Metabolite Urolithin B Inhibits Recombinant Human Monoamine Oxidase A Enzyme

Urolithins are gut microbial metabolites derived from ellagitannins (ET) and ellagic acid (EA), and shown to exhibit anticancer, anti-inflammatory, anti-microbial, anti-glycative and anti-oxidant activities. Similarly, the parent molecules, ET and EA are reported for their neuroprotection and antidepressant activities. Due to the poor bioavailability of ET and EA, the in vivo functional activities cannot be attributed exclusively to these compounds. Elevated monoamine oxidase (MAO) activities are responsible for the inactivation of monoamine neurotransmitters in neurological disorders, such as depression and Parkinson&rsquo;s disease. In this study, we examined the inhibitory effects of urolithins (A, B and C) and EA on MAO activity using recombinant human MAO-A and MAO-B enzymes. Urolithin B was found to be a better MAO-A enzyme inhibitor among the tested urolithins and EA with an IC50 value of 0.88 &micro;M, and displaying a mixed mode of inhibition. However, all tested compounds exhibited higher IC50 (&gt;100 &micro;M) for MAO-B enzyme

Synthesis and neurite growth evaluation of new analogues of honokiol, a neolignan with potent neurotrophic activity.

International audienceA versatile synthetic route is reported towards the preparation of new analogues for potent neurotrophic agent biaryl-type lignan honokiol. A focused 24-membered library of derivatives containing five different groups at 5'-position of honokiol has been prepared in fair to good overall yields. Compared to the natural product, or to analogues with a short alkyl chain in this position, these new derivatives have lost most of the neurotrophic activity