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Theoretical Study of the Mutarotation of Erythrose and Threose: Acid Catalysis ⇑ Luis Miguel Azofra , , Ibon Alkorta , José Elguero

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Theoretical Study of the Mutarotation of Erythrose and Threose: Acid Catalysis ⇑ Luis Miguel Azofra , , Ibon Alkorta , José Elguero Carbohydrate Research 372 (2013) 1–8 Contents lists available at SciVerse ScienceDirect Carbohydrate Research journal homepage: www.elsevier.com/locate/carres Theoretical study of the mutarotation of erythrose and threose: acid catalysis ⇑ Luis Miguel Azofra , , Ibon Alkorta , José Elguero Instituto de Química Médica (I.Q.M.-C.S.I.C.), Juan de la Cierva, 3, E-28006 Madrid, Spain article info abstract Article history: The acid catalysis of the mutarotation mechanism in the two aldotetroses, D-erythrose and D-threose, has Received 22 November 2012 been studied at B3LYP/6-311++G(d,p) computational level in gas phase and in solution employing the Received in revised form 18 January 2013 PCM–water model. The open-chain, the furanose and the connecting TS structures have been character- Accepted 21 January 2013 ized. To study the enhancing effect of acid groups on the electrophilicity of the carbonyl carbon atom, four Available online 29 January 2013 + situations have been considered: (i) a classical Lewis acid as BH3; (ii) a classical hard-Pearson acid as Na ; + + + (iii) classical Brønsted acids as H and H3O ; and (iv) a combined strategy using H3O and one bridge-H2O Keywords: molecule as assistant in the proton transfer process. All the acidic groups reduce the activation energy Hemiacetal reaction with exception of the Na+, which can act, in some cases, as inhibitor. It is greatly reduced by the presence DFT-calculations Lewis acid of Brønsted acids (iii) and through the combined strategy (iv). For this last mechanism, the electronic acti- À1 À1 Hard-Pearson acid vation energies are between 12 and 43 kJ mol in vacuum and between 13 and 25 kJ mol in water Brønsted acid solution, through the use of the PCM model. Proton transfer Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction ers, a- and b-, depending on the disposition of the hydroxyl group of the hemiacetal moiety.12 The equilibrium between the different isomers present in the In Figure 2 is reported the reactive process associated with the monosaccharides was observed for the first time by Dubrunfaut mutarotation of a generic D-aldotetrose. The first structure repre- in 18461 when he noticed that the specific rotation of a freshly pre- sented is the non-reactive open-chain one which evolves to a reac- pared cold solution of crystalline glucose decreased from an initial tive structure where the terminal hydroxyl and carbonyl groups value of about 110° to a constant value of 52°. This phenomenon are close and arranged to react. The TS for a reaction without catal- was later called mutarotation2 and its explanation resides in an ysis corresponds to a transition state site characterized by a intramolecular hemiacetal reaction.3,4 The species involved in the four-membered ring with angular strain.13 Two main processes mutarotation process are the open-chain configurations as well are involved in this step: on the one hand, formation of a new C– as the cyclic forms. The cyclic forms can involve five or six bonds, O bond and, on the other hand, a proton transfer from the hydroxyl and are known as furanose and pyranose, respectively.5 Mutarota- group to the carbonyl group to transform it into a new tion has been extensively studied experimentally,6–8 including the hydroxyl group. Depending on the carbonyl face attacked by the 9,10 cases of D-erythrose and D-threose. Experimental studies of hydroxyl group, two furanose forms (a- and b-) are obtained. mutarotation of glucose in acid media indicate that the transfor- Computational studies on the formation of six-membered rings mation barrier is smaller than the one in neutral media.8,11 from open-chain monosaccharides or analogues have been carried 14–17 18 19–21 Aldotetroses, D-erythrose and D-threose, are the simplest carbo- out for glucose, D-xylose, 2-tetrahydropyranol, and deriv- 22 hydrates (see Fig. 1). They are composed of a four carbon atoms atives of L-mannopyranosyl. Furthermore, some of us have studied skeleton with three hydroxyl groups and an aldehyde or hemiace- the hemiacetal formation reaction in D-erythrose and D-threose and tal group. The mutarotation of aldotetroses has only the possibility the catalytic influence of one, two, and three water molecules as of the transformation between the open-chain and the furanose assistants in the proton transfer, as well as simplified models of 12 forms. The only difference between D-erythrose and D-threose is carbohydrates, namely, methanol and 1,2-ethanediol. the configuration of the hydroxyl group attached to C2, R, and S, Related to the study of the mutarotation process, the mecha- respectively. In addition, the cyclic forms present two diasteoisom- nisms of formation of hemiacetals between methanol and formal- dehyde have been studied using Conceptual DFT (CDFT).23 The ⇑ Corresponding author. Fax: +34 91 564 48 53. modeling of catalytic strategies was carried out with two objec- E-mail address: [email protected] (L.M. Azofra). tives: (i) to explore the effect of Brønsted acids clustered with http://are.iqm.csic.es the oxygen of the carbonyl group by enhancing the electrophilicity 0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carres.2013.01.013 2 L. M. Azofra et al. / Carbohydrate Research 372 (2013) 1–8 Open-chain α-furanose β-furanose OH 0 OH 4 O O 4 1 3 O D-erythrose HO 2 1 3 2 OH OH HO OH HO OH OH OH O O O OH OH D-threose HO OH HO OH HO Figure 1. Open-chain and a- and b-furanose configurations of D-erythrose and D-threose. Numbering is in agreement with the IUPAC recommendations. O OH OH O H C O OH OH * OH HO O O HO OH OH OH OH Non-reactive Reactive open-chain open-chain TS Furanose Figure 2. Equilibrium process of mutarotation in a D-aldotetrose. One intramolecular hemiacetal reaction is involved. The asterisk in the furanose structure indicates the anomeric carbon atom. on the carbon atom of formaldehyde; and (ii) to explore the effect Atoms in Molecules (AIM)31,32 methodology with the MORPHY33–35 of this kind of charge extractors and combine with the assistance in and the AIM2000 programs.36 On the basis of this methodology, all the proton transfer through the use of water molecules.4 One of the the interactions (covalent and weak interactions) were characterized main conclusions of this study was that the energy barriers de- by the presence of a bond critical point (BCP) and the corresponding crease in all these cases, specially, when both strategies are used bond path linking the two interacting nuclear attractors. The values at the same system. Thus, the activation barriers in the reactions of the electron density and its Laplacian at the BCP allow to classify between methanol and formaldehyde were 141.3 kJ molÀ1 with the contacts as covalent or non-covalent.31,37,38 the isolated monomers, 74.7 kJ molÀ1 with the use of one bridge- À1 H2O molecule, 99.9 kJ mol with the use of the acidic group 3. Results and discussion + À1 H3O , and finally and optimally, 25.0 kJ mol with both. Here we investigate the hemiacetal formation reaction in D-ery- This section has been divided into five parts. In the first one, the throse and D-threose influenced by different acid groups: (i) a clas- stationary points (minima and TS) of the uncatalyzed reaction rep- + sical Lewis acid as BH3; (ii) a classical hard-Pearson acid as Na ; resented in Figure 2 will be discussed. The catalytic ability of a Le- + + + + and (iii) classical Brønsted acids as H and H3O . Finally, the ap- wis acid, BH3, a hard-Pearson acid, Na , and the Brønsted acids, H + + proach (iv) corresponding to a combined strategy using H3O and and H3O , will be examined in the second, third, and four parts, one bridge-H2O molecule as assistant in the proton transfer, has respectively. The last part will be devoted the simultaneous action + been take into account based on the excellent results obtained of H3O and a water molecule assisting the proton transfer process. previously. In order to establish a common reference for all the reactions, the open-chain monosaccharide species will be considered as reac- 2. Computational methods tants and their relative energies will be defined as 0.0 kJ molÀ1. The geometries used in this study have been selected from a sys- 3.1. Isolated carbohydrates: uncatalyzed process tematic conformational search of the open-chain and furanose con- 24 figurations of the aldotetroses D-erythrose and D-threose. All the The geometries of the open-chain and furanose structures have geometries have been fully optimized with the hybrid Becke,25 been selected based on a previous and systematic conformational three-parameter, Lee–Yang–Parr26 density functional (B3LYP), and search at DFT level24 and they have been used here as reactants Pople’s basis set 6-311++G(d,p).27 Initially, all the systems were con- or products. Also, in the following sections, they have been used sidered in vacuum environment, and subsequently, the solvent ef- in the construction of the reactive forms. The most stable open- fect was taken into account by optimizing the systems through chain conformation, which shows a dihedral angle (compactness) 28 the use of the Polarizable Continuum Model (PCM) using the stan- of the carbon chain of À52.0° and À171.9° for the D-erythrose dard water parameter and including the solute–solvent dispersion and D-threose, has been used as energetic reference in all the reac- interaction energy, the solute–solvent repulsion interaction energy tions studied.
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