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Formose Reaction Controlled by a Copolymer of N,N-Dimethylacrylamide and 4-Vinylphenylboronic Acid

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Formose Reaction Controlled by a Copolymer of N,N-Dimethylacrylamide and 4-Vinylphenylboronic Acid polymers Article Formose Reaction Controlled by a Copolymer of N,N-Dimethylacrylamide and 4-Vinylphenylboronic Acid Tomohiro Michitaka, Toru Imai and Akihito Hashidzume * ID Department of Macromolecular Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan; [email protected] (T.M.); [email protected] (T.I.) * Correspondence: [email protected]; Tel.: +81-6-6850-8174 Received: 8 October 2017; Accepted: 24 October 2017; Published: 25 October 2017 Abstract: The formose reaction is an oligomerization of formaldehyde under basic conditions, which produces a complicated mixture of monosaccharides and sugar alcohols. Selective formation of useful monosaccharides by the formose reaction has been an important challenge. In this study, we have investigated the formose reaction controlled by N,N-dimethylacrylamide/4-vinylphenylboronic acid copolymer (pDMA/VBA) and phenylboronic acid (PBA) because boronic acid compounds form esters with polyols, e.g., monosaccharides and sugar alcohols. We obtained time–conversion data in the presence of these boronic acid compounds, and characterized the products by liquid chromatography-mass spectroscopy and NMR measurements. pDMA/VBA and PBA decelerated the formose reaction because of the formation of boronic acid esters with products. It is noteworthy that the formose reaction in the presence of pDMA/VBA and PBA formed favorably six- and seven-carbon branched monosaccharides and sugar alcohols. Keywords: formose reaction; boronic acid compounds; N,N-dimethylacrylamide/4-vinylphenyl boronic acid copolymer; phenylboronic acid; monosaccharides; sugar alcohols 1. Introduction Carbohydrates are ubiquitous in our life and very important biological materials [1,2]. Carbohydrates can be divided into three categories based on their molecular weight; (1) low molecular weight saccharides, e.g., monosaccharides and disaccharides, (2) oligosaccharides, and (3) polysaccharides. Low molecular weight saccharides are utilized as energy source for living organisms, and oligosaccharides exhibit a number of physiological activities. Two representative polysaccharides are cellulose and starch; the former is utilized as a structural material in plants whereas the latter is employed as energy storage. Carbohydrates used in biological systems are synthesized by enzymatic reactions [1,2]. Synthesis of carbohydrates in a non-enzymatic way has been an important subject of investigation [3–6]. Especially, synthesis of monosaccharides from starting materials containing one or two carbon atoms is still remaining as a challenging subject because monosaccharides possess a number of chiral centers to be controlled [7,8]. Since monosaccharides, e.g., glyceraldehyde, erythrose, ribose, and glucose have the general formula (CH2O)n, monosaccharides can be formed by oligomerization of formaldehyde. In 1861, Butlerow [9] reported that a sugar-like product called ‘formose’ is obtained from formaldehyde by heating in water under basic conditions. This reaction is called the formose reaction. Detailed studies have revealed that the formose reaction proceeds through three major stages [10–13]. In the first stage, two formaldehyde molecules couple to form glycolaldehyde. Since this reaction is the rate-determining step, the formose reaction indicates an induction period. As glycolaldehyde acts as a co-catalyst, a variety of monosaccharides are formed dominantly through aldol reaction and aldose–ketose Polymers 2017, 9, 549; doi:10.3390/polym9110549 www.mdpi.com/journal/polymers PolymersPolymers 20172017,, 99,, 549549 22 ofof 1111 aldose–ketose isomerization in the second stage. This stage is called the sugar-forming period. After isomerizationcompletion of inconversion the second of stage. formaldehyde, This stage isthe called monosaccharides the sugar-forming formed period. are decomposed After completion mainly of conversionthrough retro of formaldehyde,-aldol and Cannizzaro the monosaccharides reactions formedin the are third decomposed stage. This mainly stage through is called retro-aldol the andsugar Cannizzaro-decomposing reactions period. in theThus, third the stage. formose This stagereaction is called yields the a sugar-decomposing complicated mixture period. of Thus,monosaccharides the formose and reaction sugar yieldsalcohols a complicatedthrough these mixture three stages. of monosaccharides The formose reaction and sugar is considered alcohols throughas the most these promising three stages. pathway The formose to form reactionmonosaccharides is considered under as the prebiotic most promising conditions pathway [14–19]. to form monosaccharidesIf the formose under reaction prebiotic proceeds conditions in a controlled [14–19]. manner, it will provide an important pathway for nonIf the-enzymatic formose reactionsynthesis proceeds of monosaccharides. in a controlled manner,Some pioneering it will provide works an on important a selective pathway formose for non-enzymaticreaction were synthesisconducted of from monosaccharides. the 1970s to Some the pioneering1980s. Shigemasa works onet aal. selective [20–23 formose] devoted reaction their wereremarkable conducted efforts from to theoptimize 1970s tothe the conditions. 1980s. Shigemasa Consequently, et al. [ 20they–23 ]first devoted isolated their a remarkable branched effortsseven-carbon to optimize sugar alcohol the conditions. from the reaction Consequently, mixture they of the first formose isolated reaction a branched catalyzed seven-carbon by barium sugarhydroxide alcohol in from methanol the reaction [24,25] mixture. Matsumoto of the formose et reactional. [26,27] catalyzed successfully by barium synthesized hydroxide in1,3- methanoldihydroxyacetone, [24,25]. a Matsumoto three-carbon et monosaccharide al. [26,27] successfully, using a thiazolium synthesized catalyst 1,3-dihydroxyacetone, in ethanol in the apresen three-carbonce of triethylamine. monosaccharide, After using these a thiazolium pioneering catalyst works, in however, ethanol in there the presence have been of triethylamine. only a few Afterstudies these on pioneeringa selective works, formose however, reaction there [28,29] have and been, onlythus, athe few selective studies on formation a selective of formose useful reactionmonosaccharides [28,29] and, by the thus, reaction the selectiveremains under formation-examined. of useful monosaccharides by the reaction remainsWe under-examined.have been working on the formose reaction controlled by molecular recognition of polymersWe have and been molecular working assemblies on the formose [30–32 reaction]. We have controlled reported by molecularthat the reaction recognition is accelerated of polymers in andreverse molecular micelles assemblies of aerosol [30-OT,–32 an]. We anionic have reportedsurfactant, that and the yields reaction ethylene is accelerated glycol as in a reverse major micellesproduct of[31] aerosol-OT,. an anionic surfactant, and yields ethylene glycol as a major product [31]. Recently, we we have have also also utilized utilized boronic boronic acid acid compounds compounds to control to control the formose the formose reaction reactionbecause becauseboronic boronicacid compounds acid compounds form esters form with esters polyols, with polyols, e.g., monosaccharides e.g., monosaccharides or sugar or sugaralcohols. alcohols. This Thisphenomenon phenomenon has hasbeen been utilized utilized as asartificial artificial molecular molecular sensors sensors for for saccharides saccharides [33 [33––3636]] and asas polymericpolymeric materials responsive responsive to to saccharides saccharides [37] [37. When]. When the formose the formose reaction reaction is carried is carried out in outthe inpresence the presence of boronic of boronic acid compounds, acid compounds, boronic boronicacid esters acid may esters be formed may be to formed stabilize to the stabilize products, the products,leading to leadingan enhance to and selectivity. enhanced selectivity.We have, thus We, have,studied thus, the studiedformose the reaction formose in the reaction presence in the of presencesodium ofphenylboronate sodium phenylboronate (NaPB) (NaPB)and an and anionic an anionic copolymer copolymer (pNaSS/NaVB) (pNaSS/NaVB) of of sodiumsodium 4-styrenesulfonate4-styrenesulfonate (NaSS) and sodium 4-vinylphenylboronate4-vinylphenylboronate (NaVB) [[32]32].. We observedobserved thatthat NaPBNaPB and pNaSS/NaVBpNaSS/NaVB decelerateddecelerated the the reaction. reaction. More More importantly, importantly, we exhibited we exhibited that the that formose the formose reaction producedreaction produced three- to three five-carbon- to five monosaccharides-carbon monosaccharides preferably preferably in the presence in the presence of NaPB of and NaPB four- and to eight-carbonfour- to eight sugar-carbon alcohols sugar favorablyalcohols favorably in the presence in the ofpresence pNaSS/NaVB. of pNaSS/NaVB However,. However, we also observed we also thatobserved NaSS that polymer NaSS deceleratedpolymer decelerated the formose the formose reaction. reaction. We have, We thus, have been, thus motivated, been motivated to use to other use boronicother boronic acid polymers. acid polymers. In this In article, this wearticle, describe we describe the formose the formose reaction reaction carriedout carried in the out presence in the ofpresence a copolymer of a ofcopolymer non-ionic ofN ,Nnon-dimethylacrylamide-ionic N,N-dimethylacrylamide and 4-vinylphenylboronic and 4-vinylphenylboronic acid (pDMA/VBA, acid Scheme(pDMA/VBA,1).
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