Enzyme-Catalyzed Synthesis of Well-Defined Macromers Built around

Chapter 9

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Enzyme-Catalyzed Synthesis of Well-Defined Macromers Built around a Sugar Core Rajesh Kumar

1,2

and Richard A.

1* ,

Gross

1

NSF Center for Biocatalysis and Bioprocessing of Macromolecules, Department of Chemistry and Chemical Engineering, Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201 Current address: INSET and Center for Advanced Materials, Department of Chemistry, University of Massachusetts at Lowell, Lowell, MA 01854

2

By using 4-C-hydroxymethyl-α-D-pentofuranose as the sugar core and lipase-catalyzed transformations, a macromer was constructed with exceptional control of substituent placement around the carbohydrate core. The key synthetic transformations performed were as follows: 1) selective lipase -catalyzed acrylation along with prochiral selection of 4-C -hydroxymethyl-1,2-O-isopropylidene-α-D-pentofuranose (diastereomeric excess up to 93%); 2) the ring-opening of εcaprolactone, ε-CL,fromthe remaining primary hydroxyl group to give an acryl-sugar capped macromer (M 11 300, Mw/Mn 1.36, initiator efficiency 50-55 %, < 5% water initiated PCL chains); 3) selective lipase-catalyzed esterification of the terminal hydroxyl of oligo(ε-CL) chains; 4) hydrolysis of the 1,2-O-isopropylidene group at the sugar core 5) homopolymerization of the corresponding macromer. In principle, the method developed isflexibleso that it can be used to generate a wide array of unusual macromers and heteroarm stars. In the absence of biocatalytic transformation, such structural control would be extremely difficult or currently impossible to obtain. n

© 2003 American Chemical Society

Gross and Cheng; Biocatalysis in Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction The synthesis and study of polymers that contain carbohydrates has captured the attention of researchers who wish to attain i) highly functional polymers ii) specific biological functions, and Hi) complex systems thatfitinto the category of 'smart' materials. ' For example, such polymers have been studied for their ability to regulate interactions with lectins, as pseudo-glycoproteins, and as carriers for drug delivery systems. Research directed towards stars and dendrimers also reflects the aspiration of scientists to create macromolecules that function with improved efficiency and precision for catalysis, drug delivery, and much more Hetero-arm or multi-arm star block copolymers have been prepared by living anionic and cationic polymerization. Structure-property studies have proven that variations in the macromolecular architecture from linear to multiarm can have dramatic effects on the morphological and physical-mechanical properties of the corresponding materials Recently, our laboratory and others have explored the use of in-vitro enzyme-catalysis for the preparation of monomers and polymers. Recent reviews and books have been published that document the rapid development of these methods. Early reports that describe the synthesis of linear chains that are attached to a multifunctional initiator have been published. The above findings provide incentive to further extend the level of control attainable during the synthesis of these products. In this work, a general route was demonstrated that permits the efficient placement of selected structures at specific positions around a carbohydrate core. A key synthetic step that makes this possible is lipase-catalyzed diastereoselective acylation. This, and the judicious choice of a certain carbohydrate substrate, permits a level of control in the construction of hetero-arm star co-polymers that was previously non-attainable or extremely difficult to realize by traditional chemical methods where a large number of protection-deprotection steps have impeded progress. ' ' 1

2,3 4

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1 2 1 3

1 4 , 1 5

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Results and Discussion The lipase-catalyzed synthesis of the macromer 4-hydroxymethylmethacryl, 4-Chydroxymethyl-l,2-0-isopropylidene-a-D-pentofuranose (HMG, 3, Scheme \ \ its use as a multifunctional initiator to prepare a polyester arm specifically linked to the other (C-5) diastereometric center (4 Scheme 1), and its homopolymerization are described. Scheme-1 presents the strategy that was used for the stereoselective incorporation of the acryl group in the sugar molecule 5


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2 R =-COC(CH)=CH2»Ri=H 3R=H,R|=-COC(CHj)=CH 3

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®\lnflmtàmaylsÊR, THF, 3$j^8h, Anmofff Ρ) VUrçyiroeffiacirjtaf$THF, 30€»flb, Νοτη^ηηΜ35 Cv) Lsusie add, BMF, DOÇ

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Scheme-1. Stereoselective Acrylation followed by ROP and Selective Acylation ofthePCL End-Group.

followed by initiation of ε-caprolactone (ε-CL)ring-openingpolymerization to build the poly(e=caprolactone), PCL, substituted acryl sugar macromer. 4-CHydroxymethyl-l,2-0-isopropylidene-a-D-pentofuranose (1) was synthesized by starting with diacetone glucose in a two step reaction. The bishydroyxmethyl glucofiiranose 1 was then subjected to acryloylation with methylacrylate in dry THF. The ability of the lipases Porcine pancreatic lipase (PPL), Candida rugosa lipase (CRL), PS-30, lipasesfromPseudomonas AK, Pseudomonas AY and Novozyme-435 to catalyze prochiral asymmetrization of 4-C-hydroxymethyl1,2-O-isopropylidene-a-D-pentofuranose, 1, was studied at 30-35°C for 8 h and the results of this work are shown in Figure 1. 22

2 3


110

1 φ ο «

Β S C

120 100 -I 80 60 40 20 0

• C - Î epimer • C-5 epimer

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Lipases evaluated Figure 1, Ability of lipases to carry out the selective acrylation of 4-Chydroxymethyl-l^-O^sopropyUdene-a-D-pentofuranose Novozyme-435 and lipase PS resulted in highly diastereo-selective monoacryl derivatization using one equivalent of vinyl methacrylate as the acyl donor (Figure 1). Furthermore, for both Novozyme-435 and lipase PS, even when a two fold excess of vinyl methacrylate was used and the reaction was prolonged to 24 h, the major product was still the monoacryl derivative (-95-97%). The Amano PS catalyzed reaction preferentially placed the acryl moiety at the C-5 hydroxyl, affording 2 as the major product (de. 78%, yield = 72%,). In contrast, Novozyme-435 resulted in acrylation at the C-Γ position (de. 93%, yield = 95%) giving 3 as the main product. The diastereomeric excess of the products 2 and 3 was calculatedfrom H NMR spectra by the difference in the integral values of the anomeric protons of the corresponding epimers at δ 5.93 and δ 6.01. Assignments of Ή NMR signals to the epimer formed was done by detail©! spectral analyses. It was observed that the protons of both C-l'and C-5 methylene groups of 1 appeared as a multiplet, one of the corresponding methylene protons shifted downfield on acrylation, i.e. in compound 3 the C-Γ methylene protons appeared as a double doublet at δ 4.32 (J= 11.76 & 11.47) while in compound 2 the C-5 methylene protons appeared as a double doublet at δ 4.34 ( J= 11.7 ). Furthermore the anomeric proton C-l of 2 for major diastereomer appeared at higher δ values ( in CDC1 , 300 MHz) at δ 6.01 ( J= 4.12) and for minor diastereomer at δ 5.90 compared to the corresponding protons in 3 the major diastereomer appeared at δ 5.93 ( J= 4.12). This clearly indicates that there is reversal of selectivity during enzyme catalyzed acrylation. Also the position of the acryl group was unequivocally established by the significant NOE effect observed in H-3 on irradiation of Η-Γ in compound 3. The sugar acryl derivative 3 was studied as a multifunctional initiator for ε-CL polymerization. Based on previous work in our laboratory " and elsewhere " , Novozyme-435 was chosen as the selective catalyst for this ring!

3

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Ill 2 4

opening polymerization (Scheme-1). Recent work by us showed that Novozyme-435 catalysis of ε-CL polymerizations is accelerated when it is performed in low-polarity organic media. Thus, the acryl sugar 3 initiated ringopening polymerization of ε-CL was performed in toluene. The H NMR spectrum (seefigure2) of Product 4frommi S hring-openingpolymerization of ε-CL initiated by 3 and catalyzed by Novozyme-435 indicates that 4 consists of the acryl sugar moiety that is linked to the carboxyl terminal of a PCL chain (M 11300, MJM 1.36). Product 4 was separatedfromnon-reacted acryl sugar by precipitating the polymer into methanol Analysis of Product 4 by H NMR and C NMR revealed that the reaction was highly regioselective at the C-5 hydroxyl. Derivatization of 4 (after purification by precipitation in methanol) with oxalyl chloride revealed that

Enzyme-Catalyzed Synthesis of Well-Defined Macromers Built around

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