Selective Fluorination in Organic and Bioorganic Chemistry

Chapter 10

Aldolases in Synthesis of Fluorosugars C.-H. Wong

Downloaded by NANYANG TECHNOLOGICAL UNIV on May 12, 2016 | http://pubs.acs.org Publication Date: February 22, 1991 | doi: 10.1021/bk-1991-0456.ch010

Department of Chemistry, Research Institute of Scripps Clinic, 10666 North Torrey Pines Road, La Jolla, CA 92037

Aldolases hold potential for convergent synthesis of fluorocarbohydrates. There have been more than 20 aldolases isolated, eight of which have been explored for organic synthesis. This presentation describes the application of fructose- 1,6-diphosphate aldolase, 2-deoxyribose-5-phosphate aldolase and sialic acid aldolase to the synthesis of fluorosugars. Fluorinated sugars are a group of compounds with potential value as pharmaceuticals or pharmacological probes (7). Although many nucleophilic and electrophilic fluorinating reagents have been reported for the synthesis of fluorosugars, most of the reagents are not readily available and are difficult to handle. We envisage that fluorosugars may be synthesized enzymatically via a convergent approach in which thefluorinatedbuilding blocks are prepared with inexpensive, safe and easy-to-handle fluorinating reagents such as inorganicfluoride,diethylaminosulfur trifluoride (DAST) (2-5), and 1fluoropyridinium triflate (4). As our interest in the development of enzymes in carbohydrate synthesis (5), we have been active in the use of aldolases for aldol addition reactions. There have been more than 20 aldolases isolated, eight of which have been explored for organic synthesis (6). Aldolases possess two interesting common features: the enzymes are specific for the donor substrate but flexible for the acceptor component, and the stereochemistry of aldol reaction is controlled by the enzyme not by the substrates. In our previous study, we have described the use of lipases, hexokinases, glycosyl transferases and rabbit muscle aldolase for the synthesis of certainfluorosugars(7). This review describes our recent development in aldolase-catalyzed reactions for the synthesis of fluorosugars. Fructose-l,6-diphosphate (FOP) aldolase (EC 4.1.2.13) The FDP aldolasefromrabbit muscle (6-8) or E. coli (9) has been used substantially in organic synthesis. This enzyme accepts a variety of aldehydes as acceptor substrates. Figure 1 illustrates the synthesis of 6-deoxy-6-fluoro-

0097-6156/91/0456-0156$06.00/0 © 1991 American Chemical Society

Welch; Selective Fluorination in Organic and Bioorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


Figure 1. Use of rabbit muscle or E. coli FDP aldolase synthesis. (a) Normal reaction, (b) synthesis of 5-deoxy-5-fluoro-D-fructose, (c) synthesis of 5-deoxy-5-fluoro-L-sorbose



158

SELECTIVE FLUORINATION IN ORGANIC AND BIOORGANIC CHEMISTRY

D-fiructose and 6-deoxy-6-fluoro-L-sorbose. The fluoroaldehydes used for the aldol reactions were prepared via a nucleophilic opening of enantiomerically pure glycidaldehyde diethyl acetyl with inorganic fluoride (9) (Figure 2). Alternatively, both enantiomers can be prepared via a lipase-catalyzed resolution of 3-fluoro-2-acetoxypropanal diethyl acetal (10). The enantioselectivity of the resolution is very high, which allows for the preparation of both enantiomers with very high enantiomeric excess. One can also prepare the "R" enantiomer via an alcohol dehydrogenase-catalyzed reduction offluoropyruvaldehyde1,3-dithiane followed by deprotection. The alcohol dehydrogenase can be from horse liver (NAD dependent) or from Thermoanaerobium brockii (NADP dependent). Regeneration of the cofactor is required for a preparative synthesis. Notice that the aldol condensation strategy can be extended to the synthesis of many other fluoroketoses using other fluoroaldehyde substrates. Several fluorohexoketoses prepared with this aldolase can be converted to the corresponding aldoses catalyzed by glucose isomerase (8). 2-Deoxyribose-5-phosphate aldolase (DERA, E C 4.1.2.4) The enzyme DERAfromE. coli has recently been cloned and overexpressed in E. coli (11). It catalyzes the condensation of acetaldehyde and Dglyceraldehyde 3-phosphate to form 2-deoxyribose-5-phosphate. The enzyme also accepts different aldehydes as acceptors and donors. In addition to acetaldehyde, propionaldehyde, acetone and fluoroacetone are substrates as donors. As acceptor substrates, a broad range of aldehydes can be used (77). Figure 3 illustrates representative syntheses of fluorinated compounds. Work is in progress to further exploit the synthetic utility of this enzyme. N-Acetylneuraminic Acid Aldolase (Sialic acid aldolase, E C 4.1.3 J) Sialic acid aldolase catalyzes the condensation of pyruvate and Nacetylmannosamine to form sialic acid, an acidic sugar involved in a number of biochemical recognition processes. Like other aldolases, sialic acid aldolase accepts a number of aldoses as substrates (12,13). Mannose, 2-deoxyglucose, and many 6-substituted or 6-modified mannose or N-acetylmannosamine, for example, are good substrates for the enzyme. We have prepared 9-deoxy-9fluorosialic acid and a 7,9-difluoroderivative of sialic acid using sialic acid aldolase as catalyst (Figure 4). In an attempt to prepare 3-deoxy-3-fluorosialic acid, it was found that 3-fluoropyruvate is not a substrate for the enzyme. Given the broad range of sugars which can be used as acceptor substrates for the enzyme, sialic acid aldolase appears to be a useful catalyst for the preparation of fluorinated sialic acid derivatives. Conclusion Enzyme catalyzed aldol condensation is a useful strategy for the convergent synthesis of fluorosugars. The fluorinated substrates can be easily prepared with readily available and easy-to-handle fluorinating reagents. With the increasing number of aldolases available, synthesis of carbohydrates and related substances based on this chemo-enzymatic strategy will experience a substantial development in the near future.



Ν:

s-

O

E

NADPH

7~Ύ

OH

NADP

DMF reflux 3 days

OAc

OEt

OEt

OEt

+

OH

> 98% ee

E: Thermoanaerobiwn brockxi Alcohol dehydrogenase

Hg

conversion

50%

40%

OH

Figure 2. Preparation of (R) and (S)-3-fluoro-2-hydroxypropanal via (a) opening of (R)-glycidal diethylacetal, (b) lipase-catalyzed resolution of a racemic precursor, (c) alcohol dehydrogenase-catalyzed asymmetric reduction

Il

O

'OEt

.OEt

Ν:

OEt

OEt

2

KHF (2 eq)

(c)

(b)

(a)



Figure 3. Use of 2-deoxyribose-5-phosphate aldolase in the synthesis of fluorinated sugars, (a) Normal reaction, (b) synthesis of l-fluoro-4hydroxy-5-methyI-hexan-2-one, (c) synthesis of 2,5-dideoxy-5fluororibose


!Ι

I ι

ι

10.

WONG

OH NHAc I.Q


HO HO

OH

A

Neu5Ac C0 -

Aldolase

2

Fv^^Jl^^^

/

161

Aldolases in Synthesis of Fluorosugars

NHAc

Ο

HO [0-V""*^~~ OH HO

HO"" AcNH

(a)

(not a substrate)

Neu5Ac

y^^C02

co -

aldolase

2

(b)

OH

OH Utl

/ OH

DDAST(5eq),

• Uj*

HO^K

+

H O - V - " " * ^ 2)Dowex50H OMe

F

0

H

(c)

HO-X^^| C

OH HO HO

Neu5Ac OH

co -

Aldolase

2

Figure 4. Sialic acid aldolase-catalyzed synthesis of fluorosialic acid and related substances, (a) Normal reaction, (b) synthesis of 9-fluoro-9-deoxyN-acetylneuraminic acid, (c) synthesis of 7,9-difluoro-7,9-dideoxy-Dg/ycm>-L-a-a//ro-2-nonulopyranosonic acid, (d) synthesis of 5-fluoro-3^dideoxy-D-g/ycero-a-D-gu/o-2-nonuIopyranosonic acid


(d)

162

SELECTIVE FLUORINATION IN ORGANIC AND BIOORGANIC CHEMISTRY

Acknowledgment I thank the contributions of many coworkers whose names are listed in the references and the compound fluoropyruvaldehyde 1,3-dithiane provided by Professor John Welch. This research was supported by the NIH GM44154-01. Literature Cited


1. Taylor, N.F. Fluorinated carbohydrates: chemical and biochemical aspects; ACS Symposium Series No. 374; American Chemical Society: Washington, D.C. 1988. 2.

Markovskij, L.N.; Pashinnik, V.E.; Kirsanov, A.V. Synthesis, 1973, 787.

3.

Middleton, W.J. J. Org. Chem. 1975, 40, 574.

4.

Hewitt, C.D.; Silvester, M.J. Aldrichim. Acta 1988, 21, 3.

5.

Wong, C.-H. Chemtracts - organic chemistry 1990, 3, 90.

6.

Toone, E.J.; Simon, E.S.; Bednarski, M.D.; Whitesides, G.M. Tetrahedron 1989, 45, 5365.

7.

Wong, C.-H.; Drueckhammer, D.G.; Sweers, H.M. In Fluorinated carbohydrates: chemical and biochemical aspects; Taylor, N.F., Ed.; ACS Symposium Series No. 374; American Chemical Society: Washington, D.C. 1988, pp 29-42.

8.

Durrwachter, J.R.; Drueckhammer, D.G.; Nozaki, K.; Sweers, H.M.; Wong, C.-H. J. Am. Chem. Soc. 1986, 108, 7812.

9.

von der Osten, C.H.; Sinskey, A.J.; Barbas, C.F., IIΙ; Pederson, R.L.; Wang, Y.-F.; Wong, C.-H. Ibid 1989, 111, 3924.

10. Pederson, R.L.; Liu, K.K.-C.; Rutan, J.F.; Chen, L.; Wong, C.-H.J.Org. Chem., in press. 11. Barbas, C.F., IIΙ; Wang, Y.-F.; Wong, C.-H. J. Am. Chem. Soc. 1990, 112, 2013. 12. Auge, C.; Gautheron, C.; David, S. Tetrahedron 1990, 46, 201. 13. Kim, M.-J.; Hennen, W.J.; Sweers, H.M.; Wong, C.-H. J. Am. Chem. Soc. 1988, 110, 6481. RECEIVED October

19, 1990


Selective Fluorination in Organic and Bioorganic Chemistry

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