Sucralose - American Chemical Society

Chapter 6

Sucralose

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How To Make Sugar Sweeter M. R. Jenner Tate and Lyle Speciality Sweeteners, P.O. Box 68, Reading, Berks, United Kingdom

The sweetness of sucrose can be increased dramatically by selective halogenation. A class of sucrose derivatives was discovered in which up to four of the available hydroxyl groups were replaced by halogens. This transformation resulted in enhancement of sweetness by up to several thousand times. The pattern of derivatisation needed to achieve high sweetness intensity was found to be very stereospecific as many of the compounds in the series are essentially tasteless. A rationalisation of the structural requirements for high sweetness intensity of sugar derivatives has been proposed. The sweetener sucralose emerged from this research programme following an assessment of many key attributes including sweetness quality and stability to the processing and storage conditions employed by the food industry. Sucrose provides an excellent starting point for the study of sweetness. Not only is sucrose the gold standard for sweetness quality but it also possesses eight functional positions suitable for derivatisation. These comprise three primary and five secondary hydroxyl groups as shown in Figure 1. During the 1960s and early 1970's sucrose and other carbohydrates were derivatised and tasted while elucidating the structural requirements for sweetness (J). This work provided insight into the hydroxyl groups responsible for the sweetness of sucrose, but it was far from comprehensive. None of the compounds evaluated in this period was sweeter than sucrose. For example, Lindley, Birch and Khan (2) reported on a series of partially methylated sucrose derivatives, as shown in Table I. Methylation of sucrose at the 6- or 6'positions appeared to have little or no effect on the sweetness of

0097-6156/91/045(M)068$06.00/0 ©1991 American Chemical Society In Sweeteners; Walters, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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In Sweeteners; Walters, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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DISCOVERY, M O L E C U L A R DESIGN, A N D CHEMORECEPTION

the molecule, while methylation at the 4- position appeared to reduce the sweetness.


Table I. Sweetness of methylated sucrose derivatives Compound Sucrose 4-O-Methylsucrose 6'-0-Methylsucrose 6,6'-Di-0-methylsucrose 4,6' -Di-O-methylsucrose 4,6-Di-O-methylsucrose Γ,6'-Di-O-methylsucrose Source: Adapted from ref 2.

Sweetness Very Sweet Sweet Very Sweet Very Sweet Sweet Sweet Sweet

From this group of compounds they concluded that the hydroxyl group on the 4- position of the sucrose molecule was particularly important for its sweetness. Lindley and Birch (3) also evaluated the sweetness of a selection of methyl ethers of methyl α-D-glucopyranoside, and α,α-trehalose as shown in Table II. Table II. Sweetness of methyl glucopyranoside and trehalose derivatives Compound Methyl a-D-glucopyranoside 2-O-methyl 3-O-methyl 4-O-methyl 6-O-methyl 2,3-di-O-methyl 3,4-di-O-methyl 4,6-di-O-methyl a, α-Trehalose 2,2'-di-0-methyl 3,3'-di-0-methyl 4,4'-di-0-methyl 6,6'-di-0-methyl 2,3,2\3'-tetra-0-methyl 4,6,4',6'-tetra-0-methyl Source: Adapted from ref. 3.

Sweetness Sweet Sweet Sweet Sweet Bitter Bitter Bitter Sweet Sweet Sweet Sweet Bitter Bitter

Monomethylation of methyl glucopyranoside did not significantly alter the sweetness of the parent compound, but introduction of two methyl groups eliminated the sweetness and caused the compounds to taste bitter, possibly due to an increase in lipophilicity of the molecule (4). A similar pattern was seen with


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the trehalose derivatives where mono-methylation of each of the hexopyranoside rings caused little change in the sweetness whereas dimethylation of the hexopyranoside units resulted in a transformation from sweet to bitter taste.


Discovery of Sweetness Enhancement During a collaborative research programme between Tate & Lyle and Prof. Hough's group at Queen Elizabeth College at The University of London in 1976, it was found that selective chlorination of certain hydroxyl groups in the sugar molecule could cause a dramatic increase in sweetness (5). This was remarkable in view of the fact that previous evaluation of many chlorinated carbohydrates, including derivatives of methyl a-D-glucopyranoside, a,α-trehalose, maltose, and lactose, had demonstrated such compounds to be extremely bitter (4). The first compound tasted in the sucrose series contained four chlorine atoms located at the 4,6, Γ, and 6'- positions which was produced by selective chlorination of sucrose using sulphuryl chloride in the three-step process shown in Figure 2a and 2b (5,6). Treatment of sucrose with sulphuryl chloride in pyridine at low temperature, -30 °C, gave the 4,6,6'-trichloro-derivative. This was treated with mesitylenesulphonyl chloride at -5 °C for 6 days to selectively derivatise the Γ- position, which was then converted to the chloride by treatment with lithium chloride in dimethylformamide at 140 °C for 18 hours. The resulting product, 4,6, l',6'-tetrachlorogalactosucrose, had a sweetness roughly 200 times that of sugar (J). This was the first carbohydrate derivative observed to be significantly sweeter than the parent molecule. The term galactosucrose was first proposed by Hough et al in 1975 to denote a sucrose derivative which had undergone inversion of configuration at the 4- position, converting the glucopyranosyl ring into a galactopyranosyl unit (7). Investigation of Structure Activity Relationships Following this observation, a series of halogenated sucrose derivatives were synthesised and tasted (8). l'-Chlorosucrose was prepared using the sequence shown in Figure 3a and 3b. The Γand 2- positions were first blocked by treatment with dimethoxydiphenylsilane in dimethyl formamide which produced the l',2-diphenylsilylene derivative in a yield of about 20% (9). Acetylation of the remaining six hydroxyl groups was followed by cleavage of the silylene group using boiling aqueous acetic acid. Tritylation of the remaining primary hydroxyl was achieved using trityl chloride in pyridine and acetylation of the remaining secondary group gave Γ-tritylsucrose hepta-acetate. Detritylation was achieved using hydrogen bromide in glacial acetic acid. This was followed by chlorination using sulphuryl chloride in pyridine


Figure 2. 4,6,1 ',6'-Tetrachlorogalactosucrose




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followed by lithium chloride in dimethylformamide. Finally deesterification using sodium methoxide in methanol gave the Γchlorosucrose which was found to be 20 times sweeter than sugar (JO). 4-Chlorogalactosucrose was prepared using the sequence described in Figure 4a and 4b. Partial tritylation of sucrose using trityl chloride in pyridine at room temperature resulted in a mixture containing 6-tritylsucrose that could be isolated by silica gel column chromatography in low yield. Acetylation of 6tritylsucrose followed by de-tritylation using boiling aqueous acetic acid resulted in a migration of the acetyl group from the 4 to the 6 position. Chlorination using sulphuryl chloride in pyridine followed by de-esterification produced the desired 4-chlorocjalactosucrose which was found to be 5 times sweeter than sugar (JO). 6*-Chlorosucrose was produced by a sequence which involved selective tritylation of the 6'-position of sucrose using trityl chloride in pyridine at room temperature followed by acetylation with acetic anhydride as shown in Figure 5a and 5b. De-tritylation was achieved using hydrogen bromide in glacial acetic acid. The resulting sucrose hepta-acetate was chlorinated using sulphuryl chloride in pyridine and the acetates removed with sodium methoxide in methanol to give 6'-chlorosucrose. This compound was found to be 20 times sweeter than sugar (8). r,6'-Dichlorosucrose was produced in a process which commenced with selective sulphonylation of sucrose with mesitylenesulphonyl chloride at -5 °C for 6 days. The major product was the 6,r,6*-trisulphonate and the desired Γ,6'disulphonate was separated out by silica gel column chromatography. Acetylation of the l',6'-disulphonate followed by treatment with lithium chloride in dimethylformamide at 140 °C for 18 hours produced the l',6'-dichloride which was de-acetylated to give r,6'-dichlorosucrose shown in Figure 6. This was found to be approximately 80 times sweeter than sugar (J). 4,r-Dichlorogalactosucrose was produced in a process which started by protecting the three primary hydroxyl groups with trityl groups and the secondary hydroxyls with acetate shown in Figure 7a and 7b. Detritylation of this tritrityl derivative with boiling aqueous acetic acid removed the primary protecting groups and caused the acetate group on the 4 position to migrate to the 6 position producing the sucrose penta-acetate with the 4,1',and 6'positions unprotected. Selective benzoylation of this penta-acetate using benzoyl chloride in pyridine gave the 6'-benzoate which was chlorinated with sulphuryl chloride and lithium chloride and deprotected to give 4, l'-dichlorofifaiactosucrose. This product was found to be 120 times sweeter than sugar (J). Other compounds in this series which were produced by similar protection sequences include 6,l',6'-trichlorosucrose which was 25 times sweeter than sugar (I J), and 4,l',6'-trichloroflfaiaciosucrose which was 650 times sweeter than sucrose, both shown in Figure 8.



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Figure 4.

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4-Chlorogalactosucrose




SWEETENERS:

OH

OH

Figure 5. 6'-Chlorosucrose



6. JENNER Sucralose

Figure 6.

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l'.e'-Dichlorosucrose


SWEETENERS:



78



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Sucralose

Figure 8. 6,l\6'-Trichlorosucrose galacto-sucrose (bottom)

(top); 4, l\6'-Trichloro-



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All of the compounds described so far are substituted at the 4,6,1' and 6'- positions. However, chlorination at the 4'- position also increases sweetness. The 4,l',4',6'-tetrachloro-derivative was prepared by the sequence shown in Figure 9a and 9b. The 4,l',6'-trichlorocompound was blocked at the 6- position by treatment with tertiary butyl diphenyl silyl chloride in pyridine. Then the 3',4'-epoxide was formed by treatment with diethylazodicarboxylate and triphenylphosphine (12). The epoxide was cleaved using lithium chloride in dimethylformamide at 90 °C for 5 hours to give a 60% yield of the 4'-chlorinated derivative. Conventional de-esterification produced 4,l',4',6'-tetrachlorogalactosucrose. This was found to be 2200 times sweeter than sugar (13). To summarise this group of compounds, it was clear that chlorination at any or all of the sites 4,1',4' and 6'- resulted in enhancement of sweetness while chlorination at the 6- position had a negative effect on sweetness. Effect of Substitution at the 6- Position Based on another series of compounds, it was found that the influence of the substituent at the 6- position was more dependent on its size than on the presence or absence of an oxygen capable of participating in hydrogen bonding. Thus 6-deoxy-4,l',6'trichlorofifalactosucrose was found to be 400 times sweeter than sucrose, the 6-0-methyl derivative was 500 times sweeter, but the 6-0-isopropyl derivative was not sweet. Presumably in this last case, the size of the isopropyl function was such as to invade an essential structural boundary, thus preventing the molecule from binding with the sweetness receptor on the taste bud. Effect of Substitution at the 2- Position Chlorination at the 2- position also had a very profound effect on sweetness, as demonstrated by the 2,6,l',6'-derivative whose production is shown in Figure 10a and 10b. Sucrose was selectively chlorinated at the 6- and 6'- positions with a yield of about 50% using methane sulphonyl chloride and dimethylformamide, initially at -20 °C for 2 hours, then at 70 °C for 10 hours. Selective acetalation with 2,2-dimethoxypropane and paratoluenesulphonic acid in dimethylformamide at 20 °C for 4 hours followed by acetylation with acetic anhydride in pyridine and removal of the cyclic acetal gave a sucrose derivative with the 1'- and 2- positions unprotected. These were chlorinated using sulphuryl chloride and lithium chloride, and the ester groups removed to yield 2,6, Γ,6'tetrachloromannosucrose. This was found to be exceedingly bitter (14) with a potency approximately equivalent to quinine, demonstrating that the presence of a hydroxyl group at the 2position was essential for sweetness.



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SWEETENERS:

OH

Figure 10.

,

,

2,6,l ,6 -Tetrachloromannosucrose


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Sucralose

Effect of Other Halogens In an attempt to determine the effect of size and electronegativity of substituents, other halogens were examined as follows. The 4,l ,6'-tribromo- and the 4,l ,4',6 -tetrabromo- derivatives (Figure 11) were produced using similar chemistry to that employed for the chlorides. The former was 800 times sweeter than sugar (J5) while the tetrabromo compound was 7500 times sweeter than sugar. Clearly the size of the bromide substituent was such as to cause a better fit of the molecule onto the taste receptor. Both the more electronegative fluoride and the larger iodide did not result in such a great enhancement of sweetness. Thus the 4,Γ,6'trifluoro-derivative was about 40 times sweeter than sucrose, whereas the 4,l\6'-triiodo-derivative was about 120 times sweeter than sugar (Figure 12). This is in contrast to the corresponding chloride which was 600 times sweeter and the corresponding bromide which was 800 times sweeter than sugar. It would appear, therefore, that bromine and chlorine have approximately the optimum molecular size and electronegativity. To further elucidate the size and electronegativity effects of substituents, some compounds containing mixed halogens were produced. For example Figure 13 shows the 4-fluoro-l ,4',6'trichloro-compound which was 1000 times sweeter than sugar in contrast to the corresponding 4,r,4',6'-tetrachloro-derivative, which was 2200 times sweeter. This indicates that the size of the 4- substituent is particularly important: the small decrease in size from chloride to fluoride at this site caused a 50% reduction in sweetness.


,

,

,

,

Effect of Size of Substituent at the 4*- Position From another series, it was found that increasing the size of the substituent at the 4'- position had a positive effect on sweetness. Thus, the 4 -iodo-4,l ,6'-trichloro-derivative (Figure 14) was found to be 3500 times sweeter than sugar as against 2200 times for the 4,r,4\6'-tetrachloro- compound. A small increase in substituent size from chloro to iodo at the 4'- position resulted in an increase in sweetness of roughly 50%. The chemistry involved in producing these mixed halogen derivatives incorporates a large number of separate processes, often employing fifteen or twenty steps. Such processes inevitably result in low yields and the majority of these compounds are clearly of academic interest only. However, following detailed evaluation including taste quality, stability and cost of manufacture, one compound was found worthy of further development. ,

,


SWEETENERS:



84

Figure 12. 4, r,6'-Trifluorogalactosucrose iodogalactosucrose (bottom)

(top); 4,1\6'-Tri-



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Sucralose

OH

OH ,

Figure 13. 4-Fluoro-l',4 ,6'-Trichlorogalactosucrose (top); 4, r,4\6'-Tetrachlorogalactosucrose (bottom)

OH

Figure 14.

OH

,

,

4 -Iodo-4 l',6 -trichlorogalactosucrose t

Identification of Sucralose The 4,r,6'-tricWoro-derivative of sucrose, shown in Figure 8, was called sucralose. Sucralose is roughly 650 times sweeter than sugar, is exceptionally stable in aqueous acidic conditions and has an excellent sweetness profile {16,17). These two attributes, stability and taste quality are of critical importance to the manufacturers of formulated foods containing high intensity sweeteners, and because of these characteristics sucralose is destined to become an outstanding commercial success.


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Explanation of the Sweetness of Sucralose In an attempt to explain the sweetness of sucralose and its relatives, Hough and Khan [18) have proposed the existence of two AH-B-X units which closely approximate the dimensions of the Kier [19) triangle. The proposed location of two of these AH-B-X triangles for sucralose is shown in Figure 15. In both cases, the equatorial 2-hydroxyl group on the galactopyranosyl unit must act as AH, the hydrogen bond donor to the receptor site. The top structure shows the involvement of the chloro group at the Γposition as the proposed hydrogen bonding acceptor, B, with the axial hydrophobic group at the 4- position in the galactopyranosyl unit acting as the third binding site corresponding to the locking group X of the Kier triangle. The lower structure shows an alternative, or probably additional AH-B-X triangle as a consequence of the relatively free rotation around the Γ- position and the overall flexibility of the molecule. In this case the 2-hydroxyl group again acts as the hydrogen bond donor, A, with the Γ-chloro group

HO

CI

OH

Figure 15. The 1',2,4-glucophore (top); the l',2,6-glucophore (bottom) (Adapted from ref. 18).



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participating as the electron accepting position, B. However, the conformation is now such as to allow the 6'-chloro group to act as the third binding site, X. It seems likely that the intense sweetness of some of these sugar derivatives is a direct consequence of the ability of the molecules to possess multiple AH-B-X binding sites. This seems to be confirmed by the intense sweetness of some of the 4'-halogenated compounds implying that a third AH-B-X triangle involving the 4'- position can participate. In conclusion, a formidable amount of research chemistry has been devoted to sucrose and the enhancement of its sweetness, particularly by the groups at Queen Elizabeth College and Tate & Lyle. The main structural features which contribute to the sweetness of this series of molecules have been elucidated, the theory being consistent with the explanation of the sweetness of many otherwise unrelated molecules. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Hough, L.; Khan R.A. In Progress in Sweeteners; Grenby, T.H., Ed.; Elsevier: London, 1989; pp 102-118. Lindley, M.G.; Birch, G.G.; Khan, R.A. J . Sci. Food Agric. 1976, 27, 140. Lindley, M.G.; Birch, G.G. J. Sci. Food Agric. 1975, 26, 117. Lee, C.K. In Developments in Food Carbohydrates, Vol 2; Applied Science: London, 1981. Hough, L.; Phadnis, S.P. Nature 1976, 263, 800. Hough, L.; Phadnis, S.P.; Tarelli, E . Carbohydr. Res. 1975, 44, 37. Fairclough, P.H.; Hough, L.; Richardson, A.C. Carbohydr. Res. 1975, 40, 285. Lee, C.K. Adv. Carbohydr. Chem. Biochem. 1987, 45, 266. Jenner, M.R.; Khan, R.A.; J. Chem. Soc. Chem. Comm. 1980, 50. Hough, L.; Phadnis, S.P.; Khan, R.A.; Jenner, M.R. UK Patent 1 543 167, 1976. Hough, L. Int. Sugar J. 1989, 91 (1082), 23. Lee, C.K. Carbohydr. Res. 1987, 162, 53. Lee, C.K. UK Patent 2 088 855, 1981. Khan, R.; Jenner, M.R. UK Patent 2 037 561, 1980. Jackson, G.; Jenner, M.R.; Khan, R.A. UK Patent 2 101 989, 1982. Jenner, M.R.; Bagley, L.; Heath, C.R. Food. Tech. Int. 1989, 273. Quinlan, M.E.; Jenner, M.R. J . Food. Sci. 1990, 55 (1), 244. Hough, L.; Khan, R.A. Trends Biochem. Sci. 1978, 3, 61. Kier, L.B. J . Pharm. Sci. 1976, 61, 1394.

RECEIVED August 27, 1990


Sucralose - American Chemical Society

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