HOCH2 Cl H C C H H HO (R)-3-MCPD H HO - H2O, - Cl in .NET

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(R)-hydroxymethyloxirane
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(S)-3-MCPD
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(S)-hydroxymethyloxirane
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Figure 6.3.11 Dehydrochlorination of 3-chloropropane-1,2-diol to hydroxymethyloxirane.
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X=OH, (R)-hydroxymethyloxirane X=Cl, (R)-chloromethyloxirane
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X=OH, (S)-hydroxymethyloxirane X=Cl, (S)-chloromethyloxirane
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Figure 6.3.12 projection.
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Structures of optically active hydroxymethyloxiranes in the Fischer
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Analogously to other vicinal chlorohydrins, (R)-3-MCPD (R1 = H, R = CH2OH, R3 = R4 = H) gives (R)-hydroxymethyloxirane by base-promoted ring closure while (S)-3-MCPD (R1 = CH2OH, R2 = R3 = R4 = H) yields enantiomeric (S)-hydroxymethyloxirane (Fig. 6.3.11). Thus, a racemic mixture of both isomers of hydroxymethyloxirane is obtained from the treatment of the acid-HVP with alkali. Structures of the chiral hydroxymethyloxiranes (also known as oxiranemethanols, oxiranylmethanols, or glycidols) in the Fischer projection are given in Fig. 6.3.12. The hydroxymethyloxiranes produced are not stable but the rate of 3MCPD decomposition is higher than the rate of the decomposition of hydroxymethyloxiranes. As a result, hydroxymethyloxiranes accumulate in some acid-HVPs treated with alkali (55). The concentration of hydroxymethyloxirane depends on the temperature and pH of the hydrolysate. For example, at 65 C and pH 9 the maximum concentration of hydroxymethyloxirane, equal to about one-third of the quantity of the original 3-MCPD concentration, is reached within 5 min. Epoxides are very reactive compounds in which the epoxide ring can open by a variety of nucleophiles (water, alcohols, thiols, amines, acids, etc.). Nucleophilic ring opening of epoxides has many of the features of an SN2 reaction (inversion of con guration is observed at the carbon at which substitution occurs; Fig. 6.3.13). Asymmetrical epoxides are attacked at the less substituted,
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CONCLUSION
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R C O
R C O
Y C R 4 R
R C O
Y C R 4 R
R C HO
Y C R 4 R
epoxide
transition state
alkoxide
-substituted alcohol
Nucleophilic ring opening of 1,2-epoxides.
less sterically hindered carbon of the ring from the side opposite to the carbon oxygen bond (45). In HVP, the oxirane rings of both optically active hydroxymethyloxiranes are opened mostly by the action of water with the formation of glycerol (55). Other nucleophiles present in HVP such as HCl, chlorides, ammonia, amino acids, and alcohols open the oxirane rings of hydroxymethyloxiranes to some extent. For example, the reaction of (R)-glycidol with HCl gives rise to (R)-chloropropane-1,2-diols. The major reaction product with ammonia was (S)-3-aminopropane-1,2-diol while (R)-3-aminopropane-1,2-diol formed from (S)-glycidol. Their racemic mixture has been found in the acid-HVP (56). Reactions of hydroxymethyloxiranes with amino group of amino acids in alkaline media lead to N-(2,3-dihydroxypropyl)amino acids (57), the reaction of hydroxymethyloxiranes with glycerol yields polyglycols (55). Accordingly, symmetric cyclization of the prochiral 2-MCPD (elimination of proton occurs from either C-1 or C-3 hydroxyl group, followed by ring closure) always leads to a racemic mixture of both optically active hydroxymethyloxiranes. It is evident (Table 6.3.2) that 2-MCPD is more stable in alkaline media than 3-MCPD. In cases 3-MCPD is totally decomposed, 2MCPD may still be found in the alkaline-treated HVPs and even becomes the major chloropropanol of these products (54). 6.3.5.2.2 Dichloropropanols Dehydrochlorination of 1,3-DCP involves symmetric cyclization (base-promoted ring closure), which leads to racemic intermediate chloromethyloxirane (epichlorohydrin) (58). (S)-Chloromethyloxirane is obtained by dehydrochlorination from the optically active (R)-2,3dichloropropan-1-ol, while (S)-2,3-DCP forms (R)-chloromethyloxirane. The result of cyclization of racemic 2,3-DCP, which occurs in acid-HVPs, is racemic chloromethyloxirane (59). The epoxide ring of the reactive intermediate chloromethyloxirane then rapidly opens with water to give 3-MCPD. Racemic chloromethyloxirane converts to racemic 3-MCPD (60). The reactions of 3MCPD in alkaline media are described previously.
CONCLUSION
Since 1989/1990, there have been a number of achievements including continual monitoring of the amount of 3-MCPD in acid-HVPs and imported soy
CHLOROPROPANOLS
sauces and issuing health hazard alerts where the concentration exceeded the guidelines. New technological approaches and new ingredients have been introduced to food manufacturers as alternatives to the classical procedures for acid-HVPs and they were adopted to minimize the concentration of 3MCPD in the nal product. In March 2001, the regulatory limit of 0.02 mg/kg, based on 40% dry matter content for 3-MCPD in acid-HVP and soy sauce, was adopted and came into force in April 2002. The results show a continuous decrease in frequency of contamination of acid-HVPs. The soy sauce contamination by 3-MCPD can be prevented by approaches based on GMP used for the production of acid-HVPs and by ensuring that 3-MCPD free acid-HVPs are used.