Dehydration biological

The aqueous layer from the ester column distillate, the raffinate from washing the ester, and the aqueous phase from the dehydration step are combined and distilled in the alcohol stripper. The wet alcohol distillate containing a low level of acrylate is recycled to the esterification reactor. The aqueous column bottoms are incinerated or sent to biological treatment. Biological treatment is common. 

Naphthalenedicarboxylic Acid. NaphthaUc acid readily dehydrates on heating to 1,8-naphthalenedicarboxyhc acid anhydride [81-84-5] (naphthahc anhydride) (41). The anhydride and its imide naphthaUmide [81-83-4] (R = H) (42) are intermediates for important dyes, pigments, optical bleaches, and biologically active compounds. 

Conversion of the C-2 amide to a biologically inactive nitrile, which can be further taken via a Ritter reaction (29) to the corresponding alkylated amide, has been accomphshed. When the 6-hydroxyl derivatives are used, dehydration occurs at this step to give the anhydro amide. Substituting an A/-hydroxymethylimide for isobutylene in the Ritter reaction yields the acylaminomethyl derivative (30). Hydrolysis affords an aminomethyl compound. Numerous examples (31—35) have been reported of the conversion of a C-2 amide to active Mannich adducts which are extremely labile and easily undergo hydrolysis to the parent tetracycline. This reverse reaction probably accounts for the antibacterial activity of these tetracyclines. 

A closely related oxygenated heterocyclic system devoid of acidic groups interestingly shows quite different biological activity. Thus, condensation of the benzofuran hydroxyketone 66 with ethyl thiomethyl acetate (67) probably proceeds initially by formation of the acylation product 68. Intramolecular dehydration leads to formation of a pyran ring. There is thus obtained the hypo-cholesterolemic agent timefurone (69) [14],... 

In biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in which the -OH is positioned two carbons away from a carbonyl group. In the biosynthesis of fats, for instance, /3-hydroxybutyry) ACP is converted by dehydration to tram-crotonyl ACP, where ACP is an abbreviation for acyl carrier protein. We ll see the reason for this requirement in Section 11.10. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

A third important reaction of alcohols, both in the laboratory and in biological pathways, is their dehydration to give alkenes. The C-0 bond and a neighboring C—H are broken, and an alkene tt bond is formed. 

As noted previously in Section 11.10, biological dehydrations are also common and usually occur by an ElcB mechanism on a substrate in which the -OH group is two carbons away from a carbonyl group. An example occurs in the biosynthesis of the aromatic amino acid tyrosine. A base first abstracts a proton from the carbon adjacent to the carbonyl group, and the anion intermediate... 

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

Omission of the 17-hydroxyl group in the 6,9-dihalo compounds apparently does not lead to loss of biological activity. Dehydration of the 6a-fluoro-17-desoxy intermediate 272 by means of NBS in pyridine... 

Glauert, A. M. (1975). Fixation, Dehydration and Embedding of Biological Specimens. North-Holland, Amsterdam,the Netherlands. 

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