Dehydration intermediates

With secondary alcohols the picture is different. By measuring rates of hydration, isomerization, dehydration, and exchange, in the system of butenes and 2-butanol, Manassen and Klein (7S) proved that the hydration-dehydration intermediate in dilute acid solution is equally bonded to two water molecules ... 

Figure 35 Conversion of LctA to the mature form of lacticidin 481 catalyzed by LtcM. (a) Thioether bridge formation in lantibiotic biosynthesis leading to dehydration (-18 Da), (b) MS time course of isotopically depleted LctA conversion to lacticidin 481 by LtcM shows that little dehydration intermediates are observed.

Several examples of aldol addition and condensation are given in Scheme 7.3. Entries 1 and 2 are typical aldol reactions of aldehydes, with and without dehydration. The reaction in Entry 1 was done with 15% KOH in aqueous solution at room temperature. The condensation reaction in Entry 2 was carried out at 80°-90°C with 1M NaOH. Entries 3 and 4 show addition and condensation reactions of acetone. Entry 3 features a clever way of overcoming the unfavorable equilibrium of the addition step. The basic catalyst is contained in a separate compartment of a Soxhlet extractor. Acetone is repeatedly passed over the basic catalyst by distillation and the condensate returns to the flask. Since there is no catalyst present in the flask, the adduct remains stable. The concentration of the addition product builds up as the more volatile acetone distills preferentially. The acid-catalyzed condensation in Entry 4 is carried out similarly. The acetone is continuously passed over the acidic resin, and the reaction is driven forward by the stability of the conjugated condensation product. In Entry 5, the final product is a (3-chloroketone, presumably formed by addition of HCl to a dehydrated intermediate. 

Recently, Barbas and Bui looked at the one-pot proline-catalysed Robinson annulation sequence to see if proline can catalyse the entire reaction sequence they found that substituted hydroxyproline derivatives 17-19 were viable catalysts for the reaction, producing the desired products in 60 to 70% ee. The corresponding smaller analog 20 gave the product in less than 10% ee. Additional work looked at analogs in which the carboxylic acid was reduced or replaced by other functional groups these compounds, which are not shown, were not viable catalysts for the entire reaction sequence. Instead, the intermediate Michael adduct or cyclized, but not dehydrated, intermediates were observed. 

These prodncts are formed through nucleophiUc attack followed by Claisen condensation (intermediate A), intramolecular cyclization and dehydration (intermediate B), and then by aromatization (after hydrolysis and decarboxylation) through involvement of the phenolic hydroxy group. At the same time, chromone 130a reacts with cyanoacetamide, A-methyl cyanoacetamide, and cyanoacetohydrazide in the presence of sodium ethoxide, affording 2-pyridones 212 in good yields [104] (Scheme 68). 

Quantitative data, when appropriate, on inhibitor residuals measured in fluid samples collected at suitable points in the field, such as dehydrators, intermediate sample points, and inlet separators should be provided. Knowledge of the residual inhibitor levels, as well as the iron and manganese levels allows the most economic rates to be established for full protection of a particular... 

CHa CHlCH CHO. Colourless lachrymatory liquid with a pungent odour. B.p. 104 "C. Manufactured by the thermal dehydration of aldol. May be oxidized to crotonic acid and reduced to crolonyl alcohol and 1-butanol oxidized by oxygen in the presence of VjOj to maleic anhydride. It is an intermediate in the production of l-butanol from ethanol. 

D-Mevalonic acid is the fundamental intermediate in the biosynthesis of the terpenoids and steroids, together classed as poly-isoprenoids. The biogenetic isoprene unit is isopentenyl pyrophosphate which arises by enzymic decarboxylation-dehydration of mevalonic acid pyrophosphate. D-Mevalonic acid is almost quantitatively incorporated into cholesterol synthesized by rat liver homogenates. 

Peachey L D, Ishikawa H and Murakami T 1996 Correlated confocal and intermediate voltage electron microscopy imaging of the same cells using sequential fluorescence labeling fixation and critical point dehydration Scanning Microsc. (SuppI) 10 237-47... 

Dehydration of the intermediate p-alkoxy- or p-hydroxy ketone can also serve to drive the reaction to the right. 

Another version of the o-aminobenzyl anion synthon is obtained by dilithi-ation of A-f-Boc-protected o-alkylanilines. These intermediates are C-acylated by DMF or A"-methoxy-At-melhyl carboxamides, leading to either 3- or 2,3-disubstituted indoles. In this procedure dehydration is not spontaneous but occurs on brief exposure of the cyelization product to acid[4]. Use of CO as the electrophile generates oxindoles. 

Thiazole carboxylic acid hydrazides were prepared in a similar way (444, 445). Thus by refluxing thioacetamide or thiobenzamide with y-bromoaceto acetic ester arylhydrazones (83) for several hours in alcohol the 4-carboxythiazole derivatives (84) listed in Table II-ll were obtained (Scheme 36) (656). This reaction is presumed to proceed via dehydration of the intermediate, thiazoline-S-oxide. 

Similarly, 5-thiazole alkanoic acids and their salts are obtained from thioamides and /3-halo -y-keto acids (695). Thus thioarylamides condensed with 3-aroyl-3-bromopropionic acid (88) in isopropanolic solution in the presence of Na COs give first 4-hydroxy-2-aryl-A-2-thiazoline-5-acetic acid intermediates (89), which were dehydrated in toluene with catalytic amounts of p-toluene sulfonic acid to 2,4-diaryl-5-thiazole acetic acid (90) (Scheme 39) (657), with R = H or Me Ar = Ph, o-, m- or p-tolyl, o-, m-, or P-CIC6H4, 0-, m-, or p-MeOC(iH4, P-CF3C6H4, a-thienyl, a-naphthyl (657). 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

Write a structural formula for the carbocation intermediate formed in the dehydration of each of the alcohols in Problem 5 14 (Section 5 10) Using curved arrows show how each carbocation is deprotonated by water to give a mixture of alkenes... 

As noted earlier (Section 4 10) primary carbocations are too high m energy to be intermediates m most chemical reactions If primary alcohols don t form primary car bocations then how do they undergo elimination s A modification of our general mech amsm for alcohol dehydration offers a reasonable explanation For primary alcohols it is... 

Like tertiary alcohols secondary alcohols normally undergo dehydration by way of carbocation intermediates... 

Section 5 12 Secondary and tertiary alcohols undergo dehydration by an El mecha nism involving carbocation intermediates... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Secondary amines are compounds of the type R2NH They add to aldehydes and ketones to form carbmolammes but their carbmolamme intermediates can dehydrate to a stable product only m the direction that leads to a carbon-carbon double bond... 

Reaction with secondary amines (Sec tion 17 11) Isolated product IS an en amine Carbinolamine intermediate can not dehydrate to a stable imine... 

Intermediates. 3,4-Dihydro-2H-pyran [110-87-2] is prepared by a ring-expanding dehydration of tetrahydrofurfuryl alcohol. It is used as a protecting agent for hydroxyl compounds and as an intermediate. 2-Methylfuran is a chemical intermediate for 5-methylfurfural [620-02-0] (151) and... 

The reaction of adipic acid with ammonia in either Hquid or vapor phase produces adipamide as an intermediate which is subsequentiy dehydrated to adiponitrile. The most widely used catalysts are based on phosphoms-containing compounds, but boron compounds and siHca gel also have been patented for this use (52—56). Vapor-phase processes involve the use of fixed catalyst beds whereas, in Hquid—gas processes, the catalyst is added to the feed. The reaction temperature of the Hquid-phase processes is ca 300°C and most vapor-phase processes mn at 350—400°C. Both operate at atmospheric pressure. Yields of adipic acid to adiponitrile are as high as 95% (57). 

The principal steps in the mechanism of polyisoprene formation in plants are known and should help to improve the natural production of hydrocarbons. Mevalonic acid, a key intermediate derived from plant carbohydrate via acetylcoen2yme A, is transformed into isopentenyl pyrophosphate (IPP) via phosphorylation, dehydration, and decarboxylation (see Alkaloids). IPP then rearranges to dimethylaHyl pyrophosphate (DMAPP). DMAPP and... 

---