Preparation enediolates

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Whereas condensation of a-hydroxy ketones such as benzoin and acetoin on heating with formamide [240] or ureas in acetic acid [239, 242] to form imidazoles such as 769 or 770 is a well known reaction, only two publications have yet discussed the amination of silylated enediols, prepared by Riihlmann-acyloin condensation of diesters [241], by sodium, in toluene, in the presence of TCS 14 [241, 242]. Thus the silylated acyloins 771 and higher homologues, derived from Riihl-... 

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

The dehydration of the enediols is a reaction subject to general acid-base catalysis. The deoxyaldosulose 7a has been isolated from 3-O-benzyl-D-glucose34 and from D-fructose35 after treatment with alkali, and from D-fructose35 and various Amadori products8 after treatment with acid. The most successful preparation of 7a has been by way of amine addition compounds an improved procedure has been reported.36 Compound 9a has been isolated as a product of the alkaline treatment of both cellobiose37 and maltose.38 The isolation of 10a has not been reported, but it has been synthetically39 prepared. 

Certain polyaryl-substituted pyrroles can be prepared by condensation of a benzoin and a benzyl aryl ketone with ammonium acetate (equation 128) (38JOC361, 71ZOR1264). The details of the mechanism are not known but one assumes enamines are involved and that the carbonyl and hydroxymethylene oxidation level of the benzoin are probably interconvertible via an enediol or related substance. A possible formulation is given in Scheme 15. 

Owing to the narrow specificity of the DHAP aldolases for the donor substrate DHAP (41), direct access to this essential compound is vital to the development of synthetic applications. Commercial offers of the compound, however, are prohibitively expensive for preparative-scale applications. A further problem is that 41 is relatively unstable in solution and, particularly at elevated pH values, readily decomposes according to an Elcb pathway via an enediol intermediate... 

The best reagents for reduction of olefinic aldehydes to olefinic alcohols are lithium aluminum hydride and sodium borohydride. Crotyl alcohol, CHjCH = CHCHjOH, and cinnamyl alcohol, CjH,CH =CHCHjOH, have been prepared in excellent yields. Cinnamyl alcohol is further reduced at higher temperatures to hydrocinnamyl alcohol. Citral, (CHj)jC =CHCHjCHjC(CH3)=CHCHO, may be selectively reduced to the ctOTesponding dienol by catalytic hydrogenation over platinum catalyst. A new method for the preparation of enediol esters of the type... 

The preparation of enols of simple aldehydes and ketones has been achieved by rDA reactions under FVP conditions. Equation (17) illustrates the formation of enols by this method. The percentage of enol reported shows that the enols of aldehydes appear to be isolated more readily than those of the ketones. The last two entries are preparations of the simplest enediols, ( )- and (Z)-ethylene-l,2-diols. A very similar process was used to unmask the protected ethylene cycloadduct (23) and produce 2-hydroxybu-tadiene (24), an elusive enol form of the more stable unsaturated ketone, as given by equation (18). ... 

Formation of the enediol(ate) of RuBP is readily assayed on the basis of exchange of solvent protons with the C3 proton of substrate (20-22). The six-carbon intermediate of the carboxylation pathway (II in Fig. 1) can be prepared by rapid quench after mixing equimolar amounts of RuBP and the carboxylase in the presence of 14CO2 (23). Availability of this labeled intermediate allows determination of an enzyme s commitment to forward processing in the carboxylation step. Decomposition, via decarboxylation, is observed as a decrease in radioactivity that can be stabilized by borohydride, whereas forward catalysis is equated with an increase in acid-stable radioactivity. 

A series of 3-hydroxy-2,5-bis(p-alkylphenyl and p-alkoxyphenyl)pyrazines has been prepared, each in small yield, from p-substituted phenylglyoxals and ammonia (283) benzil and liquid ammonia (and ammonium chloride or potassium amide) have been shown to give low yields of tetraphenylpyrazine (284) and benzoyl-formoin, which has been claimed to have an enediol [PhCOC(OH)=C(OH)COPh] content of 90%, and ammonia gave 24rydroxy-3,5-diphenylpyrazine (285). 

In the second place, oxidation by Fehling s or Tollens reagent cannot be used for the preparation of aldonic acids (monocarboxylic acids) from aldoses. Both Fehling s and Tollens reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure. 

When the cyclic enediol ester of phosphoric acid (87 equation 56) is reacted with the diol (88), the cyclic phosphate (89) is obtained in a single step. Use of the chlorophosphoryl analog (90 equation 57) effects the two-step preparation of the phospholiposterol (91).The pyrophosphate-type compound (92) has a similar reactivity. 

Bis(tributyltin) enediolates can be prepared from the reaction of acyloins with bis(tributyltin) oxide or tributyltin methoxide (equation 14-61).112... 

The carbonylation of the sodium salt of an enediolate, prepared in situ from PhC(0)C(0)Ph, with COClj gives the 1,3-dioxol-2-one derivative [270] ... 

The studies of Ban and Wakamatsu culminated in the preparation of three natural compounds from a single synthetic route (Scheme 1.15). The enediol bis silyl ether 63 was converted to the dianion and immediately alkylated with l-iodo-3-butanol to give glycol 64 as a mixture of diastereomers in 87% yield. Diol fragmentation with lead tetraacetate afforded keto lactone 65 in quantitative yield. Formation of the dithioketal and subsequent Raney nickel desulfurization then gave 66 (81%). Macrocyclic lactone 66 is the simple natural product... 

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