Dehydration involving intermediate ester formation

The acidity function criterion offers little prospect as a basis on which the reaction mechanism can be assigned as both 1-methylcyclohexanol and its acetate form olefins by unimolecular carbonium-ion processes which show a unit dependence on in acetic acid containing aqueous sulphuric acid . The ten-fold greater reactivity of the ester was explained in terms of the greater electronegativity of the protonated ester function than the oxonium ion, which more than counterbalances the lower basicity of the acetate than the alcohol. 

Kinetic studies concerning the intermediacy of esters in the dehydration of alcohols are few and evidence is less soundly based on product analysis. An interesting diversity of reaction products, suggestive of varying mechanisms, has been reported for the dehydration of a-terpineol with various acidic catalysts, wz.  

For carbonium-ion processes, unless configuration is maintained by ion association, it is unsound to assume stereochemistry of elimination from product analysis, unless it is independently shown that racemisation at the carbon bonded to the hydroxy group is slower than olefin formation. Decompositions of ester intermediates should show a preference for anti stereospecificity if they are base-induced and syn-clinal stereospecificity when they occur under thermal conditions. Typical results on the dehydration of two stereoisomers in the rigid steroid series support the above generalisations 

When dehydration is accomplished with perchloric acid in acetic acid, both alcohols give the enJo-olefin as would be expected for carbonium ion processes. An E2 reaction of an oxonium ion, by analogy with the orientation observed for base-induced eliminations of 1-methylcycloalkyl trimethyl- 

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Oxazolo[2,3-fe]oxazoles (191 R = CF3, n-C7F,5) have been prepared by acid-catalyzed dehydration of A,A-bis(hydroxyalkyl)amides <82JFC(2l)359>. The imidazo[l,5-6f]imidazole (193) was prepared by reaction of aminoacetonitrile with excess trimethylorthoformate in the presence of catalytic amounts of formic acid <84JOCl2i2>. The reaction is believed to involve the initial formation of the imidate (192) which reacts successively with aminoacetonitrile and the ortho-ester to give the observed product. Pyrazolo[5,l-Z>]oxazoles (198) have been synthesized by reaction of hydrazino alcohols (195) with the Michael adduct (194), prepared from A-isobutylidene-r-butylamine and dimethyl-methoxymethylene malonate <93JHC1529>. The reaction is believed to involve cyclization of the intermediate hydrazones (196) to give pyrazoles (197), which subsequently cyclize to the bicyclic system with loss of dimethyl malonate. 

Whereas the C2—C4 alcohols are not carboxylated under the usual Koch-Haaf conditions, carboxylation can be achieved in the HF-SbF5 superacid system under extremely mild conditions.400 Moreover, Olah and co-workers401 have shown that even methyl alcohol and dimethyl ether can be carboxylated with the superacidic HF-BF3 system to form methyl acetate and acetic acid. In the carboxylation of methyl alcohol the quantity of acetic acid increased at the expense of methyl acetate with increase in reaction time and temperature. The quantity of the byproduct dimethyl ether, in turn, decreased. Dimethyl ether gave the desired products in about 90% yield at 250°C (90% conversion, catalyst/substrate ratio =1 1, 6h). On the basis of experimental observations, first methyl alcohol is dehydrated to dimethyl ether. Protonated dimethyl ether then reacts with CO to yield methyl acetate [Eq. (5.154)]. The most probable pathway suggested to explain the formation of acetic acid involves the intermediate formation of acetic anhydride through acid-catalyzed ester cleavage without the intervention of CO followed by cleavage with HF [Eq. (5.155)]. 

Selective removal of bromine from 3/3,5,6/3-tribromo-5a-cholestane to give 3/3-bromocholest-5-ene was achieved by reaction with [T -C5H5Cr(N02)2]2. The reactions of 11/3-hydroxy-steroids with dialkylaminosulphur trifluorides depend on the substitution at C-9 and involve the formation of intermediate (11) (Scheme 1) (see ref. 232). Selective dehydration with FeCls adsorbed on silica gel allowed the conversion of 5a-cholestane-3/3,5-diol into cholesterol (80%) and 3i3-acetoxy-5a-cholestane-5,25-diol into 3/S-acetoxycholest-5-en-25-ol (72%). Other examples and additionally the hydrolysis of 5,6a-epoxy-5o -cholestan-3/3-ol to the 3/3,5a,6/8-triol (90%) were reported. Chromatographic alumina is reported to effect smooth elimination of sulphonic acids from the esters with less than normal rearrangement. Thus lanosteryl tosylate and cycloartenyl tosylate gave the respective A -compounds in yields of 90% and 45% respec-... 

The extremely unusual 1,5-oxazocine derivative (11) is formed from (12) by a sequence of reactions involving hydrolysis of the formate ester with NaHCOs, followed by ring-opening of the four-membered ring, a 1,4-dehydration reaction, and finally rearrangement of the unstable intermediate on attempted isolation by chromatography on silica. Ozonolysis of 1,4-dihydronaphthalene in methanol gives the expected peroxide. ... 

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