Acid-catalysed exchange

A series of disteroidal enol ethers [e.g. (357)], which have, in some cases, long-lasting uterotrophic activity, has been prepared by acid-catalysed exchange... 

The starting material for these aliphatic Claisen rearrangements consists of ethers with one allyl and one vinyl group. We need now to consider how such useful molecules might be made. There is no problem about the allyl half—allylic alcohols are stable easily made compounds. But what about the vinyl half Vinyl alcohols are just the enols of aldehydes (MeCHO). The solution is to use an acetal of the aldehyde in an acid-catalysed exchange process with the allylic alcohol. 

The mechanism shown in eqns. (8) and (9) is applicable to the isotope exchange of many carbon acids. The acid catalysed exchange of hydrogen atoms in aromatic molecules (ArH) follows a different mechanism [14, 27] shown in eqns. (13) and (14), viz. 

Electrophilic substitution at carbon can be effected much more readily with the three oxy-pyridines than with pyridine itself, and it occnrs ortho and para to the oxygen fnnction, as indicated below. Acid catalysed exchange of 4-pyridone in denterinm oxide, for example, gives 3,5-didenterio-4-pyridone, via C-protonation of the neutral pyridone. ... 

Measurements of acid-catalysed exchange, or of protonolysis of other groups, for example silicon, or mercury, show the rate of proton attack at C-2 to be about 1000 times faster than at C-3. The for... 

In the acid-catalysed exchange of optically active s-butyl alcohol (1), Bunton et al. (1955b) found that kex(.hjkT c = 0-5 (see Table 1), which is compatible with the direct displacement mechanism (b). However, other... 

As might be expected, tertiary alcohols undergo a fairly rapid acid-catalysed exchange of oxygen with water. There are, however, several pathways by which this can be achieved. In the case of t-butyl alcohol (13) these various reactions are ... 

An interesting tertiary alcohol is pinacol (16) in which both the rate of acid catalysed exchange and of rearrangement to pinacolone can be measured. Bunton et al. (1958b) found that both reactions follow the Hammett acidity function H0. The reaction is postulated to proceed through a carbonium ion as follows ... 

Rate Constants and Activation Parameters for the Acid Catalysed Exchange of -Substituted Benzophenones, p-XCeBU. CO18. CeHs, with Water in 20 80 (v/v) Aqueous Dioxan (0-01m HCIO4)... 

Aleksankin and Gragerov (1961) made a semi-quantitative study of the acid-catalysed exchange of some ring-substituted benzaldehydes and... 

Bunton et al. (1960) compared the acid-catalysed exchange rates of benzoic and mesitoic acids. For benzoic acid, the exchange rate is proportional to the stoicheiometric acid concentration, suggesting that a water molecule participates in the transition state. This interpretation is consistent with the large negative entropy of activation, — 30 e.u. The energy of activation is 16 kcal. mole-"1 which is in the range typical of... 

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