Carbonyl group attack

This reaction is an example of a Dieckmann cyclization The anion formed by proton abstraction at the carbon a to one carbonyl group attacks the other carbonyl to form a five membered ring... 

In addition to the typical behavior exhibited in a wide array of reactions by the carbonyl group, attack by nucleophiles and reactions leading to C—S single bonds are generally favored. The energetic reasons for this behavior have been discussed in Section II. Likewise, thiocarbonyl groups show a rich variety of cycloaddition reactions, ranging from... 

The mechanism of imine formation (Key Mechanism 18-5) begins with an acid-catalyzed nucleophilic addition of the amine to the carbonyl group. Attack by the amine, followed by deprotonation of the nitrogen atom, gives an unstable intermediate called a carbinolamine. 

When chelation is possible, this is the conformation to consider—the one with the carbonyl O and the other chelating atom almost eclipsing one another. It is the most populated because it is stabilized by the chelation, and it is also the most reactive, because the Lewis-acidic metal atom increases the reactivity of the carbonyl group. Attack is still along the less hindered pathway, but this now leads to the other face of the carbonyl group, and the stereochemical outcome is reversed. 

This simple hope is not much supported by the results with the anhydride 4.172. The X-substituents, conjugated to the a carbonyl, ought to make the reaction intrinsically selective for attack at Cp, and the Z-substituent for attack at CQ. The coefficients in the LUMO match this expectation, except for R = Me, when the values are barely different. In spite of the fact that C is still the sterically more hindered carbonyl group, attack is selectively at C for all three substituents (NaBEL, a (3 87 13, 57 43 and 83 17, respectively).399,400... 

Type-Ill cyclizations are initiated by conversion of an acetal, ketal or enol ether into an oxonium ion. Such an activated carbonyl group attacks an olefin intramolecularly in an exo reaction. Mono- and trialkylated double bonds form new C-C bonds regioselectively at the least hindered terminus according to Markovnikov s rule, and in this case endo cyclization products can be produced25. It is worth noting that acetals are also used as initiators in cation olefin cyclizations (see Section 1.5.5.3.)26. Use of activated acetals in Lewis acid catalyzed C-glycoside synthesis is also known 12c 12e. 

Formulate a possible mechanism for the acid-catalyzed dehydration of 2,5-hexanedione to 2,5-dimethylfuran. (Hint The crucial ring closure is accomplished by the oxygen of one carbonyl group attacking the carbon of the second. See also Section 18-2.)... 

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