Self acyl groups

Self-condensation of the substituted propiophenone, 15, by the pinacol reaction proceeds to give the glycol, 16, as the meso isomer. (If it is assumed that the transition state for this reaction resembles product, this stereoselectivity can be rationalized on the grounds of steric interaction compare A, which leads to the observed product, with B.) Dehydration under very specialized conditions (acetyl chloride, acetic anhydride) affords the bisstyrene-type diene (17). Removal of the acyl groups by means of base affords the synthetic estrogen, dien-... 

Secondary amines can be acylated with acyl groups bearing no a-hydrogens. Deprotonation next to the nitrogen atom and introduction of electrophiles allow the oxidation of that position. Eaiiy work showed that these anions underwent self-condensation. For example, A(,lV-dimethylbenzainide can be deproton-ated with lithium 2,2,6,6-tetramethylpiperidide (LITMP) to give Af-methyl-lV-phenacylbenzamide in 60% yield. It is clear that lithium ion is crucial to the success of this reaction. Ctown ethers prevent the reaction,and recent kinetic studies show intermediate lithium complex formation. ... 

The self-assembly properties of amphiphillic cyclodextrins esteiified at the 2- and 3-positions with lipophillic groups were examined in py-rfs and thf-rfs with the aid of Eu(fod)3. In py-fi/s the cyclodextrin molecules dimerized as indicated in Figure 2a, and the europium ion caused shifts and broadening of the primary methylene resonances. In iM-di, the cyclodextrin associates had the alignment shown in Figure 2b, and the europium shifted the methyl and methylene resonances of the fatty acyl groups". ... 

Figure 10.26 demonstrates how the neighbouring acyl group can actively participate in a mechanism to open up the lactam ring. Thus, penicillin G has a self-destruct mechanism built into its structure. 

The hydrogen atom in the NH group can also be substituted by the acyl residue.229,267-272 Reaction of benzoyl chloride with 5,5-diethyl- and 5-ethyl-5-phenylbarbituric acids proceeds in two steps. In the first step, benzoyl chloride reacts with N-nonsubstituted barbiturates, yielding N-monosub-stituted derivatives (99 R1 = Et, R2 = Et, Ph R3 = PhCO), which then undergo the self-acylation reaction [Eq. (7)]. 

Under the above mentioned conditions of the formation and peptide reaction of symmetric anhydrides, we have no indications for over-reactions like self-acylation at the urethane-masked amino group of the activated compound [71], as was occasionally observed by Merrifield [149]. 

At the plants pH values (3-7), anthocyanins are mostly present as colorless structures. But a stabilizing mechanism takes place. Such effects are provided by complexation of the color form with other species present in the solution, which can be either an identical anthocyanin molecule (self-association), one of its aromatic acyl groups substituents (intramolecular copigmentation) or another molecule (intermolecular copigmentation). 

Other heterocyclic systems studied include the phenazines [61], triazoles and imidazoles [89] and aziridenes [88]. In the last named the NH bands are more typical of alkyl amines and absorption is between 3360 and 3340 cm" in the vapour. In the liquid state self-association lowers this to 3250—3240 cm" for alkyl aziridenes. When acyl groups are substituted in the ring two NH bands appear corresponding to two different invertomers. Marked variations in both frequency and intensity have been noted by Barr and Haszeldine [60] in NH groups with fluorine substituents at the a-carbon atom. The NH frequency of Z /s-2-2-2-trifluoro-dimethylamine, for example, is 3460 cm", and the extinction coefficient is about 50 times greater than that of dimethylamine itself. 

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

An important group of acylation reactions involves esters, in which case the leaving group is alkoxy or aryloxy. The self-condensation of esters is known as the Claisen condensation.216 Ethyl acetoacetate, for example, is prepared by Claisen condensation of ethyl acetate. All of the steps in the mechanism are reversible, and a full equivalent of base is needed to bring the reaction to completion. Ethyl acetoacetate is more acidic than any of the other species present and is converted to its conjugate base in the final step. The (3-ketoester product is obtained after neutralization. 

Undesirable intermolecular reactions can be avoided during certain synthetic conversions. Thus it is often useful to carry out C-alkylation and C-acylation of compounds that form enolate anions, for example, esters with a-hydrogens. Such reactions are often complicated by self-condensation since the enolate anion can attack the carbonyl group of a second ester molecule. Attachment of the enolizable ester to a polymer support at low loading levels allows the alkylation and acylation reactions (Eq. 9-79) to be performed under... 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

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