Acylation kinetics

Keywords Acylation, Kinetic resolution, Desymmetrization, Nucleophilic catalysis... 

Acylium ion pairs, as well as the related oxonium complexes with Lewis acids, are recognized as the effective intermediates of aromatic (Friedel-Crafts) acylations. Kinetic studies apparently exclude that the electrophilic attack at the aromatic nucleus is by free acyl cations (Brown and Jensen, 1958). 

Fig. lA. tRNA acylation kinetics with total ARS preparations. Relationship between amount (in pmoles) of acylated tRNA produced in the reaction and incubation period with enzyme preparation is demonstrated. (-D—) — control ARS preparation, (—A--) — experimental preparation, (-- —) — internal control over background inclusion (BSA was added instead of enzyme in similar concentration). 

When tRNA acylation kinetics was studied on ttyptophanyl-ARS-( tRNA) and lysil-ARS-(LystRNA) pairs, linear dependence between the aminoacylation reaction and the incubation period was identified just as in the preceding case. Phenylalanine tRNA acylation kinetics also testified to linear dependence. The results of this series of experiments are presented in Fig. 1 (A, B, and C) and Fig. 2 (A, B and C). In Fig. 1, also, are presented ES effects, and in Fig. 2 the influences of IPG on the protein-synthesizing cell mechanism is presented. Figure 2 demonstrates that ES and IPG... 

Fig. 2C. tRNA acylation kinetics with total ARS preparations. Relationship between amount (in nmoles) of acylated tRNA produced in the reaction and incubation period with enzyme preparation is demonstrated. 

In acidic media, the electron sink is most often the carbocation produced from protonating the acylating agent, and therefore the sink is very hard. Attack by the Z end (harder end) of the allylic source is very fast. For enols, the Z-acylated kinetic product can be isolated. Since the Z-acylated enol is itself an allylic source (but weaker), it can be forced by more vigorous conditions to equilibrate to the more stable C-acylated product. For enamines, the Z-acylated enamine is a good acylating agent any excess of enamine will attack it and equilibrate it to the more stable C-acylated product. 

Dalaigh, C.O. and Connon, S.J. (2007) Nonenzymatic acylative kinetic resolution of Baylis-Hillman adducts. The Journal of Organic Chemistry, 72, 7066-7069 Min, S., Wen, G.Y and Hong-Bin, L. (2007) Baylis-Hillman reaction of sulfonyl aldimines or aryl aldehydes with... 

Isatin as a strategic motif for asymmetric catalysis 13CAC2131. Nonenzymatic acylative kinetic resolution of racemic amines and related compounds 12EJ01471. 

Scheme 22.1 Acylative kinetic resolution of racemic secondaiy alcohols with Fu s planar-chiral DMAP catalysts.

Scheme 22.2 Acylative kinetic resoiution of racemic secondary aicohois with a bicyciic chirai PPY cataiyst.

Miller developed peptide-based iV-methylimidazole catalysts and applied them to acylative kinetic resolution of N-acylated amino alcohol 29 (Scheme 22.6). The p-hairpin secondary structure of the peptide backbone in catalysts 30 and 31 constitutes a unique environment for effective asymmetric induction. Acylative kinetic resolution of 29 with acetic anhydride in the presence of catalyst 31 proceeded with high s values (s = up to 51). The asymmetric acylation was further extended to remote asymmetric desymmetrisation of a o-symmetric nanometer-scale diol substrate, 32 (Scheme 22.7). Catalyst 33 enabled the enantiotopic hydrojq groups in 32 to be distinguished even though they are located 5.75 A from the prochiral stereogenic centre, and 9.79 A from each other. 

Scheme 22.6 Acylative kinetic resolution of racemic secondary alcohols with Miller s peptide-based iV-methylimidazole catalysts.

According to the reaction conditions, PhCOCl reacts with ketone enolates to lead to enol benzoates (0-acylation, kinetic product) or p-diketones (C-acylation, thermodynamic product) (eq 9). ... 

---