Model benzaldehyde

A model developed by Leksawasdi et al. [11,12] for the enzymatic production of PAC (P) from benzaldehyde (B) and pyruvate (A) in an aqueous phase system is based on equations given in Figure 2. The model also includes the production of by-products acetaldehyde (Q) and acetoin (R). The rate of deactivation of PDC (E) was shown to exhibit a first order dependency on benzaldehyde concentration and exposure time as well as an initial time lag [8]. Following detailed kinetic studies, the model including the equation for enzyme deactivation was shown to provide acceptable fitting of the kinetic data for the ranges 50-150 mM benzaldehyde, 60-180 mM pyruvate and 1.1-3.4 U mf PDC carboligase activity [10]. 

Based on the model described in the previous section, an optimization strategy has been developed for substrate feeding with maximum PAC production as its objective fimction. A molar ratio of pyruvate benzaldehyde of 1.2 1 was maintained in the feed as some of the pyruvate is converted also to by-products acetaldehyde and acetoin. 

The optimal feeding profile based on the model is shown in Figure 3 and the simulation profiles are shown in Figure 4 for initial substrate concentrations of 90 mM benzaldehyde and 108 mM sodium pyruvate, and initial PDC activity of 4.0 U ml carboligase. Feeding was programmed at hourly intervals and the initial reaction volume would increase by 50% by the end of the simulated biotransformation. 

Thirteen different chiral diol ligands were used (Scheme 25), leading to a catalyst library of 104 members.121 In a model reaction benzaldehyde (51), (R = Ph) was used as the carbonyl component, HPLC being used to ascertain the enantiopurity of (92). Initially 1 mol.% of catalyst was used. In the primary screening catalysts modified by L4, L5, L6, and L7 turned out to be excellent (77-96% ee yields 63-100%). Thereafter the catalyst loading of Lm/Ti/Lra (m, n = A-l) was decreased to 0.1 mol.%, but this led to only trace amounts of product. Finally, the solvent was... 

In the present Ln(OTf)3-catalyzed aldol reactions in aqueous media, the amount of water strongly influences the yields of the aldol adducts. The effects of the amount of water on the yields in the model reaction of benzaldehyde with the silyl enol ether 2 in the presence of 10 mol% Yb(OTf)3 in THF were investigated (Eq. 2). The best yields are obtained when the amount of water present in THF is in the range 10-20 %. When the amount of water is increased, the yield begins to decrease The reaction system becomes a two phase one when the... 

A model readion of benzaldehyde, o-methoxy-aniline, and 1-phenyl-l-trimethylsiloxyethene was performed in the presence of 5 mol% of Sc(OTf)3 in... 

The search for low-molecular weight (phenoxyl)copper(II) complexes as functional models for GO, which would mimick this reactivity, had a promising start in 1996 when Tolman and co-workers (202) reported that electrochemical one-electron oxidation of Cull(,L,lil 2)(bcnzylalcoholatc) (Fig. 27) resulted in the formation of benzaldehyde (46%) and some other decomposition products of the ligand H L,Bu2 in <5% yield and probably a Cu(I) species of unknown composition. These authors suggest that a (phenoxyl)copper(II) intermediate Cull(,L,l l 2 )(bcn-zylalcoholate)]+ is formed and that the reaction sequence, as in Fig. 8, leads to the observed products. Although this represents a stoichiometric reaction, it demonstrated for the first time that GO chemistry can be successfully modeled. 

A recent example of a chemical study showing how strain effects could be important in an enzymatic reaction, dealt with the hydrolysis of benzaldehyde di-t-butyl acetal [18] (Anderson and Fife, 1971b). As shown by a Stuart-Briegleb model, substantial ground-state strain is present which would be partially relieved in the... 

In connection with the enantioselective alkylation of Pro or 4-hydroxy-proline, the azabicyclo[3.3.0]octane system 81 was obtained after reaction with pivaldehyde (81HCA2704 85HCA155). In a more complex transformation A-protected L-Pro was transformed into the same bicyclic system (Scheme 49) (81JA1851 84JA4192). The product was prepared as a model substance in the total synthesis of pumiliotoxin. A related compound 82 was prepared from 5-(hydroxymethyl)-2-pyrrolidinone (prepared from L-pyroglutamic acid) by an acid-catalyzed condensation with benzaldehyde (86JOC3140). 

Papai et al. selected as model reaction the addition of 2,4-pentanedione (acetylacetone) to trans-(R)-mtrostyvQnQ, catalyzed by the bifunctional thiourea catalyst shown in Scheme 6 [46]. The analogous Michael-addition involving dimethyl malonate and nitroethylene as substrates, and a simplified catalyst was calculated at the same level of theory by Liu et al. [47]. Himo et al. performed a density functional study on the related cinchona-thiouTQa catalyzed Henry-reaction between nitromethane and benzaldehyde [48]. 

To rationalize the enantioselectivity of the TADDOL-catalyzed HDA reaction between Danishefsky s diene and benzaldehyde, eight possible diastereomeric transition states of different regio- and stereochemistry should in principle be considered for comprehensive analysis. The cycloaddition between the model diene and benzaldehyde can take place along two regio-isomeric meta (C1-06, C4-C5 bond formation) and ortho (C1-C5, C4-06 bond formation) reaction channels. For both of these pathways, an exo- and an endo-approach can be formulated (Scheme 11) [64]. 

Fig. 19 Transition states involved in the cycloaddition (endo-mode) of the model diene with benzaldehyde, both in the absence (TS-endo) and the presence (TS-(Si)-4b, TS-(Si)-4b) of the TADDOL catalyst corresponding activation energies Occal mol ](B3LYP/6-31G(d)//B3LYP/6-31G(d) PM3)...

The energy of the localized transition state for the ortho route (uncatalyzed reaction) is 14kcal/mol higher than that of the meta channel. Therefore, the ortho channel can be excluded. Unlike the uncatalyzed transformation, the TADDOL-catalyzed HDA reaction exhibited a clear energetic preference for the endo- over the exo-approach. Thus, only endo transition states were considered. The number of possible reaction paths/transition states is thus reduced from eight to two, namely endo-approach with re- or si-face attack of the model diene to the activated benzaldehyde. 

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