Antibody 38C2-catalyzed Aldol Reactions

Aldolase antibodies 38C2 and 33F12 generated by immunization with diketone 1 are capable of accelerating more than 100 different aldol reactions [4, 8, 11, 15, 16]. Some examples of cross-aldol reactions are shown in Table 6.1. For cross-aldol reactions, a variety of ketones are accepted as donors, including aliphatic open-chain ketones (for example acetone to pentanone), aliphatic cyclic ketones (cyclopentanone to cycloheptanone), functionalized open-chain ketones (hydroxyacetone, dihydroxyacetone, fluoroacetone), and functionalized cyclic ketones (2-hydroxycyclohexanone). As with the donors, the antibodies also accept different kinds of aldehyde substrate, for example benzaldehyde derivatives 8-10, a,j5-unsaturated aldehyde 11, and aliphatic aldehydes 12 and 13 with products as indicated in Table 6.1. 

Although a variety of secondary aldols can be prepared by aldolase antibody 38C2-catalyzed cross-aldol reactions, tertiary aldols are typically not accessible via intermolecular cross-aldol reactions. For preparation of enan-tiomerically enriched tertiary aldols, aldolase antibody 38C2-catalyzed retro-aldol reactions can be used (Section 6.3.2). 

Antibody 38C2-catalyzed cross-aldol reactions. 

Antibody 38C2-catalyzed self-aldol reactions. 

Antibody 38C2-catalyzed intramolecular aldol reactions. 

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It soon became clear that aldolase antibody 38C2 catalyzed the aldol reaction with both efficiency and broad scope. For example, compounds 7 to 13 could all be synthesized in antibody 38C2 catalyzed aldol reactions (Hoffmann et al, 1998). 

Stereochemistry of antibody 38C2-catalyzed aldol reactions and kinetic resolution. 

In addition to broad-scope substrate specificity, 38C2 exhibits high enantioselectivity for the aldol reaction. Although this high degree of enantioselectivity has been observed for antibody-catalyzed ester hydrolysis reactions, it is certainly not a feature common to all such catalysts (Janda et al., 1989 Lo et al., 1997 Pollack et al., 1989 Tanaka et al., 1996 Wade and Scanlan, 1996). Furthermore, the rules for the enantioselectivity for 38C2-catalyzed aldol reactions are both simple and general (Hoffmann et al., 1998). For most ketone donors, attack occurs on the si side of the acceptor. However, when a ketone with an a-hydroxy substituent (such as hydroxyacetone) acts as donor, attack occurs on the reside (Scheme 5). 

Because an equilibrium constant is not affected by catalysis, an enzyme that accelerates a forward reaction must also accelerate the reverse or retro-reaction. Furthermore, the enantioselectivity for both reactions will be identical. Antibody 38C2 catalyzes both the forward and retro-aldol reaction, and we envisioned that it may be useful in the kinetic resolution of aldols. Because the product enantiomer from the forward aldol reaction is the substrate in the retro-aldol reaction, the opposite... 

Antibody 38C2-Catalyzed Retro-aldol Reactions and their Application to Kinetic Resolution... 

Figure 19 A success of the reactive immunization strategy. Aldolization reaction catalyzed by antibody 38C2 raised against a /3-, 3-diketone hapten.

Diketone 4 was used for immunization, and two out of twenty monoclonal antibodies produced showed the characteristic enaminone absorb-tion at 315 nm after incubation with the diketone. Only these two antibodies, 38C2 and 33F12, were aldol catalysts (Wagner et al 1995). For example, 38C2 catalyzes the aldol reaction between acetone and aldehyde 5 to give aldol 6 with cat = 6.7 X 10 3 min-1 and = 17 (Scheme 4). 

Scheme 4.11 Activation of a prodrug of the anticancer agent eto-poside via tandem retro-aldol/retro-Michael reactions catalyzed by antibody 38C2.

Although the use of enzymes in ionic liquids has been explored, the enzymatic carbon-carbon bond forming the reactions in ionic liquids has not been studied in detail. Since aldol reactions catalyzed by the aldolase antibody 38C2 in buffer... 

Aldolase antibodies 38C2 and 33F12 are able to catalyze both the aldol addition and the retro-aldol reaction [99]. These catalysts have been employed to carry out the kinetic resolution of /3-hydroxyketones [100] and have been found to catalyze the asymmetric aldol reactions of 23 donors (ketones) and 16 acceptors (aldehydes) [101]. A highly efficient enantioselective... 

Scheme 13. Diastereoselectivity of the aldol reaction between racemic aldehyde 67 and acetone catalyzed by antibodies 38C2 and 33F12...

A model for the porphyrin metalation indicates its driving force empty space for the metal ion is provided in the protein-porphyrin complex. Antibody 38C2 efficiently catalyzes a wide variety of ketone-ketone, ketone-aldehyde, aldehyde-ketone, and aldehyde-aldehyde intermolecular cross-aldol reactions. 

For the aldol reaction of acetone and aldehyde 3 at pH 7.5, aldolase antibodies 38C2 and 33F12 had (kcat/km)/kuncat 10 - The efficiency of catalysis is largely because of an entropic advantage in the antibody-catalyzed reaction, vhich is reflected as a high effective molarity, kcat/kuncat > 10 m. 

Antibodies 38C2 and 33F12 also catalyzed self-aldol condensations of pro-pionaldehyde and provided the aldol-elimination product 26, and the antibodies did not catalyze the consecutive aldol reaction of 26 with propionalde-hyde (Scheme 6.3). The antibodies did not catalyze the self-aldol reactions of aldehydes bearing a longer alkyl chain (> valeraldehyde, for example, aldehydes 3, 12, and 13). It might not be possible for the antibody to accept two... 

Antibodies 38C2 and 33F12 also catalyzed intramolecular aldol reactions and Robinson annulation (Scheme 6.4) 1,5-diketones 28 and 30 were converted into 29 and 31, respectively [15]. To explore the scope of Baldwin s rules in antibody-catalyzed intramolecular aldol reactions, 38C2 was incubated with three different aliphatic diketones - 2,4-hexanedione, 2,5-heptanedione and 2,6-octanedione. No catalysis was observed in the... 

The antibodies also catalyzed retro-aldol reactions of secondary [9, 11, 16] and tertiary aldols [10]. In these retro-aldol reactions antibodies 38C2 and 33F12 processed hydroxyketones whose stereochemistry was the same as that of the aldol reaction product. Kinetic resolution by the retro-aldol reaction therefore provided the opposite enantiomer from the forward aldol reaction (Scheme 6.5 and Table 6.2). For example, (K)-14 (> 99% ee) was obtained by the 38C2-catalyzed kinetic resolution of ( )-14 (Table 6.2) whereas... 

S)-14 (98% ee) was formed in the 38C2-catalyzed forward-aldol reaction (Table 6.1). The recovered aldols were highly enantiomerically enriched by the aldolase antibody-catalyzed kinetic resolution examples include secondary aldols 14, 17, 21, and 32-35, and tertiary aldols 36-40 (Tables 6.2 and 6.3). Kinetic resolution with antibody 38C2 usually afforded higher enantiomeric excess for the recovered aldols than the forward aldol reactions, because one enantiomer was completely consumed in the resolution. For example, (S)-32 was obtained in >99% ee by 38C2-catalyzed kinetic resolution after 67% conversion whereas in the 38C2-catalyzed forward aldol reaction (R)-32 was obtained in 58% ee. 

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