Aldehyde, acceptor

Aldolases cataly2e the asymmetric condensation of intermediates common in sugar metaboHsm, such as phosphoenolpymvic acid, with suitable aldehyde acceptors. Numerous aldolases derived from plants or animals (Class I aldolases) or from bacteria (Class II) have been examined for appHcations (81). Efforts to extend the appHcations of these en2ymes to the synthesis of unusual sugars have been described (2,81). 

D-erythro-Pentulose 5-phosphate (XLIV) has been formed by the action of transketolase on hydroxypyruvate (XLII) and D-glycerose 3-phosphate, the hydroxypyruvate being decarboxylated196 to active glycolaldehyde which then reacts with the triose phosphate by an acyloin reaction.28 The active glycolaldehyde is also formed from L-glycero-tetrulose, d-altro-heptulose 7-phosphate, D-fructose 6-phosphate, and D-i/ireo-pentulose 5-phosphate and it reacts with various aldehydes (acceptors) to give ketoses.198, 200 Thus, substitution of L-gfh/cero-tetrulose for hydroxypyruvate in the above experiment also resulted in formation of D-en/i/iro-pentulose... 

Here the hapten (Scheme 2) is a 13-diketone, which incorporates structural features of both reactants - ketone donor and aldehyde acceptor (see below, Scheme 3) - in the aldol reaction of interest. In favorable cases the hapten reacts with the primary amino-group of a lysine residue in the complementary-determining region of an antibody to form a Schiffbase 5, which readily tautomerises to the more stable vinylogous amide 6. 

Scheme 14 Equilibria in aldol reactions with ketone and aldehyde acceptors...

Typical starting materials, catalysts, and products of the enamine-catalyzed aldol reaction are summarized in Scheme 17. In proline-catalyzed aldol reactions, enantioselectivities are good to excellent with selected cyclic ketones, such as cyclohexanone and 4-thianone, but generally lower with acetone. Hindered aldehyde acceptors, such as isobutyraldehyde and pivalaldehyde, afford high enantioselectivities even with acetone. In general, the reactions are anti selective, but there are aheady a number of examples of syn selective enamine aldol processes [200, 201] (Schemes 17 and 18, see below). However, syn selective aldol reactions are still rare, especially with cychc ketones. 

At present, most enamine-catalyzed aldol reactions are reliable only with electron-poor aromatic aldehyde acceptors, hi addition, a handful of aliphatic aldehydes (e.g. isobutyraldehyde or pivalaldehyde) are often used as acceptors. The use of unbranched aldehyde acceptors is difficult, and generally only modest yields have been obtained. In addition, unsaturated aldehydes are curiously absent from the list of commonly used acceptors. On a positive side, it should be noted that even potentially racemizing a-chiral aldehydes have been employed as acceptors. As an example, in the recent synthesis of caUipeltoside C, MacMillan and coworkers were able to employ protected Roche aldehyde 113 as a starting material (Scheme 22) [204]. 

The simplest possible aldehyde donor, acetaldehyde, can also be used as the donor Very recently, Hayashi and coworkers discovered how to use acetaldehyde in crossed-aldol reactions - the trick is to use diarylprohnol as the catalyst and to optimize the reaction conditions carefully to prevent oligomerization of acetaldehyde. However, so far the acetaldehyde aldol reactions appear to be limited to aromatic aldehyde acceptors [205],... 

The first asymmetric enamine-catalyzed Mannich reactions were described by List in 2000 [208]. Paralleling the development of the enamine-catalyzed aldol reactions, the first asymmetric Mannich reactions were catalyzed by proline, and a range of cyclic and acyclic aliphatic ketones were used as donors (Schemes 24 and 25). In contrast to the aldol reaction, however, most Mannich reactions are syn selective. This is presumably due to the larger size of the imine acceptor, forcing the imine and the enamine to approach each other in a different manner than is possible with aldehyde acceptors (Scheme 23). 

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. 

Scheme 4.8 Multi-enzyme system for the facile one-pot C—C bond formation catalyzed by Fuc-1 PA from readily available DHA and an aldehyde acceptor.

The ready availability of the transketolase (TK E.C. 2.2.1.1) from E. coli within the research collaboration in G. A. Sprenger s group suggested the joint development of an improved synthesis of D-xylulose 5-phosphate 19, which was expensive but required routinely for activity measurements [27]. In vivo, transketolase catalyzes the stereospecific transfer of a hydroxyacetyl nucleophile between various sugar phosphates in the presence of a thiamine diphosphate cofactor and divalent cations, and the C2 donor component 19 offers superior kinetic constants. For synthetic purposes, the enzyme is generally attractive for its high asymmetric induction at the newly formed chiral center and high kinetic enantioselectivity for 2-hydroxyaldehydes, as well as its broad substrate tolerance for aldehyde acceptors [28]. 

Fig. 16. Mechanistic model for DHAP addition to an aldehyde acceptor by class II aldolases based on the inhibitor-liganded FucA structure (right) in comparison to earlier proposals (left)...

The intermediary cofactor bound acetyl anion equivalent can be transferred to an aldehyde acceptor, e.g. to acetaldehyde already produced during regular catalytic reaction in which optically active 3-hydroxy-2-butanone (acetoin, an important aroma constituent) is formed. Interestingly, PDCs from different sources differ in stereoselectivity [443] acetoin (I )-143 is obtained using brewer s yeast PDC (ee 28-54%) [444,445] while the enantiomeric (S)-143 is produced preferentially by PDC from wheat germ (ee 16-34%) [446] or from Z. mobilis (ee 24-29%) [445], When glyoxylate 14 (instead of 2) is subjected to decarboxylation in the presence of acetaldehyde, optically active lactaldehyde... 

A corresponding ThrA has been detected in a number of strictly anaerobic bacteria, and the enzyme from Clostridium pasteurianum has been purified and shown to be highly selective for L-threonine 150 [457]. A corresponding L-specific catalyst has also been purified and crystallized from cells of the yeast Candida humicola. Very recently, the latter enzyme was reinvestigated for synthetic purposes and found to have a very broad substrate tolerance for the aldehyde acceptor, notably including variously substituted aliphatic and aro-... 

The phase-transfer-catalyzed enantioselective direct aldol reactions of glycine donor with aldehyde acceptors provide an ideal method for the simultaneous construction of the primary structure and stereochemical integrity of P-hydroxy-a-amino acids, which are extremely important chiral units. In the first report from the Miller s group, N-benzyldnchorudinium chloride (4a) was employed as a catalyst for the reaction of 1 with heptanal, and the corresponding aldol product 21 was obtained in 74% yield, though the diastereo- and enantioselectivities were unfortunately not satisfactory (Scheme 2.18) [40]. 

The Denmark method is synthetically very valuable, because a broad range of aldehyde acceptors can be used (Scheme 6.13) [60, 61]. Aromatic and a,fl-unsaturated aldehydes react very rapidly in the presence of 5 mol% 14 as organo-catalyst. The desired aldol products (S)-19, (S)-24 to (S)-26 were obtained in excellent yields of 92 to 98% and with high enantioselectivity (up to 91% ee). 

Fructose 1,6-biphosphate aldolase from rabbit muscle in nature reversibly catalyzes the addition of dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The tolerance of this DHAP-dependent enzyme towards various aldehyde acceptors made it a versatile tool in the synthesis of monosaccharides and sugar analogs [188], but also of alkaloids [189] and other natural products. For example, the enzyme-mediated aldol reaction of DHAP and an aldehyde is a key step in the total synthesis of the microbial elicitor (—)-syringolide 2 (Fig. 35a) [190]. 

The phase-transfer-catalyzed enantioselective direct aldol reactions of a glycine donor with aldehyde acceptors provide an ideal method for the simultaneous con-... 

General Procedure for O-tert-Butyl-L-Threonine Catalyzed Cross-Aldol Reactions of Ketone Donors and Aldehyde Acceptors [2] (p. 23)... 

The use of enzymes for the aldol reaction complements traditional chemical approaches. In the early twentieth century a class of enzymes was recognized that catalyzes, by an aldol condensation, the reversible formation of hexoses from their three carbon components.3 The lyases that catalyze the aldol reaction, are referred to as aldolases. More than 30 aldolases have been characterized to date. These aldolases are capable of stereospecifically catalyzing the reversible addition of a ketone or aldehyde donor to an aldehyde acceptor. Two distinct mechanistic classes of aldolases have been identified (Scheme 5.1).4... 

Despite the broader substrate tolerance for aldehyde acceptors by DHAP-dependent aldolases, the donor substrate specificity is narrow. With the exception of DHAP, only a few donors have shown to be acceptable as weak substrates (Scheme 5.6).20a 21a 26... 

Mimetics of sialyl Lewis X, the terminal tetrasaccharide of glycoproteins and glycolipids that are known to interact with selectins in the inflammatory process, have been efficiently synthesized through the use of the enzymatic aldol condensation (Scheme 5.26).29 This straightforward approach involved the condensation of mannosyl aldehyde derivatives with DHAP in the presence of DHAP-dependent aldolases. The aldehyde acceptors are generated from mannose by protection of the anomeric center as allyl ether, followed by... 

V-acetylmannosamine, but all at less than 5% of the natural substrate d-arabinose.58a This enzyme is specific for substrates with the -configuration at C(3), and Re-face addition of pyruvate to the aldehyde acceptors is the normal mode of attack.580 Though the enzymes accept substrates with C(2) hydroxyl of R- and -configurations, it prefers the S-isomer (Scheme 5.36). 

Scheme 5.54. Thiamine pyrophosphate (TPP) addition to D-xylulose-5-phosphate provides an electron sink to stabilize the incipient carbanion, which in turn reacts with the incoming aldehyde acceptor. P = PO .

It should be mentioned that most natural aldolase enzymes can also be assayed using enzyme-coupled systems relaying the reaction to a redox process with NAD. The formation of NADH by active microbial colonies in expression libraries of mutant enzymes was detected colorimetrically in agar plates using phenazine methosulfate and nitroblue tetrazolium, which forms an insoluble precipitate. The assay was used by Williams et al. [14] and Woodhall et al. [15] for evolving sialic acid aldolases to accept non-natural aldehyde acceptors. 

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