Tetramer aldolate

Thus, pyrrole and acetone react as shown above. This involves pyrrole acting as the nucleophile to attack the protonated ketone in an aldol-like reaction. This is followed by elimination of water, facilitated by the acidic conditions. This gives an intermediate alkylidene pyrrolium cation, a highly reactive electrophile that reacts with another molecule of nucleophilic pyrrole. We then have a repeat sequence of reactions, in which further acetone and pyrrole molecules are incorporated. The presence of the two methyl substituents from acetone forces the growing polymer to adopt a planar array, and this eventually leads to a cyclic tetramer, the terminal pyrrole attacking the alkylidene pyrrolium cation at the other end of the chain. 

A (3 replacement reaction catalyzed by the PLP-dependent tryptophan synthase converts indoleglycerol phosphate and serine to tryptophan. Tryptophan synthase from E. coli consists of two subunits associated as an a2P2 tetramer (Fig. 25-3). The a subunit catalyzes the cleavage (essentially a reverse aldol) of indoleglycerol phosphate to glyceraldehyde 3-phosphate and free indole (Fig. 25-2, step s).67 The P subunit contains PLP. It presumably generates, from serine, the Schiff base of aminoacrylate, as indicated in Fig. 25-2 (step f). The enzyme catalyzes the addition of the free indole to the Schiff base to form tryptophan. The indole must diffuse for a distance of 2.5 ran... 

Formaldehyde is not available as a pure monomer because it forms trimers and tetramers in the pure state (Chapter 52). The aqueous solution formalin used to preserve biological specimens is available—it is 37% formaldehyde and mostly consists of the hydrate CH2(OH)2 see Chapters. A pure dry polymer paraformaldehyde is also available and was mentioned in Chapter 9, Neither of these is particularly useful in aldol reactions. The aqueous solution is used in the Mannich reaction that we describe shortly. It is possible to make the short-lived monomer and capture it with a lithium enojate, but this is not trivial experimentally. 

FDP aldolase catalyzes the reversible aldol addition reaction of DHAP and d-glyceraldehyde 3-phosphate (D-Gly 3-P) to form d-FDP (Fig. 14.1-1). The equilibrium constant for this reaction has a value of -104 m-1 in favor of FDP formation. The enzyme has been isolated from a variety of eukaryotic and prokaryotic sources, both in type I and type II forms[7 21). Generally, the type I FDP aldolases exist as tetramers (M.W. 160 KDa), while the type II enzymes are dimers (M. W. 80 KDa). For the... 

A more complex oligomer is obtained by treatment of butanal with KOH or NaOH in aqueous ethanol. This reaction has been shown to produce the tetramers (3)-(5) and trimer (6 equation 14), some in respectable yield, depending on the exact reaction conditions. Compound (3) is produced as a mixture of four diastereomers and probably arises from dimerization of the initial aldol condensation product, 2-ethylhexenal. Diol (4), also obtained as a mixture of four stereoisomers, appears to be a Cannizarro reduction product of (3). Lactones (5) and (6) can reasonably arise by mechanisms involving hydride transfer from a cyclic hemiacetal. 

DFT calculations have been used to study the kinetic and thermodynamic parameters of the oligomerization of formaldehyde in neutral aqueous solution linear and cyclic oligomers up to tetramer were examined, and implications for enolization and aldol reactions were also examined. ... 

The successive replacement of the enolate by aldolate moieties en route from an enolate tetramer to an aldolate tetramer might involve severe reconstruction of the skeleton. Thus, a mixture of pinacolone, its lithium aldolate with pivalaldehyde, and the enolized aldolate dianion was recently reported to cocrystallize in a 1 1 3 ratio as a heptalithium cage compound missing any cube-shaped unit [15]. Rather complex crystal structures aldolate-enolate aggregates were also found for calcium enolates [8b]. 

Aldol-type condensations are useful for making the c connection in 3-pyrroHn-2-ones, hence it makes sense to combine an aldol reaction with an amidation to form a straightforward intermolecular-type ac approach to 3-pyrrolin-2-ones. It turns out that not many aldol-lactamiza-tion approaches have been reported for making (non-tetramic acid) 3-pyr-rohn-2-ones. One example, reported by Yao and coworkers, involves the synthesis of lactam-terminated polyketides (Scheme 63 2007BMCL3426). 1,3,5-Trisubstituted 3-pyrrolin-2-ones 257 were prepared by treating esters... 

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