Aldehydes binding modes

A model accounting for the observed sense of absolute stereoinduction is based upon the coordination mode revealed in the crystal structure of the cyclometallated C,0-benzoate complex [280]. It is postulated that aldehyde binding by the a-allyl haptomer occurs such that the allyl moiety is placed between the naphthyl and phenyl moieties of the hgand, allowing the aldehyde to reside in a more open enviromnent. In the favored mode of addition, the aldehyde C-H bond projects into... 

The distance of 28 A between the pi-pi and pV-pT active sites corresponds almost exactly to a nonapeptide in extended conformation. The X-ray structure of 20S proteasome containing the ThrlAla mutant of the p subunit revealed the binding mode of the non-processed propeptide to the substrate binding cleft up to the adjacent p2 active site (Fig. 2.4) [63]. Correspondingly, this propeptide sequence was used in a first approach to design bivalent inhibitors bearing a glutamic acid aldehyde on the C-terminal position to address in a more selective maimer the p active site... 

The stereochemical outcome of the new FucA was indistinguishable compared to that obtained for the wild type. However, while the (J )-N-Cbz-aminoaldehydes yielded the anti(3i ,4i )-configured aldol adduct in high diastere-oselectivity (>2 98 syn iR,4-S)/anti(iR,4R) ratio), the (S) enantiomers depended on the aldehyde. In the extreme situation, R)-N-Chz prolinal derivatives ((J )-7a,b) gave exclusively the anti 3R,4-R) adduct whereas the S counterparts ((S)-7a,c,d) rendered the syn(3i ,4S) one. Protein molecular models were built to gain insight into the acceptor binding mode that led to this distinct stereochemical outcome [19]. 

A stereochemical study reported by Henry illustrated that the formation of aldehyde and formation of chlorohydrin occur with different stereochemistry, and this result implies that one process occurs by syn addition and one by anti addition of water and palladium across the olefin. This study is summarized in Scheme 16.24. Oxidation of the non-race-mic, chiral allyl alcohol in the absence of added chloride forms the (R)-(E)-alcohol, whereas reaction of the allyl alcohol in the presence of added chloride forms the product with stereochemistry resulting from the opposite mode of attack. Because it is known that the allylic alcohol binds to palladium with hydrogen bonding between the hydroxyl group and the bound chloride, Henry concluded that the reaction conducted in the presence of high concentrations of added diloride occurs by external attack of the oxygen nucleophile, while the reaction with low concentrations of added chloride occurs by insertion of the olefin into a Pd-0 bond. ... 

In a related work, a design of enzyme-inspired star block-copolymers with branched topologies and protein-Hke tertiary or quaternary structures was performed. These polymers incorporate hydrophihc, superhydrophobic, and polydentate metal-binding sites and self-assemble in water, their mode of assembly being controlled by the composition of the polymer. An important feature of the star block-copolymers is that they incorporate perfluorocarbons and, due to that, their emulsions in water can attract and preconcentrate O2 in the vicinity of the active metal site. Addition of Cu(II) and TEMPO leads to an effective catalytic system for oxidation of alcohols to aldehydes in water. " ... 

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