Diels-Alderases

Keywords biopolymer, Lewis acids, Diels-Alderase... 

Keywords biological Diels-Alder reaction In biosynthesis of phytotoxins, DIels-Alderase... 

The Diels-Alder reaction, probably the most widely used methodology in organic synthesis today, has contributed greatly to the development of mechanistic and theoretical chemistry. The recent discovery of a Diels-Alderase enzyme has provided insights into the mechanism of biosynthetic cycloaddition. 

The antibody-catalyzed Diels-Alder reaction developed by Schultz utilized a Diel-Alderase enzyme-like catalyst evolved from an antibody-combining site (Eq. 12.13). The idea is that the generation of antibodies to a structure that mimics the transition state for the Diels-Alder reaction should result in an antibody-combining site that lowers the entropy of activation by binding both the diene and dienophile in a reactive conformation. 

Oxidative-pericydic processes, and in particular the oxidative/Diels-Alder reaction, are quite common in nature. The so-called Diels-Alderase is usually an oxidizing enzyme, which induces, for example, the formation of a suitable dienophile such as an enone from an allylic alcohol [49]. 

The library of natural catalysts has very little to offer for the catalysis of Diels-Alder (and the reverse) reactions (Diels-Alderases)[139]. For this reason one of the intriguing areas of biomimickry, namely the formation and use of antibodies exhibiting catalytic activity, has focused on [4 + 2] reactions to try to furnish proteins possessing useful catalytic properties. Thus in early studies a polyclonal catalytic antibody raised to hapten (57)[140] showed a modest rate enhancement for the reaction depicted in Scheme 48. 

While it is one of the most important and versatile transformations available to organic chemists, there is no unequivocal example of a biological counterpart. Hence, attempts to generate antibodies which could catalyse this reaction were seen as an important target. The major task in producing a Diels-Alderase antibody lies in the choice of a suitable haptenic structure, because the transition state for the reaction resembles product more closely than reactants (Fig. 12). The reaction product itself is an inappropriate hapten because it is likely to result in severe product inhibition of the catalyst, thereby preventing turnover. 

Fig. 24 An alternative strategy for eliciting Diels-Alderase antibodies has employed freely rotating ferrocenes [63] and [64] as TSAs.

The development of tailored "Diels-Alderase" antibodies could therefore provide a complete control of the stereochemical outcome of this cycloaddition reaction. Another possibility would be the use of antibodies to reverse the normally observed Diels-Alder endo/exo selectivity [35]. 

Romesberg, F., Spiller, B., Schultz, P. and Stevens, R. (1998) Immunological origins of binding and catalysis in a Diels-Alderase antibody. Science, 279, 1929-1933. 

Seelig,B. and J as chke, A. (1999) Asmall catalytic RNA motif with Diels-Alderase activity, ChemBiol. 6, 167-176. 

Stuhlmann, F., Jaschke, A. Chai acterization of an RNA active site interactions between a Diels-Alderase ribozyme and its substrates and products. J. Am. Chem. Soc. 124, 3238-3244 (2002). 

Diels-Alder reaction Diels-Alderase Oikawa, 1995... 

The secondary double-reciprocal plot of the data from Figure 5.3.5 gives Mi-chaelis constants of 0.37 mM for the diene and 8 nm for the dienophile. The calculated maximum rate Vmax is 0.15 mM min-1, which at a ribozyme concentration of 7 xm corresponds to a kcat of 21 min-1. The highest initial rate that was measured directly corresponds to a catalytic turnover of approximately 6 min-1. With these catalytic properties the 49nt Diels-Alderase ribozyme is among the faster... 

For 9-anthrylmethanol, the ee was merely 16%, for anthracene-monoethyleneglycol it was 33%. Obviously, the size of the substituent in the 9-position plays an important role in determining the ee, whereas variation of the substituent on the maleimide does not lead to significant changes in enantioselectivity. For the 49nt Diels-Alderase it could be shown for the first time that an unnatural L-ribozyme... 

The Houk group recently compared several artificial catalytic Diels-Alderase systems, including the ribozymes described here [11]. This study came to the conclusion that in none of these artificial systems there is a significant specific stabilization of the transition state. Acceleration arises predominantly from binding of the reactants, converting a second-order reaction of diene with dienophile into a first-order reaction of the termolecular complex of host, diene, and dienophile. The simultaneous presence of the two components of an intermolecular Diels-Alder reaction within the confined space of a cavity is the driving force that facilitates the reaction. 

A11 Diels-Alderase 1A4K (mature) hapten-bound 2.4 Romesberg et al, 1998b... 

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