Cycloadditions enzyme-catalyzed

These processes are normally enzyme-catalyzed. Purefy ctemical processes are seldom encountered with carbon conq)ounds in nature. The few exceptions include the very act of phenol coupling (by which racemic compounds are obtained), cyclization reactions ofpolyprenyl compoimds (which benefit from the preferred conformation of the reaction partners, suitable for the cyclization, Wendt 2000), and Diels-Alder cycloadditions. The latter have been advocated for the biosynthesis of celastroidine A (= volubilide) from a lupane triterpene and an abietane diterpene in two different plants, Hippocratea celastroides Kunth from Mexico (Jimenez-Estrada 2000) and Hippocratea volubilis Linnaeus (Alvarenga 2000). 

The question whether Diels-Alder reactions also occur in nature can not be answered yet since special enzymes catalyzing these reactions have not been found so far [24-25]. However, artificial catalytic antibodies for Diels-Alder reactions are well known [26] and recently an all-carbon [4 + 2]cycloaddition has been observed in the biosynthesis of two phytotoxic solanopyrones 1-1 and 1-2 from the fungus Alternaria solani using a cell-free extract of this organism [27] it is highly probable that the involved enzymes will soon be isolated (Fig. 1-2). 

Andibenin B (117) is a highly oxidized meroterpenoid produced by the fungus Aspergillus variecolor. On the basis of the biosynthetic study,it was proposed that a plausible intermediate 115 from Claisen rearrangement of 114 affords the adduct 116 via the intramolecular inverse-electron demand [4 - - 2] cycloaddition as shown in Scheme 19(b). In this case, an enzyme catalyzes the cyclization of the terpene part on intermediate 114 derived from farnesyl diphosphate, and benzoates might provide the reactive dienophile 116. 

In addition, it has been discovered that there are naturally occurring enzymes that facilitate Diels-Alder type reactions within certain metabolic pathways and that enzymes are also instrumental in forming polyketides, isoprenoids, phenylpropanoids, and alkaloids (de Araujo et al., 2006). Agresti et al. (2005) identified ribozymes from RNA oligo libraries that catalyzed multiple-turnover Diels-Alder cycloaddition reactions. 

Rates for this reaction may easily be measured by disappearance of azide UV absorption. Most importantly, kinetic saturation behavior is noted with sufficient amounts of the reactants cycloaddition velocity becomes independent of substrate concentration. As is familiar from enzyme catalysis, this indicates complete occupancy of all available cucurbituril by reacting species. In actuality, the rate of the catalyzed reaction under conditions of saturation was found to be the same as that for release of the product from cucurbituril. Such a stoichiometric triazole complex was independently prepared and its kinetics of dissociation were examined by the displacement technique previously outlined, giving the identical rate constant of 1.7xl0 s under the standard conditions. (It is not uncommon for product release to be rate-limiting in enzymic reactions). 

Many of the reactions observed in molecular s)mthetic cages are reactions that enzymes are not known to catalyze examples are Diels-Alder cycloaddition and 1,3-dipolar cycloaddition. This behavior constitutes a reason to explore further the range of applications of these supramolecular assemblies. The fact that cycloaddition reactions are t) ically catalyzed is probably a result of the similarity of the transition state and the reactants and products. This characteristic of the cycloadditions makes product inhibition inevitable, unless additional reactions are operative that alter the shape and functionality of the adduct that is formed and thus reduce its association constant with the cage. 

Nucleic acid selection methods have also been exploited for the development of novel RNA enzymes or ribozymes (58). An m-vitro-selected RNA that contains the modified nucleotide 5-(4-pyridylmethyl)-uridine (Table 1) can catalyze carbon-carbon bond formation in a Diels-Alder cycloaddition, with an 800-fold rate acceleration compared with a random RNA (49). Modified RNAs that contain the same uridine modification have also been selected to mediate metal-metal bond formation in the synthesis of palladium nanoparticles (59). Modified RNAs are likely to have many other applications as novel ribozymes that catalyze important biological reactions or can be used to create novel materials. 

A somewhat different but mechanistically related reaction is the [2 -f 3] cycloaddition of a functionalized alkyne or nitrile to an azide to form a disubstituted triazole (120) or tetrazole ring (121, 122), linking the respective functionalities irreversibly (Scheme 14b). This click chemistry was used by Sharpless and co-workers (120) in 2001 as a tool to probe biochemical catalysis and substrate activation. The ease of the Cu(I)-catalyzed reaction has created a true explosion (120-160) of simple coupling-functionalization chemistry of all types of biochemical components (sugars, DNA, proteins, enzymes, substrates, inhibitors) (131, 135, 136, 139, 142, 155, 157-160), polymers (126, 134, 140, 147, 154),... 

In addition to the examples shown above, there are alternative pathways (Scheme 4) to construct cyclohexene rings by a cationic (mechanism A), a nucleophilic (mechanism B), and a radical mechanism (mechanism C). Therefore, it is difficult to distinguish the Diels-Alder reaction and alternative reactions in cyclohexene formation, and to determine enzymatic and nonenzymatic [4 + 2] adducts. Through our studies on Diels-Alderases,we recognized that Diels-Alderase catalyzes not only the formation of reactive species but also cycloaddition at the same active site. This indicates that Diels-Alderase is, at least part of them, a producer of a reactive substrate for cycloadditions. If this is general, a Diels-Alderase could be any type of enzyme such as an oxidase, a dehydrogenase, a decarboxylase, or... 

Fireflies are one of several species that emit light as a result of a retro (i.e., reverse) [2 -I- 2] cycloaddition reaction, similar to the reaction that produces the cold light in light sticks (Section 29.4). Fireflies have an enzyme (luciferase) that catalyzes the reaction between luciferin, ATP, and molecular oxygen that forms a compound with an unstable four-membered ring. When the four-membered ring breaks, an electron in... 

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