Biocatalyzed carbo Diels-Alder reactions

Biocatalysts (enzymes and proteins) and conditions resembling biological systems (micelles, cyclodextrins [CDs], molecular cages, catalytic antibodies, RNA-based mixtures of metals) have received great attention in the last few years because of the ease with which regio- and stereoselective reactions can be performed under mild conditions.  

The influence of micelle and CDs on the reactivity and selectivity of Diels-Alder reaction in aqueous medium was mainly investigated in the 1990s and has been reviewed. 

The effect of surfactants (cetyltrimethylammonium bromide [CTAB], sodium dodecyl sulfate [SDS]) at their critical micellar concentration significantly influences the yield and diastereoselectivity of Diels-Alder reactions of acrylates with cyclopentadiene performed in water at room temperature. A recent contribution on the effects of micelle on the Diels-Alder reaction was reported by Engberts who observed that in the cycloadditions of N-substituted maleimides with cyclopentadiene, sorbyl alcohol, and sorbyl trimethyl ammonium bromide, a micellar catalysis seems to be active, but if one considers the micellar rate constant, the neat effect on the rate constant of the reaction is remarkably small. 

Recent examples of Diels-Alder reactions in aqueous medium in a self-assembled coordination cage are the cycloadditions of 1,4-naphthoquinone, enclathrated in the cavity of a cage compound, with isoprene and 1,3-cyclohexadiene the reactions were accelerated 113 and 21 times, respectively, with respect to those carried out in the absence of cage. 

Currently the Diels-Alder reactions catalyzed by enzymes or antibodies in aqueous-buffered medium are a very promising topic because neither is capable of emulating the extraordinary activity and specificity of these catalysts. 

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