Yohimbine alkaloids epimerization

As can be seen by reviewing the chemistry outlined above, a variety of cleverly devised strategies have been employed for the synthesis of the structurally complex yohimbine alkaloids. One elusive problem in many of these approaches has been the proper adjustment of the C(3) stereochemistry, particularly in approaches to reserpine (2) and deserpidine (3). It is well-known that epimerization at C(3) of the yohimbine skeleton 1 can occur under acidic conditions presumably via a mechanism involving cleavage of either the C(2)-C(3) or C(3)-N(4) bond to afford the respective iminium cation 532 or a-indolylcarbinyl cation 533. In this section, we will review investigations which focus on this epimerization process. 

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