The Diels-Alder reaction is a very powerful and selective means for introducing densely packed relative stereochemistry in one synthetic step. The term “relative stereochemistry” is used reiteratively to clearly indicate the typical reaction is not stereoselectively producing one enantiomer over another. Such an outcome would require the transition state taking place in an asymmetric environment. Part One revealed how the reaction is useful in the synthesis of simple six-membered rings, however the synthetic utility spans beyond this.
Few reactions have the capacity to introduce densely packed relative stereochemistry to a molecule in one single step as does the Diels-Alder reaction, a concerted [4+2] cycloaddition. This highly synthetically useful reaction permits facile entry into complex fused ring systems endemic to some natural products exhibiting medicinal properties. This article focuses on the rudimentary elements of the Diels-Alder reaction. Students interested in a more detailed understanding of the dynamics of the reaction are encouraged to engage in the suggested readings.
The mechanism of the Wittig reaction is not without controversy. Two principal researchers, Prof. Edwin Vedejs (a former personal mentor) from the University of Wisconsin – Madison (now at the University of Michigan) and Prof. Manfred Schlosser from the Université of Lausanne, had very different points of view. The entire “argument” was based upon whether betaines are observed in the absence of lithium salts, i.e. under “salt-free” conditions.
Thus far we’ve looked at both thermodynamic and kinetic control in the Aldol Condensation. This is typically where most undergraduate Organic Chemistry professors conclude discussing the reaction. The topic of stereochemistry, however, is left mostly unexamined due to time constraints. The man pictured to the right, Professor Howard Zimmerman (1926-2012), University of Wisconsin – Madison was an exception to this rule. This article takes the Aldol Condensation one step further by discussing one of the most interesting aspects of the reaction. The article is dedicated to Professor Howard Zimmerman, one of my former mentors who helped shape me as a both a scientist and a teacher, and as a person who pursues the meaning in everything through deductive reasoning.