Total Synthesis Of Cp Compounds Man’s fascination with the many uses that can be found with the exploitation of natural substances has been demonstrated time and again throughout history, but the stage was set at the turn of the century for organic chemists to begin to focus on utilizing natural compounds for the benefit of medicinal and industrial uses. The discoveries of penicillin, aspirin, and other naturally occurring useful compounds in the earlier parts of the century set the stage for the utilization and exploitation of biologically active compounds as a molecular science. However, there are limits as to how much we can do with what nature provides us. This puts the role of the synthetic organic chemist at the forefront of synthetic compound synthesis technology. KC Nicolaou is one such leader. The main goal of Nicolaou’s lab is the complete synthesis of naturally occurring compounds, along with solid phase chemistry, molecular design, combinatorial synthesis, and biological investigations; some of the results of Nicolaou’s work include: the total synthesis of the anticancer agent Toxol, the marine neurotoxins brevetoxins A and B, the anititumor agents epothilones A and B, eleutherobin and sarcodictyins, the antibiotic vancomycin, the cholesterol-lowering CP-molecules, the immunosuppressant agent sanglifehrin A, the antibiotic everninomicin, and a number of bisorbicillinoids such as trichodimerol, bisorbicillinol, and bisorbibutenolide.
Another example of Dr. Nicolaou’s work is a paper published in Chemistry International entitled The Absolute Configuration and Asymmetric Total Synthesis of the CP Molecules (co-authored by Jae-Kyu Jung, Won Hyung Yoon, Yun He, Yong-Li Zhong, and Phil Baran.) In this paper, Nicolaou and his associates describe how their goal was both the total synthesis of these CP compounds (achieved in 1999) along with the determination of their absolute configurations; methods used in initial attempts to determine absolute configuration at different carbons included X-ray crystallography and NMR. Nicolaou set about synthesizing this compound by thinking through possible reactions that he might use to begin to build the carbon skeleton needed for this molecule. His team decided on a type-II intramolecular Diels-Alder reaction as the key step to generation of the core skeleton. The Diels-Alder reaction utilizes a dienophile in order to form new carbon-carbon bonds in a single step, in this case to form multiple ring structures.
However, Nicolaou ran into trouble when several reagent-based enantioselective approaches with the precursor failed to yield appreciable levels of the desired product. After much study of this problem, Nicolaou’s team came to the conclusion that a Lewis acid catalyst would be their best shot at inducing the asymmetry needed for this particular absolute configuration. After several more reaction steps, the team had two diastereomeric diols in a racemic mixture that were then converted to enantiomeric aldehydes with TBAF and NaIO4 – induced oxidative cleavage. After conversion to the indoline, the synthetic compound was compared to the naturally derived compound using NMR, TLC, and IR spectroscopy. However, the optical rotation of the synthetic compound was opposite in magnitude to the naturally derived CP molecule; the synthetic compound was verified as the enantiomer of the naturally occurring compound circular dichroism spectroscopy, and thus, the absolute configuration was verified. This paper parallels Nicolaou’s research goals by showing how this team determined the absolute configuration of a complex compound they synthesized from a much simpler molecule (glycidol).
It also shows the importance of techniques used to determine structure and content of complex molecules, such as NMR and IR spectroscopy. Science.