This summer I have been attempting a total synthesis of the natural compound loline. The summer commenced with running familiar reactions at larger scales to move more material forward in the route. We were able to improve the yield of the RCM (ring closing metathesis) to an impressive 89%, but this was coupled with continually inconsistent results with the preceding acylation. I then ran three new reactions to make the methyl ester carbamate that was critical to our 2011 synthesis. After some initial troubleshooting, I was able to produce the desired product in good yield. Unfortunately, the following reduction with Dibal-H had a negligible yield despite several revisions to the reaction and work up conditions. There are about four remaining steps to loline past this point, so it would be a futile pursuit to continue running this reaction.
My most recent work in the lab has been to reduce the an ester to a primary alcohol. In other words, we’re attempting to alter the oxidation state of a particular portion of the compound. To this end, we’re using the reducing agent diisobutylaluminum hydride, or Dibal-H, for the reaction. This reagent is a bit dangerous to work with it, because it is pyrophoric, meaning it spontaneously ignites in air at room temperature. This requires that we make a 1.0 molar solution of Dibal-H in a solvent, which will be added to the starting material.
It’s the end of week 9 and it’s certainly hard to believe how quickly the summer is passing. This research session has certainly had its ups and downs. The saponification/esterification sequence that previously plagued my chemical synthesis has become facile. My latest run had an overall yield of 71% over 3 steps, which is an improvement over the 65% yield that was achieved in the first generation synthesis. The following reduction, though, has proved difficult. After doing the reaction three times, I’ve maintained consistent results but the yields have been very low (20-30%). it’s been a true test of using all available skills and techniques to learn how to improve a reaction. We’ve currently devised a new plan to circumvent the alcohol with a new scheme. Unfortunately, this new plan somewhat dampens the efforts and progress we made to get to that point, but it appears to be a necessary solution.
I remember being about seven years old and asking my mom, after staring at a plastic straw for a few minutes, “Where does plastic come from?” Her response was the typical answer — plastic is made out of oil. As a child, I thought this was weird, because I knew that petroleum oil was an icky-sticky gross black liquid crud that cars needed for some reason and I couldn’t comprehend how it made the straw for a juice box, but I accepted the answer anyway.
In much of our research in the polymer lab, we’re looking to find molecular weights using the SEC-MALLS (Size Exclusion Chromatography — Multiple Angle Laser Light Scattering) and CIV (Corrected Intrinsic Viscosity). I’ve already written a blog post about CIV and I’ll save SEC-MALLS for later. But sometimes, we look at more than just the molecular weight — we’re looking at the mechanical properties of a sample of aged or fresh polymer.