Exploration Of Novel Dibromo Ketene Claisen Rearrangement

Carbonyl compounds with dibromo substitution are useful synthetic intermediates.  The bromides can be hydrolyzed to yield ketones or one can be eliminated to provide a vinyl bromide which can be a useful handle for cross-coupling chemistry.  We are interested in accessing these compounds through a novel rate-accelerated ketene-Claisen rearrangement where a highly reactive ketene-acetal intermediate is generated in-situ from a suitable tribromo precursor.  The ultimate goal of this project is to illustrate the utility of this reaction by synthesizing a variety of dibromoesters.  Additionally, we wish to explore the usefulness of these esters by carrying them on to other unique compounds of potentially high synthetic interest.  We will explore the scope of this reaction by generating a variety of ketene precursors by coupling various allylic alcohols with a known tribromo chloroether.  We dissolved bromal in ethyl ether and added methanol and phosphorous pentachloride in order to form an acetal.  We then reacted the acetal with allyl alcohol using silver triflate and silver carbonate. The presence of our desired acetal was confirmed though H NMR.  Next, we then treated with base to provide our rearranged product, thus providing promising results for this initial experiment.  We will explore the scope of this reaction by utilizing various alcohols and varying the size and bulkiness of the R group in order to learn about steric limitations of the reaction.  These reactions are important because we are exploring new types of transformations and they could potentially be useful in synthesis of other compounds.