Exploration of the reactivity of a cyclic vinylogous ester and tandem cascade sequence toward the flavonoid core structure
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The Heck coupling reaction is a well known and synthetically useful reaction involving the coupling of an aryl halide or aryl triflate with a substituted alkene via a palladium metal catalyst to make a new carbon-carbon bond. This reaction was demonstrated by R.F. Heck in the early 1970’s, and soon took a stronghold as a key reaction in synthetic chemistry. There are numerous variations on Heck chemistry, and this area of chemical space has been explored with a variety of substrates. One class of substrates that have yet to be reported in the literature are vinylogous esters (R-O-CH=CHCOR’). These substrates have a very unique electronic environment for the alkene group undergoing the Heck coupling. It is the purpose of this thesis to explore the reactivity of a cyclic vinylogous ester useful in the synthesis of flavonoids; a large class of biologically relevant molecules. The skeletal core of some flavonoids (Scheme 4) could be easily accessed through a Heck reaction between a o-allylaryl halide and cyclic vinylogous ester followed by a tandem cyclization step fashioned from a stalled intermediate of the initial 2 Heck coupling as illustrated in Scheme 14. Initial coupling attempts of 14 with 15 failed with recovery of starting material. A small variety of conditions were explored with little success which led toward a model study of 2-bromotoluene with the cyclic vinylogus ester. Reaction conditions were optimized based upon a screening of solvents, bases, and catalytic species found prevalent in the literature. The results of the model study concluded a 50% conversion to the desired product. When these conditions were applied to the tandem sequence, 14 had partially isomerized to bring the olefin into conjugation with the arene, and 15 was recovered with no desired product found in the reaction mixture. It was also found that a narrow range of temperatures was required to balance competing product formation and thermal decomposition. These results therefore conclude that the tandem sequence was not able to be demonstrated. Further, vinylogous esters are weak substrates for Heck coupling reactions most likely due to their electronic delocalization of the alkene π electrons and unique electronic nature. It is felt that this chemistry is heavily substrate dependant, and thus the desired scope of methodology involving vinylogous esters as Heck coupling partners yields a narrow chemical space of successful reactivity due to the high substrate dependence of the Heck reaction.