Ring-opening Reactions of Cyclopropanated Heterobicyclic Alkenes

Carlson, Emily
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University of Guelph

Synthetic organic chemistry centers around the development of useful reaction sequences. The primary objectives are to improve chemical transformations by making them more efficient and economical, to derive novel compounds and make them widely accessible, and to provide the scientific community with a better understanding of chemical behaviour so that new processes can be applied in the manufacture of similar structures, often with medicinal value. This thesis describes the construction of various novel heterotricyclo[,4]octanes, and their subsequent ring-opening reactions to afford uniquely functionalized and structurally appealing organic frameworks. The first part of this work concerns the preparation of some common heterobicyclic alkenes and their derivatization by palladium-catalyzed diazocyclopropanation. This produced over 40 novel cyclopropanes with satisfactory yields (64-98%) and complete exo stereoselectivity. A higher equivalency of diazomethane was required to drive the reaction to completion for substrates bearing bulky substituents near the cyclopropanation site. Later portions of this work address transition metal or acid-catalyzed ring-opening reactions of the cyclopropanated heterobicycloalkenes, focusing on 7-oxa and 7-azabenzonorbornadienes. These studies have been subdivided into three types, each providing a unique class of product(s). Type 1 ring-openings led to the formation of cis-1,2-dihydro-2-methylnaphthalenols or polycyclic γ-lactams with facile incorporation of alkyl or aryl nucleophiles from organocuprates. Aromatization could be utilized to convert the dihydronaphthalenols to 2-methylnaphthalenes. Type 2 ring-openings proceeded with thermal acid-catalyzed conditions using alcohol or carboxylic acid nucleophiles, producing 2-(XCH2)naphthalenes (X=RO, RCO2). The reaction rate was found to increase with the acidity of the reaction medium. Type 3 ring-openings were discovered in conjunction with type 2-ring opening studies when bridgehead-substituted cyclopropanated oxabenzonorbornadienes underwent expansion to benzo-fused seven-membered rings. A greater proportion of type 3 product relative to type 2 product was obtained at lower reaction temperatures. Mechanisms for each transformation were supported by intramolecular reactions of cyclopropanated azabenzonorbornadienes under type 1 ring-opening conditions, or by tethered nucleophile-bearing substrates under type 2 and type 3 ring-opening conditions. Thus, several more novel polycyclic lactams, lactones and cyclic ethers were born. As the substrates of this investigation are still largely unexplored, future work in this area is promising.

cyclopropanation, strain, heterocycle, ring-opening, transition metal, acid-catalysis, regioselective, stereoselective, synthesis, natural product, copper, palladium, bridged ring system, alkene