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Intramolecular [2 + 2] Cycloaddition Reactions of Allene-ynes: Exploring the Scope, Mechanism, and Application to the Synthesis of Carbocyclic Spirooxindoles

Osbourn, Joshua M. (2012) Intramolecular [2 + 2] Cycloaddition Reactions of Allene-ynes: Exploring the Scope, Mechanism, and Application to the Synthesis of Carbocyclic Spirooxindoles. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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The thermal allene-yne [2 + 2] cycloaddition reaction provides quick and efficient access to alkylidene cyclobutene compounds containing a fused bicyclic ring structure. The mechanism of this reaction was examined computationally in collaboration with Dean Tantillo and Matthew Siebert and experimentally in our lab. Computational studies suggest that the allene-yne [2 + 2] cycloaddition reaction proceeds via a stepwise diradical pathway. It is commonplace for researchers to postulate diradical intermediates for thermally disallowed cycloaddition reactions, but rarely are experiments conducted to support their existence. Experimental efforts to trap the postulated diradical intermediate included appending a cyclopropane to various allene-yne substrates. In two examples, products resulting from cyclopropane ring opening were isolated and characterized, thus providing experimental evidence supporting a diradical intermediate.

The scope of the thermal allene-yne [2 + 2] cycloaddition reaction was expanded to the preparation of spirocyclobutene oxindoles. Our investigation led to the discovery of a tandem thermal [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition reaction of propargyl esters to afford carbocyclic spirooxindoles. This methodology lent itself to the formation of both spirobicyclo[4.2.0]oxindoles and spirobicyclo[5.2.0]oxindoles in moderate yields. The scope of the reaction was expanded to the synthesis of potentially biologically active spirocyclobutene oxindole compounds.

Transfer of chirality from an allene to the [2 + 2] cycloadduct was possible when using a tert-butyl substituted allene; however in the case of a propargyl acetate substrate, complete racemization was observed during the tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition reaction. We hypothesize that the bulkiness of the tert-butyl group and oxindole restricts rotation at the intermediate diradical and thus prevents racemization.

The synthetic utility of the resulting [2 + 2] cycloadducts was briefly examined. The most notable transformations were conversion of the enolacetate of the spirooxindole to the corresponding ketone and α-acetoxyketone.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Osbourn, Joshua M.jmo29@pitt.eduJMO29
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairBrummond, Kay M.kbrummon@pitt.eduKBRUMMON
Committee MemberCurran, Dennis P.curran@pitt.eduCURRAN
Committee MemberMeyer, Tara Y.tmeyer@pitt.eduTMEYER
Committee MemberTantillo, Dean
Date: 27 September 2012
Date Type: Publication
Defense Date: 19 July 2012
Approval Date: 27 September 2012
Submission Date: 12 August 2012
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 306
Institution: University of Pittsburgh
Schools and Programs: Dietrich School of Arts and Sciences > Chemistry
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Allene; Cycloaddition; Diradical; Biradical; Spirooxindole, Alkylidene cyclobutene
Date Deposited: 28 Sep 2012 01:51
Last Modified: 15 Nov 2016 14:01


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