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DOI: 10.1055/s-2003-39285
Cycloadditions of Nonstabilized 2-Azaallyllithiums (2-Azaallyl Anions) and Azomethine Ylides with Alkenes: [3+2] Approaches to Pyrrolidines and Application to Alkaloid Total Synthesis
Publikationsverlauf
Received
9 August 2002
Publikationsdatum:
20. Mai 2003 (online)


Abstract
The [3+2] cycloaddition of 2-azaallyl anions with alkenes represents an attractive strategy for the synthesis of substituted pyrrolidines. Although cycloadditions of 2-azaallyl anions stabilized by aryl and ester groups have been known for more than three decades, only recently have versions bearing simply hydrogen or alkyl groups been discovered. These nonstabilized 2-azaallyl anions are generated by the low temperature transmetalation of (2-azaallyl)stannanes with alkyllithiums. The resulting nonstabilized 2-azaallyllithiums undergo cycloaddition with certain alkenes and alkynes in both intra- and intermolecular modes to yield pyrrolidine or pyrroline cycloadducts. The methodology has been extended to 2-azapentadienyllithiums, heteroatom-substituted 2-azaallyllithiums, and polymer-supported 2-azaallyllithiums. Asymmetric 2-azaallyl anion cycloadditions have also been investigated. Nonstabilized azomethine ylides may also be generated from (2-azaallyl)stannanes via an N-alkylation/destannylation or N-protonation/destannylation sequence. Together, the cycloaddition of nonstabilized 2-azaallyllithiums and azomethine ylides with alkenes allows access to a broader range of pyrrolidines, since these species have complimentary reactivity profiles.
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1 Introduction
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2 Background: 2-Azaallyl Anions
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2.1 Semistabilized 2-Azaallyl Anions
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2.2 Stabilized 2-Azaallyl Anions
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2.3 Nonstabilized 2-Azaallyl Anions
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3 Methodology Development
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3.1 Initial Attempts at Generating Nonstabilized 2-Azaallyl Anions
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3.2 Tin-Lithium Exchange on (2-Azaallyl)stannanes
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4 Cycloaddition of Simple Nonstabilized 2-Azaallyllithiums
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4.1 Preparation of (2-Azaallyl)stannanes
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4.2 Anionophiles and Quenches
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4.3 Mechanism and Stereoselectivity
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5 Variations on a Theme: Related Cycloadditions
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5.1 Cycloadditions on Solid Support
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5.2 2-Azapentadienyllithiums
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5.3 Heteroatom-Substituted 2-Azaallyllithiums
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5.4 Enantioselective Cycloadditions
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5.5 Higher-Order Cycloadditions
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6 Other Uses of (2-Azaallyl)stannanes
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6.1 Azomethine Ylide Generation and Cycloaddition
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6.2 Nucleophilic Additions to (2-Azaallyl)stannanes
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7 Synthesis of Alkaloids
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7.1 Intramolecular Cycloadditions
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7.1.1 Amabiline and Augustamine
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7.1.2 Mesembranes
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7.1.3 Coccinine
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7.1.4 Crinine and 6-Epicrinine
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7.1.5 Approach to 6a-Epipretazettine
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7.2 Intermolecular Cycloadditions
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7.2.1 Lepadiformine Isomers
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7.2.2 Lapidilectine B
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7.2.3 Indolizidine 239CD
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8 Commentary
Key words
organolithium - anionic cycloaddition - heterocycles - alkaloid synthesis - tin-lithium exchange