An Iterative synthesis of oligo-vinyl ethers and applications thereof

Date

2012-04-23

Authors

Davies, Katherine

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Abstract

An iterative protocol is a highly efficient strategy for the generation of large, complex molecules that has been applied in many different subfields of organic synthesis. The use of a tandem or cascade reaction is also an effective approach for the rapid introduction of molecular complexity into a system since the number of steps requiring independent optimization is greatly reduced. With the aim of creating new synthetic strategies to efficiently gain access to stereochemically complex small molecules, we envisioned the use of short iterative protocols to prepare reactive oligomers to which a diverse range of cascade cyclization processes could be applied. In an attempt to minimize reaction optimization and chromatographic purification steps during the development of our small molecule precursors, we first developed an iterative synthesis based on a conjugate addition/reduction sequence that has allowed us to access a diverse series of oligo-vinyl ether intermediates. Significantly, both the addition and reduction steps proceed in near-quantitative yield, and reaction co-products can be removed without column chromatography. At the same time, most of our vinyl ether intermediates are stable to silica gel, and so analytically pure samples can be prepared when desired. Except for when very sterically demanding substrates are employed as electrophiles, the intermediates are isolated as single geometrical isomers. We also developed an improved synthesis of a previously intractable class of alkynoate starting materials (4-aryl-2-butynoates) to ensure a diverse range of easily accessible monomeric building blocks were available for our use. With this effective iterative route in hand, we have several interesting small molecule targets at our disposal. We first applied our iterative route to synthesize oxygen-containing analogues of juvenile hormone III. These mono- and bis-vinyl ethers are currently undergoing biological testing (in collaboration with Dr. Steve Perlman and Dr. Michael Horst), and early results show promise as ecologically degradable insect control agents. We also developed an unprecedented 6-endo/5-exo radical cascade reaction across bis-vinyl ethers which proceeds in good yield, high diastereoselectivity, and excellent regiochemical control. This reaction represents the first cascading radical cyclization ever reported for a bis-vinyl ether system and validates our iterative approach to molecular complexity.

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Keywords

Iterative Synthesis, Natural Products, Cascade Cyclizations, Oligo-Vinyl Ether, Stereochemically Complex Small Molecules, 4-Aryl-2-Butynoates, Juvenile Hormone III

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