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Title:Synthesis and Claisen Rearrangement of Alpha-Ethoxyallyl Vinyl Ethers. Evidence for a Dipolar Transition State
Author(s):Rogers, Brian Daniel
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Organic
Abstract:The product stereoselectivity, scope, and mechanism of the Claisen-Micheal reaction (8 $\rightarrow$ 11) were explored. The products (e.g. 11) of the Claisen-Micheal reaction, which are enol ethers, are protected equivalents of the adducts obtained from the Micheal reaction of $\alpha,\beta$-enals with activated carbonyl compounds.$$\vbox{\vskip72pt}$$==>>>
The enolate anions of ethyl 2-methylacetoacetate, 2-carbethoxycylopentanone, 2-carbethoxycyclohexanone, 2-cyanocyclopentanone, and 2-cyanocylohexanone underwent O -alkylation in the THF containing polar cosolvents with $\alpha$-chloro-$\beta$-(arylseleno)alkyl ethyl ethers to produce 2-(arylseleno)alkanal vinyl ethyl acetals (e.g. 8). Oxidation of the seleno acetals with 1 equiv of m -chloroperoxybenzoic acid in dichloromethane at $-40 \rightarrow 0\sp\circ{\rm C}$ provided 2-(3-ethoxyallyl) active carbonyl compounds (e.g. 11) directly. The mechanism of the rearrangement probably involves a Claisen rearrangement of the intermediate $\alpha$-ethoxyallyl vinyl ether (e.g. 10).
The oxidation of the seleno acetal derived from cyclopentanone (Z = H) gave the putative intermediate $\alpha$-ethoxyallyl cyclopentenyl ether (45) in 18% yield. The Claisen rearrangement of 45 was found to occur 159 and 2560 times faster than cyclopentenyl allyl ether (47) in benzene-d$\sb6$ and 80% aqueous ethanol-d$\sb6$, respectively. The significant substituent and solvent effects provide experimental evidence for a pronounced dipolar character of the transition state. Further ($>$45 fold) acceleration of the rearrangements from the combined influence of a 4-ethoxy group and a cyano or carbethoxy group at C-1 indicates a synergistic interaction of the donor and acceptor substituents.
The product stereoselectivity of the selenoxide fragmentation was examined by oxidation of 2-(phenylseleno)butyraldehyde diethyl acetal (56) which provided crotonaldehyde diethyl acetal (57) as an 87:13 (E: Z) mixture. Oxidation of various 2-(arylseleno)butyraldehyde vinyl ethyl acetals showed the product stereoselectivity (with respect to diastereomers produced as a result of the formation of contiguous chiral centers via rearrangement) of the overall process (8 $\rightarrow$ 11) to vary between 88:12 (considered to be a stereoselective rearrangement) and 63:37 (ca. 34% stereoselectivity). The lack of consistent product stereoselectivity is believed to result from a high degree of C-4/O-3 bond cleavage in the transition state which reduces the free energy difference between the chair and boat transition states.
Issue Date:1987
Description:148 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.
Other Identifier(s):(UMI)AAI8721744
Date Available in IDEALS:2014-12-15
Date Deposited:1987

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