Stereoselective synthesis of alcohols containing (Z)- and (E)-olefins, dienes, enynes and styrenes: cyclic β-halogeno ether scissions using samarium diiodide as the electron-transfer agent

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Abstract
In contrast to the sodium-mediated ring scission of 2-substituted 3-chloro ethers of the tetrahydrofuran series, samarium diiodide gives olefins of high (E)-stereoselectivity and provides (E)-conjugated and unconjugated dienes, styrenes and enynes in good yield without appreciable over-reduction. Whilst the SmI2 scission of 3-chloro-2-alkyltetrahydropyrans gives(Z)-rich (Z)/(E) olefin mixtures, the 2-(alk-1′-ynyl) members give (Z)-enyne alcohols with high stereoselectivity, providing a valuable complement to the (E)-enyne synthesis employing the tetrahydrofuran series. In electron-transfer scissions using sodium, the stereochemistry of the product alcohols is related to the ground-state conformation of the cis- and trans-pyrans and -furans. The slow SmI2-mediated reactions appear to involve samarium-complexed intermediates aving structures independent of the original conformation, or of the cis- or trans-geometry of the furan or pyran, and it is the transition states from these intermediates that determine the stereochemical outcome. Scissions in the tetrahydrofuran series can be accelerated by addition of HMPA or DMPU with only a little deterioration in stereoselectivity, but in the tetrahydropyran series there are drastic changes in product stereochemistry when DMPU is added. Brief comment is made on the synthesis of tetrahydro-furan and -pyran precursors.

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