A joint experimental and ab initio study on the reactivity of several hydroxy selenides. Stereoselective synthesis of cis-disubstituted tetrahydrofurans via seleniranium ions
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Introduction
Structurally complex tetrahydrofuran units are often found in many natural products such as polyether antibiotics and a particularly challenging aspect is the stereocontrolled synthesis of the substituted tetrahydrofuran unit, especially those in which there is a cis relationship between substituents at the 2 and 5 positions.1 During the last decade several authors have prepared in a synthetically useful manner substituted tetrahydrofurans via 5-endo-cyclization of intermediate seleniranium,2., 3., 4., 5., 6., 7., 8. thiiranium9 or iodonium ions.10., 11. In the last years our efforts have been devoted to the stereoselective synthesis of oxygenated heterocyclic rings from mixtures of hydroxy selenides or by treatment of the appropriate unsaturated alcohol with a source of the electrophilic seleno species PhSe+, such as PhSeCl.12., 13., 14., 15., 16., 17., 18. Both the methods lead to oxygenated heterocyclic rings via the intermediate seleniranium ion. Recently we studied the behaviour of the intermediate seleniranium ion having two hydroxyl groups and an aliphatic R chain (Fig. 1).17 We found that the 5-endo-tet19 cyclization occurs only at high acid concentration or when the primary hydroxy group is protected.
Section snippets
Results and discussion
In order to gain deeper information about the influence of the R group in the 5-endo-tet cyclization, we have analysed the reactivity of hydroxy selenides 6 bearing, as R group, an oxygenated aliphatic chain (RCH2OPG; RCH2CH2OPG). These compounds were prepared as outlined in Scheme 1. Epoxidation of homoallylic alcohols 4 using tert-butyl-hydroperoxide and VO(acac)2 gave the syn-epoxy-alcohols with good stereoselectivity (>93:7), the configuration of the major epoxide being assigned by
Conclusion
The low yield for the cyclization of 6a,b could be ascribed both to the coordination of the positively charged phenylselenyl moiety by the benzyloxy group in the seleniranium ions and to the presence of the oxygen atom (of the benzyloxy group) that hampers the development of a partial positive charge at C-5 and then the rupture of the C⋯Se bond. Both these factors make the activation energy of the ring closure process high. In the cyclization of 6c,d both the coordination and some electronic
Experimental
Anhydrous solvents were distilled as follows: Tetrahydrofuran and diethyl ether were distilled under nitrogen from sodium benzophenone immediately prior to use. Dichloromethane was distilled under nitrogen from calcium hydride and used immediately. 1H and 13C NMR spectra were recorded on a Bruker AC-E series 250 MHz spectrometer. IR spectra were recorded on a Perkin–Elmer infrared spectrophotometer (model 1310) using KBr cells. Flash chromatography was carried out using Macherey–Nagel silica gel
Acknowledgements
Financial support from the University of Palermo (funds for selected research topics) and Italian MURST within the National Research Project, ‘Non-aromatic heterocycles in stereocontrolled processes’ is gratefully acknowledged.
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