Deselenenylation with Elimination

Several research groups have recently investigated the asymmetric version of the catalytic one-pot selenenylation-deselenenylation sequence described above. In particular, attention was devoted to the process in which the deselenenylation occurs with elimination. These experiments are simply carried out by replacing the diphenyl diselenide with a chiral diselenide and are suggested to proceed through a sequence of reaction steps similar to those proposed for the diphenyl diselenide by Tomoda [122] or by Tiecco [116] and indicated in the Schemes 36 and 37. 

All these transformations need stoichiometric amounts of selenium-containing reagents. The addition or cycliza-tion products are usually intermediates in synthetic sequences. Most target compounds do not contain a selenium moiety and there is the need for deselenenylation at some stage. Often this can be combined with a subsequent functionalization like substitution or elimination. Combinations of addition and deselenenylation reactions with only catalytic amounts of a selenium reagent are described in Section 9.11.2.8.2. 

Quite recently, Tiecco [46a, 128] reported the asymmetric version of the one-pot conversion of, y-unsaturated esters and nitriles 261 (Scheme 42) into the enantiomerically enriched allylic ethers and alcohols 276 (Scheme 45). The reactions were effected with the selenenyl sulfate produced from the camphor diselenide 26. Unfortunately, in the present case, this diselenide must be employed in stoichiometric amounts. However, it can be partially recovered at the end of the reaction. Good chemical yields and enantiomeric excesses (up to 86%) were obtained in the methoxyselenenylation-elimination reactions. Lower ee was observed when the reactions were run in ethylene glycol or in water. In the case of the hydroxyselenenylations, reaction yields were low because the addition products 275 gave rise to the lactones, which were then deselenenylated to the butenolides. These were isolated in about 30% yield. 

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