Mesylates substituted alkenes

Scheme 6.40. Influence of alkene geometry on stereoselectivity of allylic substitution of mesylates 189 and 192 with boron trifluoride-modified lower order cyanocuprate reagents.

The anti-SN2 substitution of allylic aziridines (see Section 10.5.2.1.3.2) has been extended to the synthesis of the methyl-substituted D-Ala-Ala alkene isostere 153 (Scheme 33)J91 For the corresponding (trifluoromethyl) alkene isostere, the aziridine precursor could not be formed. Instead, allylic SN2 displacement (see Section 10.5.2.1.3.1) of the mesylate 154 occurred readily to provide the Ala-D-Ala isostere 155. 

Primary tosylates are more prone than bromides to form f-butyl ethers by Sn2 displacement by the f-butoxide anion in DMSO. Sulfonate esters of flexible cyclic and secondary acyclic alcohols give predominately alkenes in the presence of f-BuOK/DMSO. With sulfonate esters of 3-hydroxy steroids, there is competition between /S-elimination and attack of the f-butoxide ion on sulfur to form alcohols mesylates are more prone to this reaction than tosylates. Sulfonate esters of 3a-acetoxy-12a-hydroxycholanate undergo mainly /3-elimination with f-BuOK/DMSO (eq 4). In this case, substitution of various other aprotic solvents for DMSO and DMSO Na+ for f-BuOK was not as effective. Treatment of both the mesylate and the tosylate of cholesterol with f-BuOK/ DMSO gives the conjugated diene, 3,5-cholestadiene, in high yield. ... 

In summary, the mesyl triflone reagent is useful for preparing 1,1-disubstituted aUcenes (eq 1) (R = R = H) or cyclopen-tenones such as dihydrojasmone. However, cyclohexenones were not formed and, while more substituted acyclic alkenes are readily obtained, they arise as ds-fraws mixtures. 

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