Guthrie, Emma Jayne
Novel routes to benzofurans using titanium-alkylidene chemistry.
PhD thesis, University of Glasgow.
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The first route looked at the alkylidenation of esters i to give -alkoxyvinylstannanes ii. A cross-coupling reaction would have given enol ester products iii, followed by an acid-induced cyclisation to give the desired benzofurans iv. (Fig. 13510A)
The approach was not successful, but during the course of investigating the route, we formed the novel dimetallic reagent v, the properties of which we examined. (Fig. 13510B)
We looked at a second route that involved the use of thioacetals vi. Alkylidenation of esters i was carried out followed in one case by cyclisation of the enol ethers vii formed, giving benzofuran viii (R1 = Ph). (Fig. 13510C)
We also successfully carried out the second route on solid phase, where Wang and Merrifield resin-bound esters ix were alkylidenated. This was the first reported incidence of Takeda-style alkylidenations on solid phase. Cleavage of the resin-bound enol esters was achieved using mild acidic conditions. This gave ketone products x. In one case ketone x was converted into benzofuran viii (R1 = CH2CH2Ph) by deprotection followed by acid treatment. (Fig. 13510D)
Finally, we carried out an alternative direct cleavage to a range of benzofurans viii, by deprotecting the resin-bound enol ether ix and then cleaving with mild acid as before. (Fig. 13510E)
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