Enol methyl ethers, dealkylation

It has been known for some time that trimethyliodosilane is an excellent reagent for the dealkylation of ethers and the saponification of esters. Two recent examples show that this can be done with enol methyl ethers21 (equation 14) and carbamates22 (equation 15) as well. 

Lead tetraacetate was employed by Stoodley and coworkers for an oxidative isomerization in their synthesis of 4-demethoxydaunomycinone (47). The diene (48) reacted with the oxirane dienophile (49) via the least hindered endo transition state to give the cycloadduct (50) in 86% yield. Hydrolysis of the silyl enol ether followed by reduction of the oxirane and introduction of the acetylene moiety gave the compound (51), which was oxidatively isomerized with LTA in acetic acid to give the quinone (52). All that remained now to complete the synthesis was conversion of the acetylene to a methyl ketone and dealkylation of the ether, llie last two steps were accomplished in an over l yield of 38%, the low yield attributable to problems in formation of the hydroxy group from the ether (Scheme 11). Bulman-Page and Ley employed LTA for a similar transformation in their synthesis of demethoxydaunomycinone and related anthracyclinones. 

Methoxyallyl cation (5a Y = OMe), derived from 2-methoxyallyl bromide (4a X = Br, Y = OMe) and silver trifluoroacetate in the presence of sodium carbonate, reacts with benzene, toluene, p-xylene or mesitylene. 2-Ethoxyallyl alcohol (4c X = OH, Y = OEt) in the presence of trifluoroacetic anhydride generates the 2-ethoxyallyl cation (5c Y = OEt). 2-Ethoxyallyl alcohol (4c) is probably more advantageous than the corresponding 2-methoxyallylic species (4a Y = OMe) because dealkylation of the intermediary oxonium ion seems to be less facile, and the ethyl enol ether is more nucleophilic than the methyl enol ether. Treatment of cyclic a-chloroenamines (4f X = Cl, Y = morpholino) and (4g X = Cl,... 

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