Dioxan effect organic esters

A simple one-pot procedure for the synthesis of Fmoc-protected aspartic and glutanoic acid tert-butyl esters is shown in Scheme tert-Butylation of unprotected Asp and Glu can be effected in dioxane with excess isobutene using 4-toluenesulfonic add as catalyst. Subsequent Fmoc derivatization results in a mixture of the mono-tert-butyl esters (co/a 65/35 for Glu and 50/40 for Asp) with an overall yield of 55-60% contaminated with small amounts of d -tert-butyl esters. This procedure has the advantage that the Fmoc group is introduced without prior isolation or purification of the aspartic and glutamic tert-butyl esters. Whilst the diesters are readily removed by extraction with organic solvents, the Fmoc-Asp/Glu(OtBu)-OH derivatives are isolated by crystallization from dichloromethane/petroleum ether. 

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