Copper-mediated cross-coupling introduction

Installation of the 6-(trifluoromethyl) group onto a purine is most widely reported to occur via copper mediated cross-coupling of a trifluoromethyl containing reagent and a 6-halopurine [120], For example, treatment of 75 with a CF3-CU solution (obtained by filtration under inert conditions of a vigorously shaken and heated mixture of CF3I, Cu and HMPA) afforded 76 in a modest yield [120] (29 %) (Scheme 28). The introduction of the filtration step eliminated a problematic reductive dehalogenation. This procedure has been used successfully elsewhere [121]. 

Since its introduction by Allred and Liebsekind in 1996 [114], copper thiophene-2-carboxylate (CuTC) has emerged as a mild and useful reagent for mediating the cross-coupling of organostannanes with vinyl iodides at room temperature. CuTC is especially effective for substrates that are not stable at high temperature. In Paterson s total synthesis of elaiolide, he enlisted a CuTC-promoted Stille cyclodimerization of vinyl iodide 131 to afford the 16-membered macrocycle 132 under very mild conditions [115]. 

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