摘要 |
<p>This invention is directed to methods of preparing compounds of formula (I): comprising the step of reacting a compound of formula (II): with a reagent of formula POX<sub>3</sub> and silica gel at a temperature greater than about 75°C, and wherein substitutions at X, PG, A, G, R<sub>1</sub> and R<sub>4</sub> are set forth in the specification.</p> |