Extent of the cyclization

The peptide resin 1 was treated for 1 h with 50% TFA in CH2C12 containing 2% anisole, washed with CH2C12, neutralized with 5% DIPEA in CH2C12, washed with CH2C12, and then DMF. The product 2 was treated with a 5-fold molar excess of HOBt in DMF, followed after 20 min with a 5-fold molar excess of DCC in CH2C12. The mixture was adjusted with DIPEA to pH 7.5. After 24 h, the resin was drained and after a neutralization step with DIPEA, the cyclization was repeated for another 24 h. The extent of the cyclization to 3 was estimated to be about 95% by the ninhydrin test. 

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