Boc deprotection of [[921|BocPhePheOEt]]
SyntheticPage 922
Submitted: April 27, 2020, published: May 4, 2020
Authors
Bart Dietrich (bart.dietrich@yahoo.co.uk)
A contribution from
Chemicals
Trifluoroacetic acid (Fluorochem)
Chloroform (HPLC grade)
Procedure
To a solution of the starting material (8.21 g, 18.6 mmol) in chloroform (30 mL) was added trifluoroacetic acid (15 mL, approx. 10 eq) and the mixture was stirred overnight. After this time, the reaction was concentrated under reduced pressure to remove most of the excess TFA. The resulting viscous oil was dissolved in chloroform (ca. 30 mL), poured into diethyl ether (ca. 400 mL) and stirred overnight. The precipitate was filtered off, washed in the filter with a few small portions of diethyl ether and dried under reduced pressure. The title compound was thus obtained as a white solid (6.95 g, 82 %). A small amount of residual diethyl ether is seen in both the proton and carbon NMR spectra.
Author Comments
- The reaction mixture can often be poured directly into diethyl ether (without first evaporating the TFA). In this case the resulting precipitate can occasionally be sticky and difficult to wash in the filter with ether. The evaporation of TFA prevents this from happening.
- The overall concentration of TFA in the reaction mixture needs to be fairly high (33% in the present case) and the TFA needs to be present in a fairly large excess over the starting material (10 eq in the present case) otherwise the reaction is slow.
Data
dH (400 MHz, DMSO-d6) 9.00 (1H, d, J 7.56, NH), 8.13 (3H, br s, NH3+), 7.35-7.23 (10H, m, HAr), 4.55 (1H, dd, J 14.16, 7.76, CH*NH3+), 4.06 (2H, q, J 7.10, CH2CH3), 4.08-4.03 (1H, m, CH*CO2Et), 3.14-2.90 (4H, m, PhCH2), 1.11 (3H, t, J 7.10, CH2CH3).
dC (100 MHz, DMSO-d6) 170.74 and 168.32 (CH-C=O), 158.29 (q, J 31.08, CF3-C=O), 136.73, 134.80, 129.60, 129.16, 128.51, 128.40, 127.18, and 126.76 (CAr), 117.29 (q, J 299.82, CF3), 60.79 (CH*CO2Et), 53.97 (CH*NH3+), 53.16 (CH2CH3), 36.92 (PhCH2), 36.79 (PhCH2), 13.94 (CH2CH3).
HRMS (ESI) m/z: [M-TFA+H]+ calcd for C20H25N2O3 341.1860; found 341.1845.
Keywords
amino acids, Boc deprotection, peptides