S-2,4,6-trimethoxybenzyl (Tmob): a novel cysteine protecting group for the N.alpha.-(9-fluorenylmethoxycarbonyl) (Fmoc) strategy of peptide synthesis (original) (raw)

1992, The Journal of Organic Chemistry

and the oil was separated into three fractions (SiOz, EtOAc). Fraction 2 was separated further by HPLC (SiOz, EtOAc; CIE column, MeOH-H20, 141) to give 6b as an oil: [ a ] %~ +go (c 0.2, CHC1,); 'H NMR, see Figure 2. Anal. Found for C13HuN06: 274.1653 (M + H) (HRFABMS). Formation of 6b and Lactam 15. A sample of synthetic N-Boc-(~S,~R,~S)-IOE~ (37 mg) was hydrolyzed with 1 N NaOH (0.1 mL) in dioxane (1 mL) at rt for 2.5 h. Solvent was removed in vacuo, and the resulting oil was treated with TFA in CHzClz (0.1 mL) for 40 min. Excess solvent was removed under Nz, and the residual material was heated with MeOH-AcCl(401) at 65 O C for 25 min. MeOH and HC1 were removed in vacuo, and the oil was treated with AczO and CSHSN (0.2 mL each) at rt for 1 h. The product was passed through a small SiOz column (EtOAc) and then was subjected to HPLC @io2, EtOAc) to give 6b, the (200 MHz, CDClJ, Figure 2. The more polar fraction gave lactam 15 (4.2 mg, 15%): [ a I m~-5O (c 0.5, CHCl,); IR (film) 1740,1699 cm-'; ' H NMR, Table 11; FABMS m/z 200 (M + H); EIMS m/z (re1 intensity) 156 (6.4), 142 (loo), 111 (64.8),100 (36.6),82 (1001, 43 (24.0). Anal. Found for C1&8N03 (M + H): 200.1287 (HRFABMS). NOE Difference Experiment on Lactams 15 and 16. Solutions of lactams 15 (4.4 mg) and 16 (3.5 mg), each in CDCl, (0.5 mL), were degassed with dry Ar, and their qualitative NOE difference spectra were recorded with an XL-200 spectrometer: relaxation delay = 10 e; number of transients = 180 (Figure 13s). Preparation of N,O-Bis(trifluomacetyl)isostatine Methyl Esters. Synthetic samples of all eight Boc-ieostatine methyl ester isomers were treated individually with TFAA and TFA at 100 OC for 5 min. Excess acid was removed under Nz, and each product was purified by HPLC @ioz, hexane-EtOAc, 51) to give 6c-13c. Optical rotations and GC retention times are listed in Table IV. Acid Treatment of Boc-(3S,4SPS)-isostatine Ethyl Ester. Four samples (13-mg each) were treated with 6 N HC1 at 110 OC for 4,11, 24, and 38 h, respectively. Solvent was removed, and each residue was treated with MeOH-AcCl(101) at 110 OC for 15 min. The methanolic HC1 was removed, and the resulting oil was treated with TFAA and TFA at 110 OC for 5 min. Each product was dissolved in 2-propanol (1 mL) for GC analysis. Acid Treatment of Boc-(3R,4SPS)-isostine Ethyl Ester. Four samples (4-mg each) were treated with 6 N HC1 at 110 "C for 4,12,36, and 42 h, respectively. The products were converted le88 polar oil (4.6 mg, 11%): [ C I ]~D +1l0 (C 0.6, CHC13); ' H NMR to the TFA ethyl ester derivatives using the procedure described above. Each sample was dissolved in 2-propanol(l mL) for GC analysis. Synthesis of Boc-(4S,5S)-2,3-anhydroisostatine Methyl Ester (20). A mixture of Boc-(3S,4S,5S)-and Boc-(3R,4S,5S)-Ist-OMe (25 mg, 0.086 mmol) was treated with 6 N HCl(1 mL) at 110 OC for 20 h. Aqueous HCl was removed under Nz, and the residue was treated with mixture of MeOH-AcCl (101) and concentrated and then treated with Boc-ON (30 mg) and Et,N (20 pL) in CHZCl2 at rt for 10 h. Solvent was removed, and the product was purified by HPLC using a phenyl column and hexane-2-propanol (201) to give pure 20 (3.6 mg, 14%): needles, mp 60 O C ; [aImD +2O (c 0.03, CHC1,); 'H NMR, Figure 21s. _ _-Anal. Calcd for Cl4H%NO4: 272.1862 (M + H). Found: 272.1859 (M + H) (HRFABMS). Acid Tktment'of 20. Four L p l e s of 20 (0.7 mg each) were treated with 6 N HC1 at 110 OC for 4, 12, 24, and 36 h. The resulting hydrolyzates were converted to TFA methyl ester derivative 20a using the procedure described above for GC analyaea: