Br[CH:2]1[C:7](=O)[CH2:6][CH2:5][N:4]([C:9]([O:11][CH2:12][C:13]2[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=2)=[O:10])[CH2:3]1.[CH3:19][O:20][C:21]1[CH:29]=[CH:28][C:24]([C:25]([NH2:27])=[S:26])=[CH:23][CH:22]=1>C(O)C>[CH2:12]([O:11][C:9]([N:4]1[CH2:5][CH2:6][C:7]2[N:27]=[C:25]([C:24]3[CH:28]=[CH:29][C:21]([O:20][CH3:19])=[CH:22][CH:23]=3)[S:26][C:2]=2[CH2:3]1)=[O:10])[C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1Compound 502 (2.0 g, 6.5 mmol) was dissolved in ethanol (20 ml). To the solution was added 4-methoxythiobenzamide (503, 1.09 g, 6.5 mmol, 1 eq) and the yellow mixture was refluxed for 17 hrs after which time TLC analysis showed full conversion of 502. The reaction mixture was concentrated in vacuo and the residue was dissolved in EtOAc, washed with 5% Na2SO4 and the organic layer was dried (MgSO4) and subsequently concentrated under reduced pressure. The resulting oil was purified by silica gel chromatography (diethyl ether, Rf 0.3) giving compound 504 in a yield of 49% as a colorless oil. Compounds having a Rb substitute can be prepared by choosing a properly substituted (Rb)-4-methoxythiobenzamide (503).
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