Scheme 4.
Synthesis of the ‘853 derivatives with alkoxy substituents in positions 6, reagents and conditions: (a) PyBOP, HOAt, DMF, microwave irradiation (5a: 56%, 5b: 54%); (b) EDC × HCl, HOAt, DMF, rt (5c: 23%, 5d: 54%, 5e: 44%).
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