Scheme 2. Synthesis of the key core scaffold 6. Reagents and conditions: (a) 5 mol% [Rh(CO)2Cl]2, DCE, RT, 18 h, then H+, 61%. (b) Dimethyl acetylenedicarboxylate, PhMe, reflux, 72 h; then DDQ, RT, 30 min, 57%.
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