Facile synthesis of a B,D-tetradehydrocorrin and rearrangement to bacteriochlorins

Abstract

Tetradehydrocorrins contain an A–D ring junction and a level of saturation intermediate between that of a corrin (characteristic of vitamin B₁₂) and that of a corrole. Self-condensation of a p-tolyl-substituted dihydrodipyrrin-acetal (DHDPA-T) in CH₂Cl₂ containing Yb(OTf)₃ at room temperature afforded the corresponding free base B,D-tetradehydrocorrin (TDC-T) as the sole macrocycle in 77% yield. The presence of a geminal dimethyl group in each of the reduced rings in TDC-T affords stability toward adventitious dehydrogenation. Treatment of TDC-T to acidic conditions results in rearrangement and loss of methanol to give a bacteriochlorin with or without a 5-methoxy substituent; the yield was 56% (5-methoxybacteriochlorin MeOBC-T), 35% (MeOBC-T) or 20% (5-unsubstituted bacteriochlorin HBC-T) with TMSOTf/CH₂Cl₂, BF₃·OEt₂/CH₃CN or BF₃·OEt₂/CH₂Cl₂, respectively. TDC-T undergoes regioselective bromination with NBS to give the 10-bromotetradehydrocorrin in 22% yield. The conversion of the (non-aromatic) tetradehydrocorrin to the (aromatic) bacteriochlorin reflects the relative stability of the two macrocycles and indicates the former may be an intermediate—kinetically trapped or transient depending on catalysis conditions—upon dihydrodipyrrin-acetal self-condensation.