Regioselective Synthesis of Aurone Derivatives via PBu₃‐Catalyzed Cyclization of 2‐Alkynoylphenols

Abstract

Aurone derivatives were synthesized in good to high yields by PBu 3 -catalyzed intramolecular 5-exo cyclization of 2-alkynoylphenols. The reaction proceeds in high regioselectivity without forming 6-endo cyclization products.

Novel Synthesis of Aurones by 2‐PyONa‐catalyzed Regioselective Cyclization of o‐(Alkynon‐1‐yl)phenols

“…( Z ) Assignment of CC in aurones was based on the 1 H chemical shift of the vinylic proton. For example, the chemical shift of the vinylic Cβ proton in ( Z )‐aurone ( 6af ) was observed as δ 6.90 ppm, which was consistent with the previous results …”

“…However, the cyclization of o ‐(alkynon‐1‐yl)phenols using K 2 CO 3 or NaOEt was problematic because it provided a mixture of aurones and flavones by 5‐ exo and 6‐ endo attack, respectively . Recently, the use of Lewis bases such as a catalytic PBu 3 and 3 equiv of Cs 2 CO 3 on the 5‐ exo cyclization of o ‐(alkynon‐1‐yl)phenols afforded aurones together with trace amounts of flavones.…”

Efficient Synthesis of (Z)‐Aurones by the Thallium(I) acetate‐Catalyzed 5‐exo Cyclization of o‐(Alkynon‐1‐yl)phenols

“…The use of K 2 CO 3 in acetone favored the 6‐ endo cyclization, whereas the 5‐ exo cyclization process was enhanced by NaOEt or K 2 CO 3 in EtOH . The use of bases such as 5 mol % PBu 3 , 3 equiv of Cs 2 CO 3 , and 10 mol % 2‐PyONa for the cyclization of o ‐(alkynon‐1‐yl)phenols afforded ( Z )‐aurones by highly regioselective 5‐ exo cyclization (Scheme ). The cyclization of o ‐(alkynon‐1‐yl)phenols using 0.3 equiv of AgNO 3 in CH 3 OH or 3 mol % AgOAc in DMF also led to 5‐ exo cyclization via dual π‐coordination of Ag(I) to give ( Z )‐aurones at room temperature.…”

Synthesis of Flavones by Thallium(III) p‐tosylate‐catalyzed Regioselective Cyclization of o‐(Alkynon‐1‐yl)phenols

“…However, the use of PBu 3 (0.05 equiv) in EtOH or Cs 2 CO 3 (3 equiv) in acetone on the cyclization of o ‐(alkynon‐1‐yl)phenols induced the 5‐ exo cyclization with a high regioselectivity to give aurones. The treatment of o ‐(alkynon‐1‐yl)phenols with AgNO 3 (0.3 equiv) in CH 3 OH or AgOAc (0.03 equiv) in DMF also induced high 5‐ exo cyclization via dual–coordination process to give aurones.…”