Imidazole, Oxazol und N‐Methylpyrazol als Dienophile in der Diels‐Alder‐Cycloaddition mit inversem Elektronenbedarf

Abstract

Unterschiedlich ist die Produktverteilung: Die N-alkylierten Imidazole 7 fuhren bevorzugt ;cu den substituierten Pyridazinen 9; mit 7b als Edukt ist das in 73% Ausbeute gewonnene 9b das einzig isolierbare Reaktionsprodukt. Dasselbe gilt fur 14, das in 72% Ausbeute das Amidin 15 liefert. Im Gegensatz da7u ubenviegt bei Einsatz von 11 als Edukt der aromatische Heterobicyclus 12 als Reaktionsprodukt, das in 62% Ausbeute isoliert werden kmn, neben nur 10% des ringoffenen Produkts 13. In den NMR-Spektren der

Diels–Alder Reaction in the Synthesis of Fluorinated (Hetero)Aromatic Compounds

“…Thus, reactions with 4,5‐unsubstituted imidazoles proceeded smoothly through the intermediate 72 giving mixtures of the 1 H ‐imidazo[4,5‐ d ]pyridazines 73 by oxidative aromatization. Pyridazines 74 were formed as a result of imidazole ring‐opening (Scheme ) . Similar ring‐opening products were also isolated in the case of reaction with oxazole and 4‐methyl imidazole (products 75 and 76 , respectively), while in the case of 1‐methyl‐pyrazole the fused product 77 was isolated.…”

“…Another remarkable example of reactivity of tetrazine 71 with electron‐rich indoles 78 and 79 was reported by Haider and Wanko . While in the case of indole 78 the fused product 80 was formed through MeSH‐elimination, the Diels–Alder reaction of 79 was succeeded by ring‐opening and pyridazine 81 was the only isolated product (Scheme ) …”

“…Most of the chemistry towards products I was recently reviewed by Cossy et al [31] [32] Similar ring-opening products were also isolated in the case of reaction with oxazole and 4-methyl imidazole (products 75 and 76, respectively), while in the case of 1methyl-pyrazole the fused product 77 was isolated. In a similar way, compound 71 reacted also with some methoxysubstituted pyridine and quinoline derivatives, in which the corresponding fused pyridazines were isolated in low yields even after prolonged heating in chlorobenzene at 130°C (12-18 d).…”

“…Synthesis of compounds 73 and 74 by reactions of tetrazine 71 with 3,4‐unsubstituted imidazoles and products 75 – 77 by reaction of tetrazine 71 with 4‐methylimidazole, oxazole, and 1‐methyl pyrazole, respectively …”

“…[34] While in the case of indole 78 the fused product 80 was formed through MeSH-elimination, the Diels-Alder reaction of 79 was succeeded by ring-opening and pyridazine 81 was the only isolated product (Scheme 32). [32] Compound 80 can also participate in Diels-Alder reactions with enamines under prolonged heating.…”