Cinnoline is an aromatic heterocyclic compound with the formula C8H6N2. It is isomeric with phthalazine.
The free base can be obtained as an oil by treatment of the hydrochloride
with base. It co-crystallizes with one molecule of ether as white silky needles, (m.p. 24-25 °C) upon cooling etherial solutions. The free base
melts at 39 °C. It has a taste resembling that of chloral hydrate
and leaves a sharp irritation for some time. Cinnoline derivatives are obtained from oxycinnolin carboxylic acid, which is formed by digesting orthophenyl propiolic acid diazo chloride with water. Oxycinnolin carboxylic acid on heating gives oxycinnoline, melting at 225 °C, which with phosphorus pentachloride
gives chlorcinnolin. This substance is reduced by iron filings
and sulfuric acid
Discovery and synthesis
The compound was first obtained in impure form by cyclization of the alkyne o
H in water to give 4-hydroxycinnoline-3-carboxylic acid. This material could be decarboxylated and the hydroxyl group reductively removed to give the parent heterocycle. This reaction is called the Richter cinnoline synthesis
Improved methods exist for its synthesis. It can be prepared by dehydrogenation
with freshly precipitated mercuric oxide
. It can be isolated as the hydrochloride
Cinnolines are cinnoline derivatives. A classic organic reaction for synthesizing cinnolines is the Widman-Stoermer synthesis , a ring-closing reaction of an α-vinyl- aniline with hydrochloric acid and sodium nitrite:
The sodium nitrite is first converted to nitrous acid which then forms the electrophilic intermediate dinitrogen trioxide. The next intermediate is the stable Nitrosamine with goes on to lose water forming the diazonium salt which then reacts with the vinyl group in the ring-closing step. A conceptually related reaction is the Bamberger triazine synthesis towards triazines.
Another cinnoline method is the Borsche cinnoline synthesis.
Cinnoline is toxic.