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Indole
Chemical formula	C₈H₇N
Molecular mass	117.15 g/mol
Melting point	52 - 54 °C
Boiling point	253 - 254 °C
Density	1.220 g/cm³
CAS number	120-72-9
SMILES	C1(NC=C2)=C2C=CC=C1

Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring.

Indole is solid at room temperature and has an intense fecal smell. At very low concentrations however it has a flowery smell and is a constituent of many flower scents (such as orange blossoms) and perfumes.

The indole structure can be found in many organic compounds like the amino acid tryptophan and in tryptophan containing protein, in alkaloids, or in pigments.

Indole undergoes electrophilic substitution, mainly at position 3. Substituted indoles are structural elements of (and for some compounds the synthetic precursors for) the tryptophan-derived tryptamine alkaloids like the neurotransmitter serotonin, melatonin, the hallucinogens psilocybin, DMT, 5-MeO-DMT, or the ergolines like LSD.

Other indolic compounds include the plant hormone Auxin (indolyl-3-acetic acid, IAA), the anti-inflammatory drug indomethacin, or the betablocker pindolol.

The name indole is derived from indigo, a blue pigment obtained from some plant species. The indigo molecule comprises two indole units joined together.

Although the indole nitrogen atom has a lone pair of electrons, indole is not basic like the amines because the lone pair is delocalised and contributes to the aromatic system.

History

Indole chemistry began to develop with the study of the dye indigo. This was converted to isatin and thence to oxindole. Then in 1866 Adolf von Baeyer reduced oxindole to indole using zinc dust, and in 1869 he proposed the formula for indole that is accepted today^[1].

Certin indole derivatives were important dyestuffs until the end of the 19th century. In the 1930s interest in indole intensified when it became known that the indole nucleus is present in many important alkaloids as well is in tryptophan and auxins, and it remains an active area of research today.

Synthesis of indoles

Leimgruber-Batcho indole synthesis

The Leimgruber-Batcho indole synthesis is an efficient method of sythesizing indole and substituted indoles. Origonally disclosed in patent #3,976,639 issued in 1976, this method is high yielding and can easily generate the substituted indoles often found in pharmaceutical drugs.

Fischer indole synthesis

The Fischer indole synthesis

One of the oldest and most reliable methods for synthesizing indoles is the Fischer indole synthesis developed in 1883 by Emil Fischer.

Other indole forming reactions

References

A. Baeyer, A. Emmerling, Chemische Berichte, 2, 679 (1869).
Indoles Part One, W. J. Houlihan (ed.), Wiley Interscience, New York, 1972.

@@ Line 45: / Line 45: @@
 ===[[Leimgruber-Batcho indole synthesis]]===
 [[Image:Leimgruber-Batcho_Indole_Scheme.png|right|350px|The Leimgruber-Batcho indole synthesis]]
-The [[Leimgruber-Batcho indole synthesis]] is an efficient method of sythesizing indole and substituted indoles.
+The [[Leimgruber-Batcho indole synthesis]] is an efficient method of sythesizing indole and substituted indoles.  Origonally disclosed in patent #3,976,639 issued in [[1976]], this method is high yielding and can easily generate the substituted indoles often found in pharmaceutical drugs.
 ===[[Fischer indole synthesis]]===
+[[Image:Fischer Indole Reaction Scheme.png|right|250px|The Fischer indole synthesis]]
 One of the oldest and most reliable methods for synthesizing indoles is the [[Fischer indole synthesis]] developed in [[1883]] by [[Emil Fischer]].