alpha-iso-Methylionone (CAS 127-51-5) is a synthetic terpenoid ketone used primarily in perfumery. Here are its key chemical characteristics:

1. Chemical Structure

• IUPAC Name:
  (E)-1-(2,6,6-Trimethylcyclohex-2-en-1-yl)pent-1-en-3-one

• Molecular Formula: C??H??O

• Core Structure:

• Cyclic moiety: 2,6,6-Trimethylcyclohex-2-enyl ring (derived from ionone backbone).

  Chain moiety: Unsaturated ketone (α,β-unsaturated enone: ?CH=CH?C(O)CH?).

  "iso" designation: Methyl group (?CH?) attached at the C1 position of the cyclohexenyl ring (distinguishing it from α-methylionone).

2. Isomerism & Chirality

• Stereoisomers: Exists as cis/trans isomers due to the double bond in the side chain (the E-isomer is most common).

• Chiral Centers: The cyclohexenyl ring has chiral centers at C1 and C6, leading to enantiomers (typically used as a racemic mixture).

3. Synthesis

Produced via acid-catalyzed aldol condensation:

Citral (or pseudoionone) + acetone → ionone intermediate.

Methylation: Selective allylic methylation at C1 of the ionone ring.

4. Key Chemical Properties

• Functional Groups: α,β-Unsaturated enone (electrophilic, Michael acceptor).

• Reactivity:

• Prone to oxidation under strong conditions.

  Can undergo Diels-Alder reactions (diene character in ring).

• Solubility: Insoluble in water; soluble in ethanol, oils.

• Boiling Point: ~295°C (decomposes).

5. Why "iso"?

The "iso" prefix indicates methyl substitution on the ring carbon adjacent to the side chain (C1), unlike α-methylionone where methylation is on the side chain.

6. Spectroscopic Identifiers

• IR: Strong C=O stretch at ~1670 cm?1; C=C at ~1600 cm?1.

• NMR: Characteristic vinyl proton signals (δ 5.5–6.5 ppm).

Practical Note: Its α,β-unsaturated ketone structure contributes to both its violet-like scent and potential skin sensitization (ability to form protein adducts). This reactivity is regulated in cosmetics (IFRA limits).