Photochemistry of dienones. Part IX. Photochemistry of (E)- and (Z)-α-ionone

✍ Scribed by Cornelis P. Visser; Hans Cerfontain

• Publisher: Elsevier Science
• Year: 2010
• Tongue: English
• Weight: 359 KB
• Volume: 100
• Category: Article
• ISSN: 0165-0513
• DOI: 10.1002/recl.19811000406

✦ Synopsis

Abstract

The photochemical reactions of (E)‐α‐ionone (1) and its photoproducts have been re‐examined. Upon irradiation with λ 254 and 313 nm a sequence of photo‐reactions takes place: initially (Z)‐α‐ionone (2) is formed and subsequently (E)‐ and (Z)‐retro‐α‐ionone (4a and b), which eventually undergo a 1,3‐ and 1,5‐acyl shift to give 3‐acetyl‐2,4,4‐trimethyl‐3‐ (5) and 6‐acetyl‐1,3,3‐trimethyl‐2‐vinylcyclohexene (6), respectively. Evidence is presented for the intermediacy of the photo‐enol 3a in the photoconversion of the α‐ionone to retro‐α‐ionone. The multiplicities of the reacting excited states for the (E) → (Z)‐α‐ionone and the α‐ionone → retro‐α‐ionone conversions (using triplet photosensitization) and for the 1,5‐acyl shift of retro‐α‐ionone (using CIDNP) have been shown to be triplet, singlet, and singlet, respectively. Upon irradiation of 1 with λ 366 nm, instead of the shorter wavelengths, the reaction stops at the (E)‐ and (Z)‐retro‐α‐ionone stage. Variation in the solvent did not alter the course of the photoconversion.