Synthesis, Olfactory Evaluation, and Determination of the Absolute Configuration of the 3,4‐Didehydroionone Stereoisomers

Abstract

The synthesis of 3,4‐didehydroionone isomers 4, (+)‐6, and (−)‐6 and of 3,4‐didehydro‐7,8‐dihydroionone isomers 5, (+)‐7, and (−)‐7 was accomplished starting from commercially available racemic α‐ionone (1). Their preparation of the racemic forms 4–7 was first achieved by mean of a number of chemo‐ and regioselective reactions (Schemes 1 and 2). The enantio‐ and diastereoselective lipase‐mediated kinetic acetylation of 4‐hydroxy‐γ‐ionone (10a/10b) provided 4‐hydroxy‐γ‐ionone (+)‐10a/(±)‐10b and (+)‐4‐(acetyloxy)‐γ‐ionone ((+)12b) (Scheme 3). The latter compounds were used as starting materials to prepare the 3,4‐didehydro‐γ‐ionones (+)‐ and (−)‐6 and the 3,4‐didehydro‐7,8‐dihydro‐γ‐ionones (+)‐ and (−)‐7 in enantiomer‐enriched form. The absolute configuration of (+)‐12b was determine by chemical correlation with (+)‐(6S)‐γ‐ionone ((+)‐3) and with (−)‐(6S)‐α‐ionone ((−)‐1) therefore allowing to assign the (S)‐configuration to (+)‐6 and (+)‐7. Olfactory evaluation of the above described 3,4‐didehydroionone isomers shows a significant difference between the enantiomers and regioisomers both in fragrance feature and in detection threshold (Table).

Sesquiterpenes from the Marine Red Alga Laurencia saitoi

“…On the other hand, no NOESY correlations of HÀC(6) with the adjacent Me(12)/Me(14) were observed, while correlations of HÀC (4) with Me(12)/Me(14) could clearly be discerned in the NOESY spectrum. This was consistent with a cis-orientation between HÀC(4) and Me (12) and Me(14), and a trans-orientation between HÀC(6) and Me (12) and Me(14). The above spectroscopic evidence established the relative configuration and resulted in the Tables 1 and 2) with those of compound 1 suggested a close structural similarity between them.…”

“…This was confirmed by the spin systems observed in the HMBC spectrum (Fig. ), including the correlations from Me (12) and Me(13) to C(6), C(10), and C(11), from Me(14) to C(6), C(7), and C(8), from CH 2 (5) to C(3), C(4), and C(6), from CH 2 (9) to C(7), C(8), and C(10), and from HÀC(6) to C(4), C(5), C(7), C(11), C(12), and C(14). Another spin system was also established by the HMBC data to expand from CH 2 (15) to C(1), C(2), and C(3).…”

“…The 13 C-NMR, along with the DEPT experiments, revealed the presence of 13 C-atoms including three Me, three CH 2 , and four CH groups, as well as three quaternary C-atoms. Comparison of the NMR data of 4 with 4-hydroxy-g-ionone [12] suggested that it had the same cyclohexane ring sub-unit, which was confirmed by the HMBC data (Fig.). The relative configuration of 4 at C(2) and C(6) was shown to be the same as of g-ionone by analysis of the NOESY spectrum and detailed NMR data comparison [12] [13].…”

Natural sesquiterpenoids

“…32 Racemic a-ionone has been used as starting material for syntheses of the enantiomeric forms of a-and c-damascone 33 and of 3,4-didehydroionone stereoisomers. 34 Enantioselective syntheses of (+)-ricciocarpin A 35 and striatenic acid 36 have been achieved. A total synthesis of the trinor-cyclonerolidane sesquiterpene (−)-chokol A has been reported.…”

Carotenoids and Related Polyenes, Part 12 First Total Synthesis and Absolute Configuration of 3'-Deoxycapsanthin and 3,4-Didehydroxy-3'-deoxycapsanthin

“…Next, the C 15 -Wittig salt 24 was prepared for the synthesis of 3,4-didehydroxy-3Ј-deoxycapsanthin (2). EmmonsHorner reaction of 3,4-didehyro-b-ionone (21), prepared 18) from commercially available a-ionone, with triethyl phosphonoacetate in the presence of NaH provided the ester 22 (81%) as an isomeric mixture (9E/Z ca. 85/15), which without separation, was reduced with LiAlH 4 to afford the alcohol 23 in 96% yield.…”