Synthesis of Alkylphenols and -catechols from Plectranthus albidus (Labiatae)

Article abstract of DOI:10.1002/hlca.19930760509

In the preceding paper, we described the isolation and structure elucidation of a series of even-numbered phenol- or pyrocatechol-derived 1-arylalkane-5-ones.To establish the assigned structures unambiguously and to have larger quantities available for physiological testing, the following compounds were prepared: in the alkylphenol series, 1-(4'-hydroxyphenyl)tetradecan-5-one (2a), 1-(4'-hydroxyphenyl)hexadecan-5-one (2b), and 1-(4'-hydroxyphenyl)octadecan-5-one (2c); in the alkylcatechol series, 1-(3',4'-dihydroxyphenyl)decan-5-one (3a; not isolated as a natural compound), 1-(3',4'-dihydroxyphenyl)dodecan-5-one (3b), 1-(3',4'-dihydroxyphenyl)tetradecan-5-one (3c), 1-(3',4'-dihydroxyphenyl)hexadecan-5-one (3d), 1-(3',4'-dihydroxyphenyl)octadecan-5-one (3e), and 1-(3',4'-dihydroxyphenyl)icosan-5-one (3f); in the alkenylphenol series, (Z)-1-(4'-hydroxyphenyl)octadec-13-en-5-one (4a) and (E)-1-(4'-hydroxyphenyl)octadec-13-en-5-one (4b); in the alkenylcatechol series, (E,E)-1-(3',4'-dihydroxyphenyl)deca-1,3-dien-5-one (1) and (Z)-1-(3',4'-dihydroxyphenyl)octadec-13-en-5-one (5).All compounds proved to be identical with the previously assigned structures.Compound 1 was synthesized by regioselective aldol condensation of heptan-2-one with (E)-3-(3',4'-dimethoxyphenyl)prop-2-enal (6d; Scheme 1), the phenols 2a-c and the catechols 3a-f by addition of the corresponding alkyl Grignard reagent to 5-(4'-methoxyphenyl)- or 5-(3',4'-dimethoxyphenyl)pentanal (17c and 18c, resp.; Scheme 4), and the olefins 4a, 4b and 5 from 17c or 18c via the 9-O-silyl-protected 13-(4'-methoxyphenyl)- or 13-(3',4'-dimethoxyphenyl)tridecanals (26 and 27, resp.) and Wittig olefination as the key steps (Scheme 5).