Synthesis and photochemistry of (2S,4R)- and (2R,4R)-1,2,3,4-tetrahydro-4-ethyl-1,1,4-trimethyl-3Z-ethylidene-2-naphthalenols

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Abstract

The synthesis and photochemistry of optically pure (2S,4R)- and (2R,4R)-1,2,3,4-tetrahydro-4-ethyl-1,1,4-trimethyl-3Z-ethylidene-2-naphthalenols 10a and 11a are described. Naphthalenols 10a and 11a were prepared in seven steps from 2-phenylbutanenitrile. Synthesis is mainly based upon acylation of isobutene by the optically active 2-methyl-2-phenylbutanoyl chloride (5), and a Wittig reaction, while the ethylidene naphthalenones 8a and 9a are separated by HPLC. Under inert conditions, irradiation of both alcohols gives rise to E-Z isomerization, followed by photodegradation reactions, leading to the formation of indane 12 by the elimination of formaldehyde. Irradiation of oxygenated solutions of 10a reveals a rapid photoaddition of oxygen to the double bond, leading to the stereo- and regioselective formation of hydroperoxide 13 and epoxy alcohol 15a, which hydrolyses on standing to triol 16a. The observed stereochemistry is explained by the stereoselective coordination of the hydroxyl group at the chiral allylic site with the attacking electrophilic oxygen. In the case of 11a a more complicated reaction mixture was obtained in which, analogously, the epoxy alcohol 15b and triol 16b were present. Semi-empirical MNDO, AM1 and PM3 calculations revealed that, in the preferential ground state conformation, the hydroxyl group occupies an orthogonal position with respect to the exocyclic double bond. Fluorescence measurements revealed that the reactive state in the photochemistry of these naphthalenol systems under inert conditions is the triplet state. In the photoreactions of optically pure compounds 10a and 11a, we were unable to identify products analogous to 2, which are claimed to originate from the racemic precursors 1 via a planar 1,3 OH shift from a singlet (in the double bond) excited state. Clean photochemical reactions were observed only when 10a and 11a were irradiated in oxygenated solutions, resulting in stereo- and regiospecific photoproducts.
Original languageEnglish
Pages (from-to)307-317
Number of pages11
JournalRecueil des Travaux Chimiques des Pays-Bas
Volume113
Issue number6
DOIs
Publication statusPublished - 1994

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