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■ Organomercury Chemistry of Iridoid Glucosides — Part 3

Andrea D’Annibale,* Carlo Iavarone,* and Corrado Trogolo*

*Centro CNR per lo Studio della Chimica delle Sostanze Organiche Naturali, Dipartmento di Chimica, Università degli Studi di Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy

Abstract

The hydroxymercuration (OM) reaction of nonconjugated diene system of aucubin (1) with at least 2 equivalents of Hg(II) salts involved not only the enol ether Δ³ double bond, but also the less reactive Δ⁷ cyclopentene double bond; the unique intermediate was observed to be the organobismercurial derivative (10). Surprisingly, the reductive demercuration (DM) of 10 with NaBH₄ gave a mixture of three products isoeucommiol (3), 6-hydroxy-8,9-dihydroxymethyl-2-oxabicyclo[3.2.1]oct-3-ene (5), and 7,8-dihydro-8-α-hydroxyisoeucommiol (6). The way these different compounds may be formed from common intermediate (10) is discussed. The aptitude to reversion showed by the OM reaction of the cyclopentene double bond of 1 was confirmed by the analogous tendency observed in the OM/DM of 3; this latter reaction afforded a good yield of the reaction product 6-hydroxy-8,9-dihydroxymethyl-2-oxabicyclo[3.2.1]octane (12) only when the DM of organomercurial intermediate (13) was carried out in basic conditions. Lastly, when the DM of monomercurial intermediate (2) was performed with Zn/CH₃COOH, this led to the conservation of the hidden 1,5-dialdehydic system of 1 and the formation of aucubigenin (16), i.e. to the achievement of a nonenzymatic hydrolysis of 1.