Photochemical reactivity of 6α-hydroxy-7-keto neoclerodane diterpenoids

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Abstract

The photochemical reactivity, in methanol at λ = 254 nm, of two 6α-hydroxy-7-keto neoclerodane, isoeriocephalin (1) and teucrolivin B (2) was evaluated. From the first compound, two new products were obtained: the 6β-hydroxy epimer (3) and the ɛ-lactone (4). The second one yielded exclusively the new spiro γ-lactone (5). The formation of these new products can be explained by the well-known radical mechanism Norrish type I.

Introduction

The species belonging to the genus Teucrium (Lamiaceae) have afforded a great number of diterpenoids and more than 220 neoclerodane diterpenes have been described, all differing in the functional groups on the neoclerodane or 19-nor-neoclerodane skeleton [1]. These secondary metabolites are of interest because of their ecological role as antifeedant against some species of insects [2], [3], [4], [5], and potential role in the medicinal properties of the plants [6], [7], [8]. As part of our ongoing studies on neo-clerodane diterpenoids, we are interested in obtaining synthetic derivatives of the more abundant compounds in order to investigate how the structural modifications influence the biological activities of these compounds.

Section snippets

Results and discussion

In the frame of our works on the photochemical behavior of diterpenoids [9], [10], we decided to investigate the reactivity of two neoclerodanes both with a 6α-hydroxy-7-keto structural moiety: isoeriocephalin (1) isolated from Teucrium dunense [11] and teucrolivin B (2) isolated from Teucrium orientale and whose biological activity as antifeedant is already known [12].

Isoeriocephalin (1) contains in its structure several photochemical active sites such as furan ring, esters and ketone.

General

Ir spectra were determined with a Perkin-Elmer 257 instrument. 1H and 13C NMR spectra were obtained on Bruker AMX-600 operating at 600.13 and 150.9 MHz for proton and carbon, respectively. DEPT experiments were acquired on a Bruker AMX-300 spectrometer. Measurements were made on solution in CDCl3, chemical shifts were referred to TMS set at 0 ppm, and coupling constants are given in Hz. Heteronuclear two-dimensional 1H-13C correlations, one-bond HMQC (heteronuclear multiple quantum correlation)

Acknowledgments

This work was supported by Italian Government Project PRIN 2003 (Sostanze Naturali ed Analoghi Sintetici con Attività antitumorale) and Palermo University funds.

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