Santonic acid: Zn –HCl – ether reduction and ceric ammonium nitrate oxidation


1. Introduction
(2 )-a-Santonin (1, Figure 1), a naturally occurring sesquiterpene lactone, is an attractive starting material for the synthesis of several natural products, transformational studies and stereochemical correlations (Paknikar et al. 1994; Furtado et al. 1998; Blay et al. 2004; Natarajan et al. 2007; Arantes et al. 2009; Lamm et al. 2009; Li et al. 2010). (2 )-Santonic acid (2, Figure 1), easily accessible in a single step from (2 )-a-santonin (Birladeanu 2003) has, however, been explored to a very limited extent (Hortmann et al. 1968; Hortmann & Daniel 1972; Naik et al. 1987; Moyano et al. 2005; Williams et al. 2008). (2 )-Santonic acid (2) appears
to be a good precursor for bicyclic skeletons present among natural products. An aim of this work was to seek entry into bicyclo[3.3.0]octane systems using Zn –HCl – ether reduction (Toda et al. 1972) to obtain pinacol (3) (Figure 1) followed by acid-catalysed molecular rearrangement. Entry into bicyclo[4.3.0]nonane skeleton has been reported (Hortmann & Daniel 1972; Naik et al. 1987), and it was our interest to explore an alternate entry into this skeleton via oxidation of 2 with ceric ammonium nitrate (CAN).

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