Manipulating the enone moiety of levoglucosenone: 1,3-Transposition reactions including ones leading to isolevoglucosenone (original) (raw)
The [3,3]-sigmatropic rearrangement of derivatives of the epimeric allylic alcohols arising from the 1,2-reduction of levoglucosenone became the focal point of our initial studies and efficient means for obtaining the first of these substrates are shown in Scheme 1. Specifically, then, enone 1 was subjected to 1,2-reduction in methanol at 0 °C using sodium borohydride in the presence of cerium trichloride and in this way a ca. 11:1 mixture of the anticipated and epimeric allylic alcohols 3 and 4 was obtained in 91% combined yield. 6 These could not be separated by conventional means and so the mixture was acetylated under standard conditions and the product esters 5 6 (92%) and 6 6 (8%) were each isolated in pure form using flash chromatographic techniques and then fully characterized. Each was treated with potassium carbonate in methanol and thus regenerating the corresponding and now epimerically pure allylic alcohols 3 (97%) and 4 (92%), respectively. The spectral data acquired on these were in agreement with those reported in the literature. 6 The preferential formation of epimer 3 in the initial reduction step is entirely consistent with previous observations and necessarily arises through selective delivery of hydride from the sterically less congested α-face of the enone moiety in substrate 1 (the corresponding β-face being shielded by the presence of the sterically demanding 1,6-anhydro-bridge).
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