Reaction of levoglucosenone with (±)-α-terpineol and its acetate

Abstract

In continuation of our studies [1, 2], we focused on the potential of levoglucosenone in the synthesis of menthane derivatives as a separate problem and examined the possibility for direct introduction of a menthane fragment into levoglucosenone molecule via reaction with (±)-α-terpineol. A short synthetic route to (±)-α-terpineol is the twostep procedure utilizing large-scale initial compounds, isoprene and methyl acrylate [3]. The transformation of (±)-α-terpineol into a chiral subunit requires its preliminary optical resolution and introduction of functional groups that could ensure subsequent transformations. It is known that dehydration of α-terpineol leads to the formation of a number of isomeric dienes, including α-terpinene whose yield in some cases attains 25% [4]. The use of such a mixture of menthane derivatives as diene component and of levoglucosenone as dienophile is attractive from the viewpoint of simultaneous solution of both problems. For this purpose, a toluene solution of levoglucosenone and diene mixture obtained from (±)-α-terpineol according to the procedure described in [4] was heated at 160°C in a sealed glass ampule (metohd a). In fact, after 9 h, we succeeded in isolating from the reaction mixture Diels–Alder adduct V in 3% yield [calculated on the initial (±)-α-terpineol]. Increase of the reaction time reduced the yield of V, and we failed to detect it in the reaction mixture after 30 h, presumably because of its decomposition. We also tried to use as starting compounds (±)-α-terpineol (I) and (±)-α-terpinyl acetate (II), expecting generation of α-terpinene (III) in situ under the given Diels–Alder reaction conditions. In the first case, no cycloaddition products were detected in the reaction mixture, and in the second case adduct V was formed in 4% yield in 28 h (method b). The structure of V was determined on the basis of the H and C NMR data. The coupling constant between 5a-H and 9a-H equal to 8.6 Hz indicates cis junction of the bicyclo[2.2.2]octane and pyran fragments. The signal assignment was confirmed by the HMBC technique. The 9a-H proton (δ 1.87 ppm) showed couplings with C (δC 68.29 ppm), C (49.29), C (200.96), C (138.26), and C (35.59), and cross-peaks of 5a-H (δ 2.90 ppm) with C, C (δC 44.98 ppm), C (135.53), and C (20.80) were