Asymmetric induction in the Diels–Alder reactions of α-hydroxy acyl nitroso compounds

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Abstract
The hydroxamic acids 7, derived from a series of α-hydroxy acids 5, have been oxidised with periodate to form transient, chiral acyl nitroso compounds 8, which were trapped in situ with cyclopentadiene and cyclohex-1,3-diene to give mixtures of diastereoisomeric, Diels–Alder cycloadducts 9 and 10, respectively. Cycloaddition at 0 °C occurred with moderate stereoselectivity, e.g. both the mandeloyl nitroso compound 8a with cyclopentadiene and the tert-butylglycoloyl nitroso compound 8d with cyclohexadiene gave ca. 5:1 mixtures of diastereoisomers. Much higher diastereoselectivities (cycloadduct ratios ca. 10:1 for 9a, 9d and 10d) were observed at –78 °C. The mandeloyl nitroso compound 8a, which can form an intramolecular hydrogen bond between the hydroxy and nitroso groups, showed higher stereoselectivities than its O-methyl ether 8b. The major cycloadduct 13a of the (S)-mandeloyl nitroso compound 12 and cyclohexadiene was degraded to the bicyclic oxazine 15 of known absolute configuration. Formation of the cycloadduct 13a as the major product is consistent with preferential endo addition of the hydrogen bonded nitroso compound 12 from the face anti to the phenyl group.