Diastereoselectivity and assignment of absolute stereochemistry in the aza-Diels–Alder reaction of a sulfonylimino acetate with 1-methoxy-3-trimethylsilyloxybuta-1,3-diene

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Abstract
N-[(1R)-(–)-camphor-10-ylsulfonyl]imine 1b generated in situ from the corresponding brominated glycinate 4b reacts with Danishefsky's diene under thermal conditions to give a high yield of diastereoisomeric Diels–Alder adducts 7b and 6b in a 2.04 : 1 ratio. Under Lewis acid catalysis at –78 °C the diastereoselectivity improves slightly to 2.30 : 1 using titanium tetraisopropoxide. However, the sense of asymmetric induction is reversed when using diethylaluminium chloride as the catalyst. Determination of the absolute stereochemistry of the adducts 6b and 7b was achieved by comparison with pipecolic acid derivatives 10b and 12b and showed that 7b had the (2R)-configuration at the newly formed chiral centre and 6b the (2S)-configuration.