Diastereoselectivity and Reactivity in the Diels-Alder Reactions of α-Chloronitroso Ethers

The structure of the α‐chloronitroso ether 1, obtained from the hydroximolactone 2 and tert‐butyl hypochlorite (89%), was established by X‐ray crystallographic analysis. The [4 + 2] cycloadditions of 1 with the dienes 3 and 8–11 led to the N‐unsubstituted 3,6‐dihydro‐2H‐1,2‐oxazines 6 and 12–16 in high enantiomeric excess (Table 1). Due to the additional α‐alkoxy group, the reactivity of 2 is much superior to the one of known α‐chloronitrosoalkanes. The reactive conformation of 1 was deduced from the X‐ray analysis as well as the high diastereoselectivity of the cycloadditions. The importance of the α‐alkoxy group was evidenced from the similar reactivity of the racemic α‐chloronitroso ethers 25–27 which were prepared from the hydroximo ethers 28–30 and tert‐butyl hypochlorite.