Intramolecular iron(II)-catalyzed nitrogen transfer reactions of unsaturated alkoxycarbonyl azides: A facile and stereoselective route to 4,5-disubstituted oxazolidinones

Intramolecular Fe-11-calalyzed reactions of various unsaturated alkoxy-carb onyl azides are described. The reactions occur in the presence of stoichiom etric amounts of trimethyl silyl chloride employing ethanol as the solvent. The corresponding 2-alkenyloxy-carbonyl azides 5, 9, 18, 20,, 22, and 24 g ave the products 7/8, 10/11, 19, 21, 23, and 25 of an olefin chloroaminatio n in moderate to good yields (47 - 72 %). The facial diastercoselectivity o f the ring closing C-N-bond forming step is good both in cyclic (20, 24) an d in acyclic substrates (5, 18, 22) (> 90% rls). The subsequent chlorine at om transfer occurs selectively in cyclic systems (20, 24) and in systems (9 b, 18) which exhibit a conformational bias in the postulated radical interm ediate 14. The lifetime tau of this elusive intermediate was estimated from the loss of stereochcmical information in conformationally unrestricted sy stems (9a, 22) and from the data obtained with a radical clock (31 --> 32). 2-Alkynyloxycarbonyl azides 34 and 36 also yield chloroamination products which are obtained exclusively as the (Z)-isomers 35 and 37 (81-99% yield). The products of the tert-butyl-substituted substrates 38 undergo an immedi ate rearrangement/solvolysis reaction in the reaction mixture and gave the 5-alkoxy-oxazolidinones 39 (93-99% yield).