Unusual Enantiopure Heterocyclic Skeletons by Lewis Acid Promoted Rearrangements of 1,3‐Dioxolanyl‐Substituted 1,2‐Oxazines

Lewis acid promoted rearrangements of different 4‐alkoxy‐substituted 1,2‐oxazines syn‐1 are reported. Depending on the nature of this alkoxy group different reaction pathways are possible either providing bicyclic 1,2‐oxazinones 2 or the novel tricyclic products 3–5. A mechanistic rational describing the role of the 4‐alkoxy group is presented. The key step for formation of tricyclic skeletons 3–5 is a 1,2‐alkyl shift. Hydrogenation reactions of these tricyclic compounds gave unsaturated 1,2‐oxazines 12 and 13 or tetrahydrofurans 15a–c/16a–c depending on the time of hydrogenolysis. Tetrahydrofuryl‐annulated 1,2‐oxazine 12 was used for further transformations into complex substituted tetrahydrofurans. Reduction with sodium cyanoborohydride and subsequent cleavage of the N,O‐bond by hydrogenation furnished aminofuran derivative 19. Alternatively treatment with a strong base such as n‐butyllithium afforded imidoester 21 via a Beckmann‐type fragmentation.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)