1,3‐Dipolare Cycloaddition elektrophiler Azide an cyclische Keten‐N,X‐acetale. — Stickstoff‐Extrusion und Ringerweiterung der [3 + 2]‐Cycloaddukte

1,3‐Dipolar Cycloaddition of Electrophilic Azides to Cyclic Ketene N,X‐Acetals. — Extrusion of Molecular Nitrogen and Ring Expansion of the [3 + 2] Cycloadducts. The electrophilic azides 2 react with cyclic ketene N,X‐acetals of type 7 , e.g. 13, 15, 18, 21, 23, 25 , and with the alkylidene‐dihydroindoles 28 as well, to produce, besides molecular nitrogen, ring‐expanded products of type 11 and 12 , e.g. 14, 16, 19, 22, 24, 26 , and 29 (path A ), and/or N ‐sulfonylimines 10 , viz. 17, 20, 27 , and 31 , besides diazo compounds (path B ). The configurations of 16b, 19, 24a , and 26a are elucidated by means of X‐ray diffraction analyses. The envelope conformations of the hetero rings of 19, 29 undergo ring inversion with rates in the range of the 1 H NMR time scale [ 19 : Δ G # c (285 K) = 54.8 kJmol −1 , 29 ; Δ G # c (301 K) = 61.4, Δ G # c (314 K) = 60.7]. The formation of the ring‐expanded products 11 and 12 is interpreted in terms of an initial [3 + 2] cycloaddition leading to unstable spiro compounds 8 . Opening of their dihydro‐1,2,3‐triazole ring generates the zwitterions 9 which lose molecular nitrogen with concomitant ring expansion by a 1,2‐shift of the more soft one of the atoms N and X.