Preparation and rearrangements of some bicyclic isoxazolines

Aromatic substitution via new rearrangements of heteroaromatic N-oxides

“…Thus, while the ylide could indeed still be arising from the car bene as in Scheme V in the acetylene reaction, or from 26 via the ring-contraction mechanism, the 3-alkylated product 19 clearly owed its origin to a different mechanism. One possibility is that intermediate 25 (Scheme IV) underwent ring closure to 29, but the objection to the intervention of 25 would still remain. A much more likely process in our opinion is a concerted rearrangement of 26 to 29 via a symmetry-allowed [ 28 + T2a + T4S] process (c).33 A diradical mechanism (d) cannot be ruled out, however.…”

Pyrrolizidine and Indolizidine Syntheses Involving 1,3-Dipolar Cycloadditions

“…In fact, they produce bicyclic cycloadducts which are subjected to cleavage of the isoxazole nucleus and subsequently to an appropriate sequence of transformations involving recyclization. Apart from two early occasional reports, 14,15 the first systematic work in this field is due to Tufariello's research group. [16][17][18][19][20] Since its copious results have been amply reviewed, 10,21,22 only a paradigmatic example concerning the total synthesis of dl-ret-ronecine 1 is reported here in detail (Scheme 1).…”

1,5-Dipolar cyclizations

“…Heterocyclic -oxides,158 i.e., nitrones in which the azo-385 methine moiety is incorporated into a ring, react as 1,3-dipoles with acetylenes to give 4-isoxazolines; however, these are in general not isolable under the reaction conditions and rearrange to carbonylazomethine ylides via acylaziridine intermediates. 154,159,160 Since the chemistry of this class of azomethine ylides has been the topic of several recent reviews145,161 and since their chemistry rarely involves 1,5-dipolar cyclization reactions,162 they will not be considered here.…”