P. Couture et al., KETENE ACETALS FROM THERMOLYSIS OF ARYLOXY METHOXY OXADIAZOLINES - EVIDENCE FOR CARBONYL YLIDE INTERMEDIATES, Canadian journal of chemistry, 75(3), 1997, pp. 326-332
Thermolysis of oxadiazolines (5) in benzene at 110 degrees C leads to
ketene acetals (11) as minor products. Carbonyl ylide intermediates (6
), and oxiranes (7), presumably in equilibrium with those ylides, are
implicated as unstable precursors of the ketene acetals although none
of the oxiranes (carbonyl protected alpha-lactones) were isolable and
only one of the ketene acetals was isolable in pure form. The evidence
points to the two-step sequence of thermolysis of oxadiazolines, name
ly, initial cycloreversion to N-2 and carbonyl ylide (6), rather than
concerted fragmentation to N-2, acetone, and carbene (12). The first-f
ormed ylide does fragment to carbene and acetone in a second step that
competes with oxirane formation. A tentative mechanism for reaction o
f 7 with 12, to afford 11, is advanced.