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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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783<br />

784<br />

+<br />

N<br />

O<br />

+<br />

CH3<br />

−<br />

N<br />

O<br />

Ph<br />

CH3<br />

−<br />

Ph<br />

KOH, TBAI<br />

toluene, reflux, 1 h<br />

REACTIONS WITH CYCLOPROPANES 393<br />

N<br />

Ph<br />

784 (54%)<br />

787<br />

N<br />

A: 40%<br />

B: 25%<br />

Ph<br />

CH 3<br />

O<br />

CH 3<br />

O<br />

A: KOH, TBAI, toluene, reflux<br />

B: NaOMe, MeOH, r.t.<br />

Scheme 2.326<br />

O<br />

N<br />

Ph<br />

786 (31%)<br />

N<br />

Ph<br />

788<br />

16%<br />

19%<br />

N<br />

789<br />

-<br />

16%<br />

A possible mechanism for the observed transformation <strong>in</strong>cludes the sequence<br />

outl<strong>in</strong>ed <strong>in</strong> Scheme 2.327: (i) propargyl (A) – allene (B) tautomerization,<br />

(ii) 8π-cyclization (C), (iii) N–O cleavage (diradical D), (iv) diradical recomb<strong>in</strong>ation<br />

(cyclopropanone derivative E), <strong>and</strong> (v) one or two step cyclizations of the<br />

azadienyl cyclopropanone <strong>in</strong>to azep<strong>in</strong>one F. The occurrence of cyclopropanones<br />

(type E), as <strong>in</strong>termediates, is supported by the formation, <strong>in</strong> some cases, of<br />

iso<strong>in</strong>doles (type I) (789) as m<strong>in</strong>or products (Scheme 2.327) (139, 850, 851).<br />

2.11. REACTIONS WITH CYCLOPROPANES<br />

Reaction of nitrones (790) with 1,1-cyclopropanediesters (791) <strong>in</strong> the presence<br />

of Yb(OTf)3 affords tetrahydro-1,2-oxaz<strong>in</strong>es (792) via a homo 3 + 2 dipolar<br />

cycloaddition (Scheme 2.328) (852).<br />

+<br />

+<br />

+<br />

Ph<br />

CH3<br />

O<br />

CH 3<br />

CH3<br />

O

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