electron demand DielsꢀAlder reactions with enamine12 and
ynamine13 dienophiles. The studies reported to date have
been derived largely from the pioneering efforts of Okatani
(Sugita)12 or Igeta and Ohsawa,13 sometimes suggesting
modest utility, and have only rarely12eꢀg,13b focused on the
parent 1,2,3-triazine (1)14 itself. As a result and in a con-
tinuation of our efforts to explore heterocyclic and acyclic
azadiene DielsꢀAlder reactions and their applications,15ꢀ18
herein we report our examination of the scope of the inverse
electron demand DielsꢀAlder reactions of the parent 1,2,3-
triazine (1) including the first disclosure of its unique cap-
abilities for participating in previously unexplored [4 þ 2]
cycloadditions with heterodienophiles.
[4 þ 2] cycloaddition versus aromatization and that may
account in part for the modest conversions.
Similarly, the reaction of 1 with ynamines proceeded well
as reported13 but required warming the reaction mixtures at
60 °C (CHCl3, 3ꢀ12 h) for complete reaction (eq 2). Other
dienophiles examined, including ketene acetals ((EtO)2Cd
CH2, xylene, 140 °C, 24 h), ethoxyacetylene (dioxane, 100 °C,
24 h), phenylacetylene (dioxane, 100 °C, 24 h), and enol
ethers (Ph(OMe)CdCH2 and Ph(OTMS)CdCH2, dioxane,
100 °C, 24 h), failed to react with 1 under the conditions
examined.
Figure 1. DielsꢀAlder reactions of select heterocyclic azadienes.
More interesting was the reactivity of 1 toward hetero-
dienophiles. Amidines, imidates, and related reagents react
Consistent with prior reports,12 the reaction of 1 with
enamines (CHCl3, 60 °C) proved more limited than the
analogous reactions of 1,2,4- or 1,3,5-triazines, providing
modest yields of the expected pyridine product (eq 1).
Products derived from liberated pyrrolidine addition to
the starting 1,2,3-triazine were detected that reflect a slow
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