198 Lcvcl2- Case 30
Experimental Data
1. LUMO energy levels and atom coefficients in 1,2,4,5-tetrazines 1 (obtained
from AMI computational studies).
NHAc
0.59~
N -...:N
11
1
N /- N
0.53~
SCH3
1a
ELUMO = -1.39 eV
NHBOC
0.59~
N -...:N
11
1
N /-N
0.54~
SCH3
1b
ELUMO = -1.36 eV
2. Ethyl vinyl eher: EHoMo -8.0 eV; C-1 coefficient 0.280; C-2 coefficient 0.720).
Discussion
Mechanism of the resction
Ifwe consider the general reactivity between tetrazines 1 and electron-rich dienophiles indicated in Scheme 30.1, the reaction between ethyl vinyl ether (EDG =
OEt and R
2 = H) and 1 should yield products with the structure 4 (Scheme 30.2).
However, 4 does not look like the expected cycloaddition adduct between 1 and
ethyl vinyl ether. In fact, the expected [4+2] adduct should be more like bicyclic
product 5, with faur nitrogen atoms in a six-membered ring and an ethoxy group
incorporated somewhere in the structure. We could then consider that, if 5 has
been formed at first instance, some further transforma6ons of this product have
arisen after the cycloaddition step.
Scheme30.2
NHR
1
N~N
11
1
NYN
SCH3
1
+
R
1 =Ac, BOC
N)tNHR1 HH
11
N
OEt
SCH3
5
N?:R1
11
N b
SCH3
4