Biomedical Engineering Reference
In-Depth Information
FIGURE 5.1
Benzoxazole library
69
.
sequence (Scheme 5.27) [30]. The intramolecular cycloadditions of these strained
ring systems with tethered alkene and alkyne dienophiles led to novel heterocyclic
scaffolds.
N
-Allylations of bicyclo[1.1.0]butanes
70
proceeded smoothly under mod-
ified phase-transfer conditions (allyl bromide, Bu
2
NHSO
4
, 50% NaOH aq., toluene),
and the alkylated intermediates spontaneously underwent further transformations
that were dependent on the electronic nature of the allylic substituent (Scheme 5.27).
When bicyclo[1.1.0]butane
70
was substituted with alkyl groups (R
3
alkyl), the
reaction proceeded via an Alder-ene pathway to the spirocyclic butenes
72
. Inter-
estingly, under the same reaction conditions, the
N
-allyl bicyclobutane with R
3
=
=
aryl substitution gave the unprecedented 3-azatricyclo[5.1.1.0]nonanes
73
by means
of a formal [2
+
2] reaction pathway. It was also observed that only bicyclobu-
tanes conjugated to aromatic rings (R
1
=
aryl) participated in these domino reaction
sequences.
