In 1996 Uemura and co-workers reported the isolation and
structural characterization of two novel alkaloids. Pinnaic
acid (1),1 which was isolated from the Okinawan bivalve
Pinna muricata, was found to be a specific inhibitor of
cytosolic phospholipase A2 (cPLA2) with an in vitro IC50 of
0.2 mM. cPLA2 is involved in regulating inflammation and
thus represents a potential target for drug discovery. Halichlorine
(2), which was isolated from the marine sponge
Halichondria okadai Kadota (Figure 1),2 inhibits the expression
of vascular cell adhesion molecule-1 (VCAM-1) with
an IC50 of 7 µg/mL and consequently has potential for the
treatment of arteriosclerosis, asthma, and cancer.3 As is
evident from examination of their structures, both pinnaic
acid and halichlorine possess several interesting features, one
of which they have in common being the azaspiro[4.5]decane
ring system.
Because of their intriguing structures and biological
activities, these alkaloids have attracted considerable attention
in the synthetic community. However, although a rather large
number of groups have published their respective approaches