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neat or in solution. This decomposition was qualitatively assessed by collecting the 1 H NMR
spectrum of 280a at different times. A solution of 280a in CDCl3 stored at rt for 5 days resulted
in appearance of additional broadened resonances in the upfield region (1.0 – 2.0 ppm). The
decomposition was accelerated when 280a was stored neat. In contrast, storing 280a in
deoxygenated frozen benzene r at -10 °C proved advantageous in extending its shelf life up to
three months. Unfortunately, these observations rendered the newly synthesized fulvenes
unsuitable for biological evaluation because the results may be compromised by the presence of
impurities. The pathway of decomposition has not been determined, however it is well known
that simple alkyl-pentafulvenes readily polymerize in presence of traces of acid (Scheme
4.25). 184 Additionally, polymerization of pentafulvenes has been triggered by reaction with
molecular oxygen. 185
Scheme 4.25 Acid catalyzed polymerization of pentafulvenes.
R 1 R 2
H +
R 1 R 2
+
H
H
R 1 R 2 R 1
R 2
281 282 283
R 1
+
R 2
R 1 R 2
The electron density of the fulvene moiety in 270a and 270f is likely responsible for the
observed decomposition. Based on the structure of HMAF (252, Scheme 4.23), it was
hypothesized that an acyl group may impart greater stability to these compounds by reducing the
electron density of the fulvene. To examine this effect we sought to prepare fulvene 286 (Scheme
4.26). To this end, the cyclocarbonylation reaction of amide tethered allenyne 145e was briefly
examined.
r Benzene was deoxygenated prior to freezing by bubbling nitrogen via a needle.
101
R 1
R 2
284
n