ABSTRACT
The use of Diels-Alder cycloadditions of 2-pyrones was applied to the synthesis of many useful compounds, such as barrelene,
colchicine,
etc. This chapter discusses the some selective cycloaddition reactions of photochemically excited 2-pyrones with various alkenes, together with the photochemistry of 2-pyrone itself. It is well known that the 2-pyrone ring itself undergoes a wide range of photochemical reactions, such as an electrocyclic ring-opening reaction of
to give
by way of ketene formation
,
valence isomerization
of
to give
and dimerization
of
to give
,
, and
(Schemes 1, 2, and 3). The valence isomerization reaction of 2-pyrone
provided a path to the synthesis of cyclobutadiene
,
which was also synthesized in the hemicarcerand as a host compound.
