(125a) Mechanistic Insights Into Ring-Opening and Decarboxylation of 2-Pyrones in Liquid Water and Tetrahydrofuran Solvents

Triacetic
acid lactone (Figure 1, 1),
5,6-dihydro-4-hydroxy-6-methyl-2H-pyran-2-one (Figure 1, 2), and
4-hydroxy-6-methyltetrahydro-2-pyrone (Figure 1, 3) are 2-pyrones that
we have recently shown to be compounds derived from biomass that may serve as
intermediates for the production of biorenewable chemicals. Using 1 as
the feedstock, we have demonstrated that a diverse range of commercially
valuable end products and chemical intermediates (e.g., 2 and 3)
may be obtained through various thermal and catalytic strategies.

Figure 1. 2-pyrones studied: triacetic
acid lactone (1); 5,6-dihydro-4-hydroxy-6-methyl-2H-pyran-2-one (2);
4-hydroxy-6-methyltetrahydro-2-pyrone (3).

The
molecular structures of these 2-pyrones differ from one another by varying
degrees of unsaturation in the pyrone ring, and we have found that they display
different reactivities to ring-opening and decarboxylation under identical
reaction conditions. Significantly, we have observed the thermally-activated
ring-opening and decarboxylation of both 1 and 2 in liquid water
at relatively low reaction temperatures (< 373 K) without the aid of a
catalyst. Also, it was found that while 3 does not undergo ring-opening
and decarboxylation under the reaction conditions employed, 3
selectively dehydrates to form parasorbic acid. The apparent activation energy
barriers for the thermally-activated ring-opening and decarboxylation of 1
and 2 in liquid water were measured to be 58 ± 12 kJ mol^-1 and
42 ± 18 kJ mol^-1 (95% confidence intervals), respectively. While
acidic conditions appeared to promote the ring-opening and decarboxylation of 2
with water or tetrahydrofuran as the solvent, the reactivity of 1 was
unchanged in the presence of an acid catalyst in either solvent.

Results
from density functional theory calculations suggest that both 1 and 2
first undergo keto-enol tautomerization to form β-ketone intermediates
prior to ring-opening. The ring-opening of 1 likely proceeds through the
nucleophilic attack of water on the C=C bond at the 5 position in the ring,
forming a β-keto acid which subsequently decarboxylates through a
six-membered cyclic transition state. It is further proposed that the tautomer
of 2 undergoes ring-opening and decarboxylation through a two-step
retro-Diels-Alder (rDA) reaction, proceeding through a zwitterionic
intermediate. Based on these proposed mechanisms, it is suggested that some
degree of unsaturation of the ring is necessary to enable initial keto-enol
tautomerization and subsequent ring-opening and decarboxylation of these
2-pyrone structures to occur. Significantly, the presence of a C=C bond in the
4 position in the pyrone ring appears to be particularly significant in that
this functional group allows for 2-pyrones to ring-open and decarboxylate
through rDA. Using 4,6,6-trimethyl-3,6-dihydro-2H-pyran-2-one
as a probe molecule, we further demonstrate experimentally that similar to the
experimental results obtained for 2, this structure undergoes
ring-opening and decarboxylation in the presence of liquid water without the
aid of a catalyst, showing
that rDA occurs independent of the nature of the functional group at the 4
position in the ring.

Accordingly,
we establish general reactivity rules for 2-pyrones and provide molecular-level
relationships to elucidate the factors that influence ring-opening and
decarboxylation chemistry.  These mechanistic insights provide guidance
for the selective conversion of reactants structurally analogous to 1, 2,
and 3 for example, in terms of solvent selection and reaction conditions
(e.g., temperature, acidity) for the production of targeted chemicals.