| Summary: | The photochemistry of β-keto ester 28 has been studied in solution as well as the
crystalline state. In both media this compound undergoes Yang photocyclization, followed by
a spontaneous retro-aldol ring opening to afford δ-keto ester 42 as the major photoproduct,
with both benzoyl and methoxycarbonyl groups in axial positions. [Figure.] The observed photoreactivity of β-keto ester 28 was correlated to its X-ray crystal
structure. Crystallographic analysis revealed that only one γ-hydrogen in the molecule lies
within a reasonable distance (2.42 A) from the ketone oxygen for abstraction to occur. The
angular parameters associated with γ-hydrogen abstraction as well as the geometric
.parameters associated with the reactivity of intermediate 1,4-hydroxybiradical were derived
from the crystallographic data.
The (-)-menthyl, (-)-bornyl and (+)-fenchyl esters of β-keto acid 36 were prepared to
explore asymmetric induction by covalent chiral auxiliaries. In the solution phase photolyses
of all chiral esters studied, two possible diastereomeric δ-keto esters [figure] analogous to δ-keto ester 42 were formed in unequal
amounts. Low diastereoselectivity (14-28% de) in the
solution phase photolyses reflected the extent of
asymmetric induction due to the direct influence of the chiral auxiliary. Asymmetric induction in the
solid state photolysis was studied for the (-)-bornyl and (+)-fenchyl β-keto esters. The solid
state photoreaction, in which chiral information is provided by the rigid crystal environment,
proceeded with high diastereoselectivity (>95% de) at quantitative conversion for the
(-)-bornyl ester and with low (18% de) diastereoselectivity for (+)-fenchyl ester. The
diastereoselectivity observed in the solid state was correlated with the X-ray crystal structure
data for the corresponding compound.
The ionic chiral auxiliary approach was also tested in the photoreaction of β-keto acid
36. Owing to its thermal instability, β-keto acid 36 was prepared in situ at low temperature
and its chiral salt with optically active R-(+)-α-methylbenzylamine was obtained. The solid
state photolysis of this chiral salt, followed by diazomethane work up, led to the methyl
β-keto ester 42 with an excellent enantiomeric excess (>99%) at quantitative conversion of
the starting material. === Science, Faculty of === Chemistry, Department of === Graduate
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