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Abstract

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.