Diastereomer-Differentiating Photochemistry of β-Arylbutyrophenones: Yang Cyclization versus Type II Elimination

Citations Over TimeTop 18% of 2005 papers

Abstract

The diastereomers of ketones 2 and 3 are shown to exhibit distinct photochemical reactivities due to conformational preferences; while the anti isomers of 2 and 3 undergo efficient Yang cyclization in 75-90% yields with a remarkable diastereoselectivity (> 90%), the syn isomers predominantly undergo Norrish Type II elimination. The differences in the product profiles of the diastereomers are consistent with a mechanistic picture involving the formation of precursor diastereomeric triplet 1,4-biradicals in which the substituents at alpha and beta-positions stabilize the cisoid (cyclization) or transoid (elimination) geometry. The fact that such a diastereomeric relationship does indeed ensue at the triplet-excited-state itself is demonstrated via the nanosecond laser-flash photolysis of model ketones 1. The diastereomeric discrimination in the product profiles observed for ketones 2 and 3 as well as in the triplet lifetimes observed for ketones 1 can both be mechanistically traced back to different conformational preferences of the ground-state diastereomeric ketones and the intermediary 1,4-biradicals. Additionally, it emerges from the present study that the syn and anti diastereomers of ketones 2 and 3 represent two extremes of a broad range of widely examined butyrophenones, which lead to varying degrees of Yang photocyclization depending on the alkyl substitution pattern.

Related Papers

• → Diastereomeric 2-fluoro-4-methyl-1,3,2-dioxaphosphorinan-2-thiones. Retention stereochemistry in nucleophilic substitution at thiophosphoryl phosphorus(1975)4 cited
• → ChemInform Abstract: COHALOGENATION OF UNSATURATED COMPOUNDS. XV. SYNTHESIS AND STUDY OF SOME REACTIONS INVOLVING THE ELIMINATION OF DIASTEREOMERIC PAIRS OF ESTERS OF 2,3‐DICHLORO‐2,3‐DICHLORO‐3‐FLUOROISOBUTANOIC, 2,3‐DICHLORO‐2,3‐DIFLUOROPROPANOIC, AND 2,3‐DIHALO‐3‐FLUOROBUTANOIC ACIDS(1980)
• → ChemInform Abstract: Diastereoselective Complexation of Benzyl Alcohols by Chromium Hexacarbonyl. Different Reactivities of the Diastereomers in an Acidic Medium.(1987)
• → ChemInform Abstract: Novel Synthesis of Selenones: Application to the Synthesis of Alkyl Cyclopropanecarboxylates.(1988)
• → ChemInform Abstract: Diastereoselective Synthesis of Heteroaromatic Glycine Derivatives.(2009)