Chemical Prey Luring in Jackson's Chameleons.

Lizards in the family Chamaeleonidae have been described as wiping a viscous substance from a pouch (the temporal pouch) at the angle of the jaw on branches and then capturing flies that land near the area where the wiping occurs. We confirmed the presence of this pouch in Jackson's chameleons. Histological work suggested that the material contained within is a result of decomposition of food and sloughed skin that has been trapped in the pouch rather than a glandular secretion.

The reaction of singlet oxygen with enecarbamates: a mechanistic playground for investigating chemoselectivity, stereoselectivity, and vibratioselectivity of photooxidations.

Photochirogenesis, the control of chirality in photoreactions, is one of the most challenging problems in stereocontrolled photochemistry, in which the stereodifferentiation has to be imprinted within the short lifetime of the electronically excited state. Singlet oxygen (1O2), an electronically excited molecule that is known to be sensitive to vibrational deactivation, has been selected as a model case for testing stereoselective control by vibrational deactivation. The stereoselectivity in the reaction of 1O2 with E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for the mode selectivity ([2 + 2]-cycloaddition versus ene-reaction) and the stereoselectivity in the oxidative cleavage of the alkenyl functionality to the methyldesoxybenzoin (MDB) product.

Control of chirality by cations in confined spaces: Photooxidation of enecarbamates inside zeolite supercages.

On photooxygenation of the optically active Z/E enecarbamates 1 (X = i-Pr) and 2 (X = Me) equipped with the oxazolidinone chiral auxiliary in methylene-blue (MB)-incorporated, alkali-metal (M = Li, Na, K, Cs, Rb), exchanged Y-type zeolites (MY-MB), oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (%ee) and/or the sense (R or S) of the stereoselectivity in the formation of the MDB product depends on the choice of the alkyl substiuent (i-Pr or Me) at the C-4 position of the oxazolidinone chiral auxiliary, the Z/E configuration of the alkene functionality in the enecarbamates, and the type of alkali metal in the zeolite. Most significantly-the highlight of this study-is the reversed sense (R or S) in the stereoselection when the photooxygenation is run in CDCl3 solution versus inside the MY-MB zeolite.

Stereoselective photooxidation of enecarbamates: reactivity of ozone vs singlet oxygen.

[reaction: see text]. Oxazolidinone-functionalized enecarbamates show contrasting behavior upon oxidation by singlet oxygen and by ozone. The observed stereoselectivity difference indicates that the oxidation with ozone is subject to classic steric effects, whereas the very high selectivity in the photooxidation with singlet oxygen is derived from vibrational deactivation.

Stereocontrol within confined spaces: enantioselective photooxidation of enecarbamates inside zeolite supercages.

Dye-exchanged Y zeolite is shown to be an effective medium to control the stereoselectivity in the photooxygenation of chiral oxazolidinone-functionalized Z/E-1 enecarbamates. An enantioselectivity (ee) as high as 80% was observed in the methyldesoxybenzoin (MDB) product, obtained in the methylene-blue-exchanged NaY zeolite at room temperature. The efficacy of the asymmetric induction in the MDB product depends on the Z/E geometry of the alkene, the Z-isomer being more effective than the corresponding E-isomer.

Temperature and solvent control of the stereoselectivity in the reactions of singlet oxygen with oxazolidinone-substituted enecarbamates.

Oxazolidinone-functionalized enecarbamates react stereoselectively with singlet oxygen to give methyldesoxybenzoin (MDB) in moderate to high enantiomeric excess. The stereochemical outcome depends on the E/Z substrate geometry, temperature, and solvent variables. The analysis of the differential activation parameters suggests a large contribution from the entropy term in determining the enantioselectivity.

Stereochemical features of the physical and chemical interactions of singlet oxygen with enecarbamates.

Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in (1)O(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with (1)O(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects.

A supramolecular "ship in bottle" strategy for enantiomeric selectivity in geminate radical pair recombination.

[reaction: see text] Reactions in which zeolites are modified with chiral inductors to serve as media for chiral induction are often limited by the propensity of both substrate and inductor to occupy the same supercage. Herein, we report a "ship in bottle" strategy utilizing the thermal decomposition of dioxetanes obtained from oxazolidinone-substituted enecarbamates for the enantioselective generation of methyl desoxybenzoin (MDB). Photoexcitation of the supramolecular geminate molecular pair results in enrichment of the opposite enantiomer of MDB.

Kinetic studies of human tyrosyl-DNA phosphodiesterase, an enzyme in the topoisomerase I DNA repair pathway.

Tyrosyl-DNA phosphodiesterase (TDP) cleaves the phosphodiester bond linking the active site tyrosine residue of topoisomerase I with the 3' terminus of DNA in topoisomerase I-DNA complexes which accumulate during treatment of cancer with camptothecin. In yeast, TDP mutation confers a 1000-fold hypersensitivity to camptothecin in the presence of an additional mutation of RAD9 gene [Pouliot, J.J.