ABSTRACT
Nonbiaryl axially chiral 2-pyridones were synthesized and employed for light-induced electrocyclic 4π ring closure leading to bicyclo-ß-lactam photoproducts in solution. The enantioselectivity in the photoproducts varied from 22 to 95% depending on the reaction temperature and the ability of the axially chiral chromophore to form intramolecular and/or intermolecular H-bonds with the solvent. On the basis of the differential activation parameters, entropic control of the enantiospecificity was observed for 2-pyridones lacking the ability to form H-bonds. Conversely, enthalpy played a significant role for 2-pyridones having the ability to form H-bonds.
Subject(s)
Light , Pyridones/chemical synthesis , Thermodynamics , Crystallography, X-Ray , Cyclization , Hydrogen Bonding , Models, Molecular , Molecular Structure , Pyridones/chemistry , StereoisomerismABSTRACT
Atropisomeric benzoylformamides 1 undergo Type II reaction leading to cis-2 and trans-2 oxazolidin-4-one photoproducts. The N-C(Aryl) chiral axis is maintained during the course of the phototransformation in spite the reaction proceeding through a near planar intermediate(s). As the rotational barrier of the N-C(Aryl) chiral axis in the cis-2 and trans-2 photoproducts is lowered when compared with the reactant 1, the isolated optically pure trans-2 isomer is converted to the ent-cis-2 isomer without affecting the C-5 stereogenic center, resulting in resolution of the cis-2 enantiomers.
ABSTRACT
Alpha-oxoamides 1 with o-tert-butyl substitution on the N-phenyl moiety were found to be stable axially chiral atropisomers. These axially chiral alpha-oxoamides undergo enantiospecific photochemical gamma-Hydrogen abstraction in CHCl(3) to yield beta-lactams with high enantioselectivity (e.r. approximately 90:10) in solution. The extent of enantioselectivity was found to be dependent on the reaction temperature.
