Synthesis, asymmetric with circularly polarized

Asymmetric Synthesis with Circularly Polarized Light... 

Inoue Y, Tsuneishi H, Hakushi T et al (1996) First absolute asymmetric synthesis with circularly polarized synchrotron radiation in the vacuum ultraviolet region direct photoder-acemization of (e)-cyclooctene. Chem Commun 2627... 

Kawasaki T, Sato M, Ishiguro S et al (2005) Enantioselective synthesis of near enantiopure compound by asymmetric autocatalysis triggered by asymmetric photolysis with circularly polarized light. J Am Chem Soc 127 3274—3275... 

Excitation with circularly polarized light (CPL) is an interesting direct method of asymmetric synthesis, which is based on preferential excitation of one of the enantiomers thanks to a difference in the molar absorption coefficients towards left or right CPL at a given wavelength.585 Unfortunately, only a small enantiomeric excess (ee) can be generated by this method. Some scientists, nevertheless, believe that... 

Bernstein, W. J., Calvin, M., Buchardt, O., Absolute Asymmetric Synthesis. 1. On Mechanism of Photochemical Synthesis of Nonracemic Helicenes with Circularly Polarized Light Wavelength Dependence of Optical Yield of Octahelicene, J. Am. Chem. Soc. 1972, 94, 494 498. 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

Cpl-induced asymmetric photochemistry is a way to conduct absolute asymmetric synthesis. The field is well developed. The method depends on the differential absorption of left and right circularly polarized radiation by the enantiomeric species of the reactants and thus on their g factor. The kinetic schemes are well developed, and for mechanistic questions as well as for the determination of molecular chiroptical constants cpl irradiation can be used with success. Unfortunately, the smallness of the enantiodifferentiating g factor prevents the method from being a match to the methods exploiting diastereomeric intermolecular interactions for thermal synthesis purposes. 

The process of synchronous enantiomeric enrichment of both reactant and product using circularly polarized light (CPL), for which the term new absolute asymmetric synthesis (NAAS) has been coined, was studied in detail for the case of methyl norbornadiene-2-carboxylate 41This photorearrangement (irreversible NAAS) of racemic 41 produces the chiral product 42. When 41 was irradiated with r-CPL at 290 nm, enantiomeric enrichment by (-l-)-41 and (+)-42 occurs. The inverse process was observed for /-CPL. The enantiomeric enrichment of 41 increases up to 100% enantiomeric excess (ee) at nearly 100% conversion, and the ee of 42 gradually decreases to zero at the end of reaction. These results suggest that one can obtain about 40% ee of 41 and 42 at 50% conversion if a starting material with an anisotropy factor g g = Ae/e) of unity is used. 

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