Circular Dichroism and Optical Rotatory Dispersion — Principles and Application to the Investigation of the Stereochemistry of Natural Products

Abstract

According to Table 3, the olefination of aldehydes via phosphorylids also gives good results; however, this method is totally unsuccessful with ketones. This is where the new directed aldol condensation becomes really valuable.It would be interesting in this connection to of the invesfigations.(CGH&P-=CH-CH=NR, like the metalated Schiff's bases, react with ketones in the desired manner. The princ@les of circular dichroism and of oprical rotatory dispersion are described. Examples are given to illustrate the use of these methods for the determination of the configurations and conformations of natural and synthetic products. Some limitations of the octant rule are mentioned. HistoryThe phenomenon of optical activity was discovered by Arago 121 in 1811 whilst experimenting with quartz, and was subsequently studied in some depth by Biot. In 1815, Biot 131 found that this property is not peculiar to crystals, but is also exhibited by liquids such as oil of turpentine or a solution of camphor in alcohol. Two years laterL41 he reported that the angle of rotation becomes greater when the measurement is carried out at shorter wavelengths; this observation laid the foundation for spectropolarimetry. Though a few prominent chemists and physicists later used this method, it remained largely unknown to those not specialized in it until about 1953.Lowry[5I attributes the long period of stagnation to the invention of the Bunsen burner (1866), which made it extremely easy to produce (practically) monochromatic light, i.e. that of the sodium D-line (589nm). Instead of using a monochromator, which had been particularly troublesome in the previous century, one had only to hold a piece of rock salt into the flame if one wanted to measure the optical rotation of The Bunsen burner was thus responsible for the delayed development of spectropolarimetry, and it was left to Carl Djerassi in the USA to bring it back to life in 1953. Within a few years, he and his co-workers carried out measurements on more substances 171 than had been investigated in all the preceding 140 years. This was partly made possibIe by the first commercial spectral polarimeter 181, which had two important characteristics: firstly, it allowed measurements to be extended into the UV range, so that the optical rotatory dispersion (ORD) could be measured even within the absorption bands of many chromophores, and secondly, the instrument was extremely simple to operate. A number of automatic recording spectropolarimeters are now commercially available 191, and such instruments are an essential part of the equipment of any natural products institute. [15] The plane in which the electric vector is propagated is generally known as the "vibration plane" of linearly polarized light. The corresponding magnetic vector is perpendicular to this plane. However, most of this author's work was not published until the middle of the last century. See also [17].[17] P . Drude: Lehrbuch der Optik. Hirzel, Leipzig 1900, p. 379. direction, whereas in the case of circularly polarized l...

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