OrganicChemists' Helping Hand
RESEARCH Bothered by Organic Structural Problems? Typical problems solved by r o t a t o r y dispersion I.
measurements
Locating carbonyl groups in polycyclic systems
No physical tool could tell the difference between these two before, b u t now rotatory dispersion can. Also shows the nature of A^B ring junction—whether the angular substituent is alpha or beta. C«H 17
Φ Ο
Cholestan-2-one II.
Cholestan-4-one
Analytical problems
If optical rotation in the sodium D line is too small to b e of help, run a rotation in the ultraviolet region. Increase may be 10 to 100 times that of [Λ]». Epoxyumbellulone has [a In —4°. But at 290 m/*, its specific rotation is + 4 4 7 0 ° . Another on* is 19-nortestosterone, having rotations of + 4 4 ° and + 2 3 3 8 % respectively. I!l.
Kinetic problems
C h a n g e in rotation in sodium D line in this reaction is too small, the two differing by only 17°. But in UV region, their rotation differs more than 1000°. Rate of change of this rotation can be used in kinetic studies. acid , or ' base
Coprostan-4-one ( cis isomer ) IV.
Φ
Cholestan-4-one ( trans isomer; more stable )
Conformational and stereochemical changes
These two differ only in orientation of t h e isopropenyl group. they give two completely distinct dispersion curves.
V.
And yet,
Absolute configuration
In one method, the rotatory dispersion curve found is compared with t h e curve of a model steroid or other reference ketone. Another way is with haloketones. H e r e , the sign of the dispersion curve c a n b e predicted. If a bromine, for example, is a d d e d axially to t h e right of the carbonyl in cyclohexanone, the curve is negative; if to t h e left, positive. Comparing the predicted sign with the one actually found tells you the configuration. It hasn't missed yet.
^
Br
Λ
Gives A ( -(- ) curve
40
Solving problems in configuration and stereo chemistry is easier with rotatory dispersion
C&EN
AUG.
12,
1957
Gives A ( — ) curve
OOLVING
ORGANIC
STRUCTURAL
and
stereochemical problems should b e a bit easier now, thanks to rotatory dis persion—which represents the measure ment of optical rotations at different wave lengths in the ultraviolet region. T h r e e years of work with hundreds of compounds and a spectropolarimeter by Carl Djerassi and coworkers at W a y n e State University shows h o w rotatory dispersion can be used to: • Locate carbonyl groups in poly cyclic compounds. • Solve analytical and kinetic p r o b lems more easily. • Find small differences in conforma tion and stereochemistry which can't be detected by other methods. • Help determine absolute configura tions. Rotatory dispersion measurements as such aren't new. Some were m a d e in the early 19th century. But u n t i l a few years ago, only the phenomenon itself was looked into. A n d this chiefly by physical chemists, not by organic researchers with an eye on applying it to their own problems. Optical rotation is usually measured at t h e sodium E) Une ( 5 8 9 m^,) ; [α]ι>. But rotations in the ultraviolet region are always larger—as m u c h as 10 to 100 times, says Djerassi—and com pounds with a small [
