Switching of Diastereoisomers

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

Making the other diastereoisomer would require the cis alkene. This is not a problem in itself, but more of a challenge for the catalyst, because now it has to distinguish between two similar groups (Et and Me) in order to oxidize one face of the alkene enantioselectively (for the trans alkene, the selection is between either Et and H or Me and H switching Et for Me makes no difference to the outcome). 

First, we switch the COOH and OH groups the molecule produced is now of opposite configuration so we need to do another switch to get back to the configuration we started with. This time, we switch H and OH. The first and third structures are identical, so if we now redraw the whole molecule, it is obvious that the two molecules are diastereoisomers the left-hand structure is a meso-compound. 

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