Chiral Helicenoid Diarylethene with Highly Diastereoselective Photocyclization¹

Abstract

A novel photochromic helicenoid diarylethene (R)-1-[1-(1-methoxymethoxyethyl)-2-naphtho[2,1-b]thienyl]-2-(2,4,5-trimethyl-3-thienyl)hexafluorocyclopentene was synthesized enantioselectively. It showed highly diastereoselective photocyclization (90% de) and a large change (950 degrees) in the specific optical rotation value at 633 nm upon UV light irradiation in ethyl acetate.

Chiral photochromism based on 6π-electrocyclization

“…The change in specific optical rotation of 21 was, however, somewhat smaller than we had expected (9501), owing to the lower number of the condensed aromatic rings. 30 The change in absorption spectra upon photoirradiation is shown in Fig. 19.…”

A Unified Strategy for Exceptionally High Diastereoselectivity in the Photochemical Ring Closure of Chiral Diarylethenes

“…[17] We have also developed the diastereoselective ring closure of photochromic diarylethenes in solution by applying the concept of the allylic 1,3-strain [18][19][20][21] to the hexatriene moiety, which gave 88-94 % diastereomeric excess (de) by using only one chiral unit to control the stereochemistry. [22][23][24][25] Herein, we report on a photochromic system that undergoes a highly diastereoselective ring closure with up to 100 % de.…”

“…Compound 3 showed only 95 % de, although it was expected to show the highest de value because of the strong electronic repulsion between the oxygen atoms on the side arms and the sulfur atom, in addition to the repulsion between the side arms-though it is still higher than our previous values (88-94 % de). [22][23][24][25] Next we examined the solvent effect on the diastereoselectivity of these compounds. In n-hexane, although the de values were still high, they are all slightly lower than those observed in ethyl acetate.…”

Chiral Photoresponsive Tetrathiazoles That Provide Snapshots of Folding States

“…[7][8][9][10] The E-Z photoisomerization of the azobenzene unit serves to denature the helical structure in a dynamic manner, [7,8] which was also integrated with guest-recognition capability. [9,10] Whereas photochromes that show 6p-electrocyclization reaction such as diarylethenes [11] have been incorporated into helicenes with a rigid geometry, [12,13] chiral helical structures with the photoresponsive 6p system have never been demonstrated. The 6p-electrocyclization reaction proceeds in a quite different way from that of azobenzene with respect to the change in geometry and electronic state, which would expand the response behaviors of photoresponsive foldamers.…”

“…A number of asymmetric reactions including diastereoselective [12,13,23] and enantioselective [24] electrocyclizations have been reported for diarylethenes in solutions, crystals, and supramolecular assemblies. These diarylethenes often tether chiral side units or adopt helicene structures to induce a chiral structure in the backbone, mainly by repulsive intramolecular interactions.…”