Diastereoselective photocycloaddition using memory effect of molecular chirality controlled by crystallization

Abstract

Naphthamides derived from L-proline, which exist as a mixture of several diastereomers in solution, converged to single diastereomer by crystallization, and the conformational transformation was controlled after the crystals were dissolved in the solvent at low temperature, where the frozen conformation was retained long enough for subsequent asymmetric reaction.

State of the Art in Crystallization‐Induced Diastereomer Transformations

“…[137] This concept they repeatedly used in combination with CIDT. [34,138] 2-Alkoxy-1naphthamides 191 a, b exist as a mixture of diastereomers due to a hindered rotation of bonds attached to the carbonyl (Scheme 70). [138] If crystallized from CHCl 3 -hexane or from a melt, crystals of a single (S, aR)-diastereomer could be obtained in unspecified yields.…”

“…The group of Sakamoto has demonstrated that besides a direct photochemical reaction of chiral crystals, photocycloadditions in a solution, if conducted at low temperatures, offer a readily option of rendering a labile axial chirality into configurationally stable compounds [137] . This concept they repeatedly used in combination with CIDT [34,138] . 2‐Alkoxy‐1‐naphthamides 191 a , b exist as a mixture of diastereomers due to a hindered rotation of bonds attached to the carbonyl (Scheme 70).…”

Solid‐Phase Conversion of Four Stereoisomers into a Single Enantiomer

“…Sakamoto et al. used this difference in stability for a related experiment . They studied a system with two stereocenters, of which one was enantiopure and could not epimerize, while the second one could epimerize in solution, resulting in a solid‐phase that contained only a single diastereomer.…”

Combining Diastereomeric Resolution and Viedma Ripening by Using a Racemic Resolving Agent

“…[10] Sakamoto et al reported the use of this difference in stability to obtain a single stereoisomer of an atrop-diastereomer where the desired stereoisomer could not be epimerized whereas the undesired one could be epimerized in solution at a low temperature resulting in the eventual formation of the pure desired stereoisomer. [11] These examples of Viedma ripening show that deracemization is possible for systems with multiple chiral centers. The present work therefore aims to combine diastereomeric resolution with the Viedma ripening process, allowing the use of a racemic mixture of the resolving agent.…”

“…Sakamoto et al. reported the use of this difference in stability to obtain a single stereoisomer of an atrop‐diastereomer where the desired stereoisomer could not be epimerized whereas the undesired one could be epimerized in solution at a low temperature resulting in the eventual formation of the pure desired stereoisomer [11] …”