Asymmetric synthesis starting from an achiral material and in the absence of any external chiral agent has long been an intriguing challenge to chemists [1] and is also central to the origin of optical activity in Nature.[2] Stereospecific solid-state chemical reactions of chiral crystals formed by spontaneous crystallization of achiral materials are defined as "absolute" asymmetric synthesis, and most of the successful examples of such transformations involve photochemical reactions. [1,3] If the molecular chirality generated by chiral crystallization is retained even after the crystals are dissolved in a solvent, the "frozen" chirality is effectively transferred to the optically active products by various types of asymmetric reactions, besides the solid-state photochemical reaction. [4,5] Herein, we provide the first example of an asymmetric intermolecular photochemical reaction in solution through transfer of the chirality generated by chiral crystallization of an achiral naphthamide.2-Alkoxy-1-naphthamides 1 were chosen to perform this asymmetric synthesis because the bond rotation between the naphthalene ring and the C=O(NR 1 R 2 ) group corresponds to enantiomerization of 1, and the rate is considerably affected by the substituents on both the naphthalene ring and the amide group (Scheme 1).[6] Naphthamides comprising a bulky Scheme 1. Enantiomerization of naphthamides 1 a, b upon rotation about the naphthalene-C( = O) bond.
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.
Asymmetric synthesis starting from an achiral material and in the absence of any external chiral agent has long been an intriguing challenge to chemists [1] and is also central to the origin of optical activity in Nature.[2] Stereospecific solid-state chemical reactions of chiral crystals formed by spontaneous crystallization of achiral materials are defined as "absolute" asymmetric synthesis, and most of the successful examples of such transformations involve photochemical reactions. [1,3] If the molecular chirality generated by chiral crystallization is retained even after the crystals are dissolved in a solvent, the "frozen" chirality is effectively transferred to the optically active products by various types of asymmetric reactions, besides the solid-state photochemical reaction. [4,5] Herein, we provide the first example of an asymmetric intermolecular photochemical reaction in solution through transfer of the chirality generated by chiral crystallization of an achiral naphthamide.2-Alkoxy-1-naphthamides 1 were chosen to perform this asymmetric synthesis because the bond rotation between the naphthalene ring and the C=O(NR 1 R 2 ) group corresponds to enantiomerization of 1, and the rate is considerably affected by the substituents on both the naphthalene ring and the amide group (Scheme 1).[6] Naphthamides comprising a bulky Scheme 1. Enantiomerization of naphthamides 1 a, b upon rotation about the naphthalene-C( = O) bond.
An asymmetric S(N)Ar reaction was performed by using molecular chirality generated and amplified by the spontaneous crystallization of achiral naphthamides; the chirality was retained in a cold solution, caused by slow racemization, and was transferred to stable axially chiral materials with high enantiomeric excesses.
N-(2-Methoxy-1-naphthoyl)piperidine afforded chiral crystals by spontaneous crystallization, and the molecular chirality of the crystals was retained after dissolving them in a cooled solvent. An asymmetric photocycloaddition reaction with a diene was performed using the provisional chiral molecular conformation derived from these chiral crystals.
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