Photochemical & Photobiological Sciences www.rsc.org/pps COMMUNICATION Asymmetric photocycloaddition of naphthamide with a diene using the provisional molecular chirality in a chiral crystal†‡ Masami Sakamoto,* Fumitoshi Yagishita, Ayako Saito, Shuichiro Kobaru, Atsushi Unosawa, Takashi Mino and Tsutomu Fujita Received 1st February 2011, Accepted 9th May 2011 DOI: 10.1039/c1pp05053b N-(2-Methoxy-1-naphthoyl)piperidine afforded chiral crys- tals by spontaneous crystallization, and the molecular chi- rality 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. The use of a chiral crystalline environment is an excellent strategy for obtaining optically active compounds from achiral compounds. 1,2 Recent advances in the use of a variety of solid- state reactions with chiral crystals have progressed to such an extent that this approach can now be regarded as an important branch of organic chemistry. 3 Furthermore, a new methodology using molecular chirality in crystals as a source of chiral memory in solution has recently been explored (Fig. 1). Fig. 1 Chiral crystallization of achiral or racemic materials and asym- metric synthesis using provisional chirality in fluid media. The provisional molecular chirality derived from chiral crystals can be effectively transferred to optically active products with various asymmetric reactions in fluid media. 4–6 Two requirements Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan. E-mail: sakamotom@faculty.chiba-u.jp † This article is published as part of a themed issue in honour of Yoshihisa Inoue’s research accomplishments on the occasion of his 60th birthday. ‡Electronic supplementary information (ESI) available: Experimental procedure, spectral data and crystallographic data. CCDC reference number 810675. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1pp05053b must be met for asymmetric synthesis to become possible: chiral crystallization of the starting materials and slow racemization at a controlled temperature. We found that the provisionally chiral naphthamide derived from chiral crystals reacted with a diene leading to a cyclobutane on irradiation. This reaction provides the first example of an asymmetric photochemical reaction for a naphthalene/diene system using the chirality of a chiral crystal. Previously, we reported naphthamide 1, which was prepared from 2-methoxy-1-naphthalene carboxylic acid and piperidine, and crystallized in the chiral space group P2 1 2 1 2 1 (Fig. 2). 4 Both of the enantiomorphic chiral crystals were easily prepared selectively by spontaneous crystallization. The rate of racemization of 1 after dissolving the chiral crystals in a solvent was measured based on changes in the CD spectrum. 4 The activation free energy (DG π ) was calculated from the temperature dependence of the kinetic rate constant between 5 and 15 ◦ C as 21.2 ± 0.2 kcal mol -1 in THF. These facts indicate that the racemization of 1 is too fast to be resolved in the usual manner. The lifetime can be lengthened by lowering the temperature so that the racemization is sufficiently slow, and the reaction can be used to accomplish asymmetric synthesis. We reported the 4 + 4 cycloaddition of excited 9-cyanonaphthalene with the ground state of 1, leading to an optically active 4 + 4 adduct in high enantioselectivity. Fig. 2 Racemization of naphthamide 1 by Ar–(C O) bond rotation and asymmetric generation by spontaneous crystallization. We examined the photocycloaddition of 1 with 2,5- dimethylhexa-2,4-diene (2), in which naphthamide 1 absorbs the irradiation light and a singlet excited state of 1 reacts with diene 2. Yang et al. reported the photochemical cycloaddition This journal is © The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 1387–1389 | 1387