Effect of Wavelength on the Photoisomerization of Vitamin K2

Abstract

trans-Vitamin K2(trams-VK2), widely used as a medicine for hemostasis, is manufactured by chemical processes as its mixture with cis-VK2, among which trans-VK2 is isolated and cis-VK2 is discarded. During the course of our investigation on the isomerization of unsaturated bonds by a tunable laser instrument, we found that laser excitation successfully converted cis-VK2 into trans-VK2. cis-VK2 exhibits absorption in 200~500 nm. The irradiation of its diisopropyl ether solution (1.5 wt%, 3 mi) under nitrogen with lasers of various wavelength in 200~800 nm (10 Hz, 2 mj/pulse, fwmh/5 ns) resulted in the isomerization to trans-VK2. Among the lasers examined, the most efficient isomerization was accomplished with lasers in the region of 355~435 nm (355, 366, 375, 404, 414 and 435 nm). Excitation of either cis- or trans-VK2 afforded their photostationary mixture comprising of trans- and cis-VK2 in 7 : 3. It is noteworthy that the isomerization between cis- and trans-VK2 takes place highly selectively at the unsaturated bond closest to the quinone ring among four unsaturated bonds in the tetraprenyl chain. Furthermore, from the irradiation mixture, a small amount of menachromenol, a cyclized product, was isolated as a by-product. We have proposed a reaction mechanism on the photoisomerization of cis- and trans-VK2. © 1989, The Chemical Society of Japan. All rights reserved.