Synthesis of 11Z-8,18-propano- and methano-retinals and their interaction with bovine opsin

[Chemical Biology Studies Using Vitamin A Analogs]

"Retinoid" is the general term for vitamin A derivatives and chemical compounds that act like vitamin A. Vitamin A are composed of four isoprene units and are named according to their terminal functional group, such as retinol (OH, 1), retinal (CHO, 2), and retinoic acid (CO₂H, 3). Vitamin A usually refers to retinol. In the past few decades, major advances in research on vitamin A have improved our understanding of its fundamental roles and physiological significance in living cells. In this review, three types of chemical biology studies using vitamin A analogs are described: (1) conformational studies of the chromophore in retinal proteins (rhodopsin, phoborhodopsin, and retinochrome), especially the conformation around the cyclohexene ring; (2) structure-activity relationship studies of retinoic acid analogs to create new signaling molecules for activating nuclear receptors; and (3) development of a new channelrhodopsin with an absorption maximum at longer wavelength to overcome the various demerits of channelrhodopsins used in optogenetics, as well as the stereoselective synthesis of retinoid isomers and their analogs using a diene-tricarbonyliron complex or a palladium-catalyzed cross-coupling reaction between vinyl triflates and stannyl olefins.

An improved synthesis of benzocycloalkanone derivatives

A convenient and improved annulation method for the synthesis of bicyclic ketones was developed. A 2,2-dimethyl-6-(2-phenylsulfonyl)ethylcyclohexanone was converted into a sulfonylester by the addition of ethyl acetate and subsequent dehydration. A Dieckmann type condensation of the sulfonylester followed by desulfonylation afforded the 8,8-dimethyl-1,2,3,4,5,6,7,8-octahydronaphthalene-2-one in good yield. This annulation method was also applicable for the synthesis of the benzocyclooctanone derivative.

Evidence for the archaebacterial-type conformation about the bond between the beta-ionone ring and the polyene chain of the chromophore retinal in chlamyrhodopsin

Previous studies using retinal analogs (Wada, A., Sakai, M., Kinumi, T., Tsujimoto, K., Yamauchi, M. and Ito, M. (1994) J. Org. Chem. 59, 6992-6997; Wada, A., Sakai, M., Imamoto, Y., Shichida., Y., Yoshizawa, T. and Ito, M. (1993) Chem. Pharm. Bull. 41, 793-795) revealed that both retinochrome and visual pigments share a common chromophoric conformation in which the ring region of retinal is twisted ca. 50 degrees with respect to the plane of the polyene chain, suggesting a highly conserved 6-s-cis conformation throughout rhodopsin-like pigments in animals. By contrast, 6-s-trans conformation was commonly observed or suggested in archaebacterial rhodopsins examined thus far. Here we have reconstituted in vivo both the photoreceptor for photobehavioral responses of the unicellular alga Chlamydomonas reinhardtii and the second archaeal sensory photoreceptor phoborhodopsin from a series of retinal analogs. Results exclusively point to the conclusion that in both photoreceptors retinal has the coplanar 6-s-trans conformation, though recent molecular cloning studies revealed no homology between Chlamydomonas photoreceptor (chlamyrhodopsin) and archaeal rhodopsins.

Determination of 6s-trans conformation of retinal chromophore in sensory rhodopsin I and phoborhodopsin

8,16-Ethanoretinal 3 was synthesized from 2-methylcyclohexanone. From the binding experiments of 3 and 8,18-ethanoretinal 2 with apoproteins of sensory rhodopsin I and phoborhodopsin, it was found that retinal was incorporated as 6s-trans conformation in the both proteins.