Synthesis of the Nakanishi Ring-Locked Retinoid

An optimized synthetic route to prepare ring-locked retinoid 1a has been developed. We fully describe a purification protocol that provides isomerically pure 1a in support of on-going proof of concept studies for the development of therapeutic agents to treat human ADRP. Additionally, we have found that isomerically pure 1a can be stored in amber vials under argon at for use over time (up to six months) without degradation. Thus, enabling 1a to be an accessible and valuable biological tool. Previous studies using P23H mutant opsin as an in vitro model of human Autosomal Dominant Retinitis Pigmentosa (ADRP) found that ring-locked retinoid 1a (Figure 1, carbon numbering shown based on 11-cis-retinal) was able to act as a pharmacological chaperone [1–3]. Compound 1a induced the mutant protein to fold properly and undergo normal cellular transport and degradation suggesting 1a could be a potential therapeutic agent for the prevention of ADRP [1–3]. This ring-locked analog of 11-cis-retinal has also been used extensively to elucidate the mechanism of photoactivation of rhodopsin [4–6]. Figure 1: Ring-locked retinoid derivative of 11- cis-retinal. We required a reliable synthesis of isomerically pure compound 1a. Its synthesis, along with the spectral data, has been reported twice in the literature [1, 7], in addition to the syntheses of a number of related ring-locked retinoids [4–6, 8]. The original synthetic route reported by Nakanishi resulted in the preparation of mixtures of multiple geometric isomers of 1 (E/Z-isomers among C7-C8, C9-C10, and C13-C14) and did not describe specific HPLC conditions for resolution of the final isomerically pure products. Our synthetic strategy differs from the two reported syntheses in two fundamental ways (discussed below) enabling ready access to 1a. Two key bond-forming events distinguish this synthesis: the C7-C8 double bond was originally established via a Julia olefination, while the connection between C13 and C14 was accomplished via a Petersen olefination. In each case, both E- and Z-isomers of the desired products were reported to have formed [7]. In contrast, our use of a Wittig reagent in the formation of the C7-C8 double bond afforded the E-isomer as the sole product in the former olefination; and in the case of the latter, the requisite Petersen reagent, a silylated acetaldehyde tert-butylamine, was not readily accessible in our hands, and an alternate methodology was instead utilized [7, 9]. A Horner-Emmons-Wadsworth strategy (utilizing diethyl cyanomethylphosphonate) was found to be an advantageous