Propan-1,3-diol (PD) and propan-1,2-diol (propylene glycol, PG) are very similar compounds because their structures, safety data, and anti-microbial activities are almost the same. Actually, both compounds are made up of three carbon atoms and two hydroxyl groups. Regarding their safety, they do not have serious hazard data for animals, and LD 50 values (in rats) of both are similar. As for the anti-microbial activity, minimum inhibitory concentration (MIC) values of both PD and PG are approximately 10% (v/v). In this study, we used the preservatives-effectiveness test (PET) to evaluate the anti-microbial activities of PD and PG, because both compounds are used in cosmetics as preservatives. The results indicated that PD was more effective as an anti-microbial agent compared with PG, and the effect of PD was marked against Escherichia coli and Pseudomonas aeruginosa. Scanning electron microscopy (SEM) images showed that the membrane of Escherichia coli was injured by PD and PG, but the damage by PD was more marked. The damage of the cell membrane may be the cause of high anti-microbial activity of PD in PET. These results suggest that PD has greater potential as a preservative, and PD should be recommended as an additive for food and medicine.

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The plant alkaloids, iso-6-spectaline and spectaline, isolated from the Cassia or Senna genera contain a characteristic 2,6-disubstituted piperidin-3-ol scaffold. Although both natural products are reported to exhibit a variety of interesting biological activities, few stereo-selective schemes for the construction of the 2,6-disubstituted scaffold have been reported. Following our previous studies regarding the synthesis of ()-spectaline, herein we report the first convergent synthesis of ()-iso-6-spectaline using a cross-metathesis under thermal conditions where the cis-2,6-disubstituted piperidin-3-ol scaffold is condensed with a long alkyl chain containing a terminal olefin. The cis-2,6-disubstituted piperidin-3-ol used in the synthesis was prepared simply via Pd(II)-catalyzed diastereoselective cyclization. It was confirmed that ()-spectaline, an epimer of ()-iso-6-spectaline, was selectively synthesized by the cross-metathesis reaction under less intense thermal conditions starting from the same cis-2,6-disubstituted piperidin-3-ol derivative.

Benzalkonium chloride (BAC) is a useful preservative for ophthalmic solutions but has some disadvantageous effects on corneal epithelium, especially keratinocytes. Therefore, patients requiring the chronic administration of ophthalmic solutions may suffer from damage due to BAC, and ophthalmic solutions with a new preservative instead of BAC are desired. To resolve the above situation, we focused on 1,3-didecyl-2-methyl imidazolium chloride (DiMI). As a preservative for ophthalmic solutions, we evaluated the physical and chemical properties (absorption to a sterile filter, solubility, heat stress stability, and light/UV stress stability), and also the anti-microbial activity. The results indicated that DiMI was soluble enough to prepare ophthalmic solutions, and was stable under severe heat and light/UV conditions. In addition, the antimicrobial effect of DiMI as a preservative was considered to be stronger than BAC. Moreover, our in vitro toxicity tests suggested that DiMI is safer to humans than BAC. Considering the test results, DiMI may be an excellent candidate for a new preservative to replace BAC. If we can overcome manufacturing process issues (soluble time and flushing volume) and the insufficiency of toxicological information, DiMI may be widely adopted as a safe preservative, and immediately contribute to the increased well-being of all patients.