Abstract
An efficient synthesis of 2,2,3,3,11,11,12,12-octamethyl-1,4,7,10,13-pentaoxacyclohexadecane (1, OM16C5) is described, which affords over an order of magnitude improvement in yield over the previously reported method. The first X-ray crystal structure of 1, as a complex with NaSCN, is also reported.
There is much interest in sodium-selective ionophores [1,2] for use in electrodes [1,3] and optodes [4]. One such ionophore is 2,2,3,3,11,11,12,12-octamethyl-1,4,7,10,13-pentaoxacyclohexadecane (1, octamethyl-16-crown-5 or OM16C5), which was first prepared in 1996 by Suzuki and coworkers [1]. The synthetic procedure as reported and shown in Scheme 1 involved three steps starting from pinacol. The first two steps proceeded in excellent (95%) and good (ca. 81%) yields, respectively. However, the cyclization step, involving the addition of diethylene glycol dimesylate to 2,3,3,9,9,10-hexamethyl-4,8-dioxa-2,10-undecanediol (2) using NaH in THF, proceeded poorly with a low (3.7%) isolated yield, making the overall yield for the three steps starting from pinacol approximately 2.8%. We were interested in this crown ether for alkali-metal extraction studies, and sought to improve the yield so that larger (potentially gram) quantities of material could be readily prepared. We report here a modification of the synthetic procedure that boosts the conversion of 1 from 2 over 15-fold, by using three good- to high-yielding steps that allows 1 to be obtained from 2 in a combined yield of 59%. We also report for the first time a crystal structure of 1, as the NaSCN complex
