Mild and scalable synthesis of phosphonorhodamines

Since its discovery in 1887, rhodamines have become indispensable fluorophores for biological imaging. Recent studies have extensively explored heteroatom substitution at the 10' and a variety of substitution patterns on the 3', 6' nitrogens. Although 3-carboxy-substituted rhodamines were first reported in 1887 and their 3-sulfonated derivatives in 1896, the 3-phosphono analogues have never been reported. We recently reported the synthesis of 3-phosphono fluoresceins, which possessed nearly identical spectral properties to the parent carboxy dyes, but showed enhanced water solubility. However, synthesis and purification were difficult, and yields were low (<17%). Here, we report a mild, generalizable, and scalable synthetic route to 3-phosphonorhodamines. We explore the substrate scope and investigate mechanistic details of the acid-free condensation. Tetramethyl-3-phosphonorhodamine (pTMR) derivatives can be accessed on the 1.5 mmol scale in up to 98% yield (2 steps). Phosphonorhodmines show a 12- to 500-fold increase in water solubility over 3-carboxy and 3-sulfonorhodamine derivatives and have excellent chemical stability. Phosphonates allow for derivatization, and esterification of pTMR allows intracellular delivery with localization profiles that differ from 3-carboxyrhodamines. The free phosphonate can be incorporated into a molecular wire scaffold to create a phosphonated rhodamine voltage reporter, phosphonoRhoVR. PhosRhoVR 1 can be synthesized in just 6 steps, with an overall yield of 37% to provide >400 mg of material, compared to a 6-step, ~2% yield for the previously reported RhoVR 1. PhosRhoVR 1 possesses excellent voltage sensitivity (37% ΔF/F) and a 2-fold increase in cellular brightness compared to RhoVR 1.