Scheme 3. Overall Synthesis Scheme for (p-Phenolyl) Dodecylguanidinium Bromide (14) and for the Corresponding ¹⁵N₂-Labeled Compound (14′).

Not shown here is the deprotonation step needed to form the corresponding free-base compounds 15 and 15′. This was carried out as in Scheme 1. Reagents and conditions: (A) reflux, 8 h, H₂SO₄ (cat), methanol; (B) Ba(OH)₂ (1/2 equiv), 24 h, methanol; (C) 2 N HCl, ether; (D) SOCl₂, dichloromethane (DCM); (E) AlCl₃, anisole, CH₂Cl₂, DCM; (F) H₂NNH₂, tBuOK, 48 h, dimethyl sulfoxide (DMSO); (G) LiAlH₄, 5 h, THF; (H) PhP₃, CBr₄, 12 h, DCM; (I) potassium phthalimide, reflux, 2 h, dimethylformamide; (J) H₂NNH₂, reflux, 12 h, absolute ethanol; (K) S-methyl thiourea hydroiodide, 2 h, absolute ethanol; (K′) S-ethyl thiourea-¹⁵N₂ hydrobromide, 2 h, absolute ethanol; (L, L′) 48 % HBr, reflux, 6 h.