Oxazoles XXI. Three Ester Ammonium Salts Resulting from the Rupture of an Oxazoline Ring

Abstract  

The synthesis and characterisation of three ammonium salts that are formed via a ring opening of a 2-oxazoline (i.e., a 4,5-dihydro-1,3-oxazole) are described. Compound 1, derived from 2-phenyl-2-oxazoline (Phox), was isolated as a major reaction product during an unrelated synthesis using Phox which was being carried out in the presence of NO₃ ⁻ ions. The isolated compound is identified as the nitrate salt of the terminal ammonium ester [H₃NCH₂CH₂OC(=O)Ph]⁺ (1). The X-ray crystal structure (C₉H₁₂N₂O₅) of 1 is revealed to be orthorhombic (space group Pna2₁) with a = 21.1304(8), b = 7.5975(2) and c = 6.8629(3)?. The analogous material 2a, obtained from the related oxazole 4,4-dimethyl-2-phenyl-2-oxazoline (MePhox) under identical conditions to that of 1, contains the [H₃NCMe₂CH₂OC(=O)Ph]⁺ cation. The X-ray crystal structure (C₁₁H₁₆N₂O₅) of 2a is monoclinic with a = 6.1270(3), b = 6.3603(3), c = 16.1632(8)? and β = 90.662(3)o and of space group P2₁. Chiral derivative 2b, was isolated from a wet CH₂Cl₂/THF solution upon treatment of MePhox with (S)-mandelic acid. This material was shown to also contain the [H₃NCMe₂CH₂OC(=O)Ph]⁺ cation, a lattice water molecule and an (S)-mandalate anion. The X-ray crystal structure of 2b (C₁₉H₂₃NO₅·H₂O) is orthorhombic (space group P2₁2₁2₁) with a = 6.4733(3), b = 11.4661(5) and c = 24.5791(11)?. One proposed mechanism by which all three materials are formed was first presented in seminal work by Deslongchamps. The compounds reported herein are representatives of a class of rarely isolated intermediates in heterocyclic ring opening reactions and they are also the first examples of (ω-ammoniumethyl)(aryl) esters to be structurally characterised.