Analogues of the Cyclic Hydroxamic Acid 2,4-Dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one: Decomposition to Benzoxazolinones and Reaction with β-Mercaptoethanol

Article abstract of DOI:10.1021/jo00005a025

Analogues of the aglucones of naturally occurring cyclic hydroxamic acids (2,4-dihydroxy-1,4-benzoxazin-3-ones) from Gramineae (Poaceae) have been synthesized by the reductive cyclization of the ring-substituted methyl α-(o-nitrophenoxy)-α-methoxyacetates, followed by demethylation of the C-2 methoxy group with BBr3 or BCl3 to reveal the 2-hydroxy group.A structure-activity series was produced by varying the substituent at C-7 on the aromatic ring .The pK_a values for the hydroxamic acid and the phenol moieties were determined for each member of the C-7 series.They correlated well with ? in a linear free energy relationship (LFER) yielding values of ρ = 0.71 (with ?_p) for pK_a1 (the hydroxamic acid) and ρ = 1.6 (with ρ_m) for pK_a2 (the phenol).A LFER also existed between the rate constants for the unimolecular decomposition of these hydroxamic acids to benzoxazolinones and ?⁺ (ρ = -1.1).The rates of hydroxamic acid reduction to lactams by β-mercaptoethanol were also investigated.It was found that only compounds with electron-rich aromatic rings and specifically an oxa functionality para to the hydroxamic acid nitrogen atom (compounds 1 and 3 - 5) had measurable rates of reduction. 1H NMR spectra recorded during this reaction in D2O buffers (pD 9), however, showed that compounds 1, 2, 6 - 9 (the only ones investigated) formed a hemithioacetal at C-2 even though only 1 has a measurable rate of reduction by the same thiol.The remarkable rate enhancement provided by an oxa functionality suggests that reduction occurs by direct attack of thiolate on the hydroxamic nitrogen of a resonance-stabilized ion pair.