Pseudoacids. I. 4- and 5-Oxoacids

Certain 4- and 5-oxoacids may exist in their cyclic lactol (or pseudoacid) forms. These commonly occur in compounds with proximate carboxylic acid and carbonyl (aldehyde or ketone) functions for the formation of five- or six-membered rings. Examples include trans-2,3-disubstituted aliphatic, (Z)-2,3-olefinic and o-disubstituted aromatic acids. Crystal structures of compounds in these categories are reported: trans-4-methyl-3-oxo-6-hydroxytetrahydropyran-3-carboxylic acid (6), monoclinic, C2/c, a = 25.412 (5), b = 6.291 (1), c = 10.757 (2) Å, β = 104.84 (3)°; penicillic acid (7), 4-methoxy-5-hydroxy-5-(2′-propenyl)dihydrofuran-2-one, tetragonal, P4₂/n, a = b = 15.83 (2), c = 7.016 (11) Å; mucochloric acid (8), (Z)-3,4-dichloro-5-hydroxydihydrofuran-2-one, triclinic, P1, a = 6.227 (5), b = 8.085 (5), c = 12.369 (9) Å, α = 99.50 (5), β = 102.38 (6), γ = 90.29 (6)°; 2-methanoylbenzoic acid (9), 3-hydroxy-1-(3H)-isobenzofuranone, monoclinic, P2₁, a = 4.006 (1), b = 11.489 (2), c = 7.347 (1) Å, β = 97.50 (3)°; 2-ethanoylbenzoic acid (10), 3-hydroxy-3-methyl-1-(3H)-isobenzofuranone, orthorhombic, P2₁2₁2₁, a = 5.199 (6), b = 9.651 (14), c = 15.950 (17) Å; 2-(2′-oxoethyl)benzoic acid (11), 3-hydroxy-3,4-dihydroisobenzopyran-1-one, monoclinic, P2₁/n, a = 4.651 (3), b = 11.886 (7), c = 14.312 (11) Å, β = 90.86 (6)°. These compounds also exist in the cyclic forms in chloroform solution. A trimeric cyclic trioxane structure, analogous to paracetaldehyde, is confirmed as the solid form of 5-oxopentanoic acid (1), triclinic, P1, a = 5.640 (4), b = 8.571 (8), c = 18.962 (13) Å, α = 78.68 (6), β = 84.34 (5), γ = 80.38 (6)°. In solution (NMR), mixtures of the open aldoacid, trimeric acid and cyclic pseudoacid exist. In both furanoid and pyranoid pseudoacids, endocyclic lactol C—O bond lengths are lengthened (1.46–1.48 Å), while the exocyclic C—O(H) bonds are shortened (1.38 Å). Pseudoacids commonly form hydrogen-bonded chains linking the lactol hydroxy and carbonyl groups, but 3-hydroxy-3,4-dihydroisobenzopyran-1-one forms distinctive hydrogen-bonded dimers.

-1