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) A, beta = 104.84 (3) degrees; penicillic acid (7), 4-methoxy-5-hydroxy-5-(2'-propenyl)dihydrofuran-2-one, tetragonal, P4(2)/n, a = b = 15.83 (2), c = 7.016 (11) A; 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) A, alpha = 99.50 (5), beta = 102.38 (6), gamma = 90.29 (6) degrees; 2-methanoylbenzoic acid (9), 3-hydroxy-1-(3H)-isobenzofuranone, monoclinic, P2(1), a = 4.006 (1), b = 11.489 (2), c = 7.347 (1) A, beta = 97.50 (3) degrees; 2-ethanoylbenzoic acid (10), 3-hydroxy-3-methyl-1-(3H)-isobenzofuranone, orthorhombic, P2(1)2(1)2(1), a = 5.199 (6), b = 9.651 (14), c = 15.950 (17) A; 2-(2'-oxoethyl)benzoic acid (11), 3-hydroxy-3,4-dihydroisobenzopyran-1-one, monoclinic, P2(1)/n, a = 4.651 (3), b = 11.886 (7), c = 14.312 (11) A, beta = 90.86 (6) degrees. 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) A, alpha = 78.68 (6), beta = 84.34 (5), gamma = 80.38 (6) degrees. 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 A), while the exocyclic C-O(H) bonds are shortened (1.38 A). 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.