Journal of Chemical Crystallography, Vol. 33, Nos. 5/6, June 2003 ( C  2003) Pseudoacids. III: formation and structures of new cyclic oxocarboxylic acids W. John Cooper, (1) Tyler N. Smith, (1) Andrea K. Barker, (1) Jason A. Webb, (1) and Edward J. Valente (1)∗ Received July 1, 2002 Crystal and molecular structures for aryl- and benzyl-substituted 3-hydroxy-3,4- dihydroisobenzopyran-1-one are described. For the 4,4-dimethyl derivative, two monoclinic modifications are produced on sublimation: (1) P 2 1 /c, a = 23.668(6) ˚ A; b = 11.749(4) ˚ A; c = 7.215(2) ˚ A; β = 96.70(2) ◦ ; V = 1992.6(10) ˚ A 3 ;(2) P 2 1 /c, a = 11.509(4) ˚ A, b = 12.429(4) ˚ A, c = 6.933(2) ˚ A, β = 91.12(3) ◦ , V = 991.6(6) ˚ A. In both, hydroxy groups are pseudoaxial, and molecules form dimeric complementary but noncooperative hydro- gen bonds. In the 6-methoxy derivative (3), P 2 1 /c; a = 14.185(7) ˚ A; b = 4.105(4) ˚ A; c = 15.598(13) ˚ A; β = 98.59(6) ◦ ; V = 898.1(10) ˚ A 3 , the hydroxy group is pseudoequa- torial, and the intermolecular hydrogen bonding forms infinite chains. In the 4,4-dimethyl- 7-nitro derivative (4), C 2/c, a = 19.365(4), b = 8.425(2), c = 13.464(4), β = 92.62(2), V = 2194.2(7) ˚ A 3 , the hydroxy group is pseudoaxial, and the intermolecular hydrogen bonding forms chains. Compared to the unmethylated compounds, the interbond angles within the ring at the dimethylated carbon are contracted. KEY WORDS: Hydrogen bonding; oxocarboxylic acids; pseudoacids; dimethyl effect. Introduction Pseudoacids are formed by the cycliza- tion of certain 4- and 5-oxocarboxylic acids. The resulting bifunctional compounds have hy- drogen bond donors and acceptors more dis- tantly separated than those of normal carboxylic acids. In some cases, hydrogen-bonded dimers form between the pseudoacid lactol carboxyl and hydroxy groups of adjacent molecules which show three-dimensional intermolecular recog- nition (complementarity) features. These are (1) Department of Chemistry and Biochemistry, Mississippi College, Clinton, Mississippi. ∗ To whom correspondence should be addressed. E-mail: valente@ mc.edu distinct from though analogous to the two- dimensional dimers formed commonly between carboxylic acids. In other pseudoacid compounds, infinite hydrogen-bonded chains form between oxocarboxylic acids. 1 Pseudoacid systems are even more complex than normal carboxylic acids since cyclization produces a stereogenic cen- ter and the open-cyclic equilibrium can have anomeric consequences. For example, pyranoid lactols can have pseudaxial or pseudoequatorial hydroxy groups which may influence the hydro- gen bonding possibilities. In this study, the synthe- ses of a group of arylpyran pseudoacids intended to address various structural questions in pseu- doacid chemistry are described. Ring-chain tautomerisms are potentiated in open molecular systems possessing two functional 375 1074-1542/03/0600-0375/0 C  2003 Plenum Publishing Corporation