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4Z)-Dipyrrinones, which are the component chromophores of yellow pigment of jaundice, are known to self-associate strongly in nonpolar solvents (Afassoc -25 000 M at 25 °C in CHCI3), forming planar dimers in which the monomers are linked tightly by four intermolecular hydrogen bonds. When the chromophore has an attached propionic or longer chain acid group, it forms a new type of stacked dimer through a network of six hydrogen bonds in which the carboxyl group of one dipyrrinone is tethered to the other dipyrrinone. Thus, xanthobilirubic acid and its homologs strongly self-associate as stacked dimers in contrast to its methyl ester, which forms planar dimers.The stacked dimers are recognized by large (1 ppm) shieldings of their NH resonances in their 'H-NMR spectra, as compared with planar dimers. They are also recognized by unusually large optical rotations and exciton coupling in the circular dichroism spectra when a stereogenic center is present in the alkanoic acid chain. In CHCI3, (J3S)methylxanthobilirubic acid (1) has [a]20D = -314°and Ae™4 = -10.9, Ae!Jg" = +5.7, whereas its methyl ester (6) has [a]20D = +62°; Ae™0x < 1.

Diethylaminosulfur trifluoride (DAST) reacts with dialcohols to give difluorides, sulfite esters or cyclic ethers depending on the number of carbons separating the two alcohol groups. Vicinal and 1,3-diols give large amounts of sulfite ester products while butane-1,4-diol gives almost exclusively the cyclic ether tetrahydro furan. Terminal dialcohols longer than four carbons give primarily difluoride products. Semiempirical calculations indicate a preference for cyclic intermediates when four or less carbons separate the two alcohol moieties. These cyclic intermediates lead directly to the cyclic ethers and sulfite ester products.Aminosulfur trifluorides such as diethy laminosulfur trifluoride (DAST) react with alcohols to replace the hydroxy group with a fluorine.In polyhydroxy compounds such as sugars only one hydroxy group is usually replaced by fluorine.1b,2 Replacement of two hydroxy groups by fluorine has been observed in a few instances.2c For example, Middleton reported a 70% yield of 1,2-difluoroethane (1) for the reaction of DAST with ethylene glycol. lo Results and discussionWe investigated the reaction of DAST with ethylene glycol and found that ethylene sulfite (2) was also produced in 20% yield. 1 2 3.5 1 .o 90% overall yield Reaction of DAST with 2-methylpentane-2,4-diol gave primarily difluoride 3 and the sulfite esters 4a, b in 80% yield. + DAST -+yF + 3 11 .o 4a 2.2 4b 1 .o Paper 41032836

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