Synthesis of trans/syn‐ and trans/anti‐Dimeric Uracil

Abstract

and working-up losses amounted t o 6.3 g = 2.9%. [Yields calculated with respect to reacted ( I ) . ] The reaction products were fractionated in a 2-mpackedcolumn. (2a) + (2b) (z 1 :l), b.p. 74-74.5 'C,%O torr, n: = 1.4898. (2a) + (26) stillcontained 4 % of cis,cis-l,3-cyclooctadiene. This additional compound can be detected gas chromatographically only after hydrogenation. (3), b.p. 67 "CjlOO torr, ng = 1.4801. Received: January 2, 1969 [Z 930 IE] German version: Angew. Chem. 81, 186 (1969) dimers (3a) and (4a) [(3a), yield 90%. m.p. from 32OoC (dec.); (4a), yield 75 %, m.p. from 300 O C (dec.); the product that separates during the reaction is purified by washing with water]. (4a) is less stable thermally than (3a); when boiled with 2 N HCI it is cleaved to uracil. IR spectroscopic comparison of (3a) and (4a) with a product considered[5] to be a mixture of (3a) and (4a) showed that product was (3a) and not a mixture thereof with (4a); a mixture is first formed, but (4al decomposes under the reaction conditions.

The Photochemistry of Uracil: A Reinvestigation

“…Analysis of the chromatogram produced indicated that the peaks corresponding to the methylated dimers of the t,a isomer were greatly diminished in area, as compared to the corresponding peaks assigned as belonging solely to methylation products of the t,s dimer. This is agreement with expectation, as Richter and Fahr (50) remarked on the greater thermal lability of the t,a cyclobutane dimer of Ura, as compared to the t,s form. This is also in agreement with the thermal behavior of the t,a (and c,a ) CBDs formed in photoreactions of thymine; the half‐life of the t,a CBD of thymine at 100°C in neutral solution, with respect to reversion to thymine, is only a few minutes (see, e.g.…”

“…In contrast, Jennings and co‐workers found that refluxing of the dimer they identified as t,a in boiling water for 24 h resulted in conversion of this compound to uracil, while the t,s isomer were stable to this treatment. The agreement of this behavior with the previously reported analogous thermal properties for these two dimers (50) was used by Jennings et al. to confirm the identifications of the t,a and t,s isomers based on IR spectra.…”

“…They indicated that the identity of the isomer corresponding to each spectrum was verified by comparisons with IR spectra provided to them by Professor E. Fahr, in whose laboratory authentic samples of each of the four dimers had been prepared via nonphotochemical synthetic methods (50,51). Varghese similarly published four IR spectra that he designated as being for the α and β forms of the c,s dimer, the c,a dimer and the t,a cyclobutane dimer of Ura and indicated that the latter dimer had been synthesized using the procedure of Richter and Fahr (50); the IR spectrum of the t,s CBD was not included in the paper. However, comparison of the IR spectra given in the two papers indicates that the spectrum that Jennings et al.…”