Add time:09/10/2019         Source:sciencedirect.com

In this extension of earlier work, the main compounds studied have been di-tert.-butylsilylene (DTBS) derivatives of hydroxylic and phenolic acids. Preparative procedures were based on those originally applied by Trost and Caldwell [Tetrahedron Lett., 22 (1981) 4999] to diols. Most substrates gave apparently quantitative yields in analytical-scale preparations. Derivatives containing five-, six-, and seven- membered rings were prepared. With a variety of substituted salicylic acids, good gas chromatographic separations were obtained for pairs or sets of isomeric substrates: examples included 3-allyl- and 3-propenylsalicylic acid; carvacrotic and thymotic acid; 1- and 3-hydroxy-2-naphthoic acid and 2-hydroxy-1-naphthoic acid. 2,5-Dihydroxyterephthalic acid yielded a bis-DTBS derivative. Among other substrates studied were 3-(2,3-dihydroxypropyl)salicylic acid and its 2-methyl homologue, some catechol derivatives, and the drugs mephenesin and chlorphenesin. The mass spectra showed general features established earlier. Most salicylate DTBS derivatives gave abundant molecular ions and base peaks resulting from loss of C4H8. Isomers afforded very similar mass spectra because of the preponderant fragmentations of the DTBS groupings. (However, DTBS derivatives of the isomeric catechols, methyl 2,3-and 3,4-dihydroxybenzoate, showed distinctive differences in their mass spectra.) The bis-DTBS derivatives of the two (3-dihydroxyalkyl)salicylic acids examined were exceptional in that fragmentations of the side-chain moieties preponderated in the mass spectra.The results confirm the value of DTBS derivatives for the characterisation of hydroxy-acids. They also show that bis-DTBS derivatives, even with masses above 450, are quite satisfactory for gas-liquid chromatography, with typical retention indices in the region of 3000. Most fragment ions from bis-DTBS derivatives, in the group studied, retain one intact cyclic silylene moiety.

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