The vibrational spectra of some 1,2,4-trioxanes present two characteristic bands at 790 and 880 cm-'. On the basis of "0-isotopic substitution and comparison with analogous compounds, these bands have been assigned to coupled C-0 and 0-0 stretching modes of the C-0-0 element.
Introduction.
-Ever since the discovery that arteannuin (1) is a potent antimalarial agent [ 1-31, there has been a growing interest in the synthesis, chemistry, and physical properties of the intrinsic structural feature of this unique natural product, namely the 1,2,4-trioxane ring and its congeners [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. From a study of the vibrational spectra of 1 and its lactol derivatives, it has been concluded [18] that the peroxide linkage contained therein is characterized by a frequency at 722 cm-'. This is a plausible value since the 0-0 stretching mode is expected to lie between 600 and 900 cm-' [ 191 [20]. Moreover, this mode is usually identified by two criteria; a Raman-active component which should show as a strong polarized band, and a corresponding IR band which should be either absent or weak. However, a caveat has been issued, namely that the 0-0 stretching mode cannot be regarded as a good group frequency because it is strongly coupled with other modes [2 11. Consequently, we decided to investigate the vibrational spectra of appropriately labelled bicyclic 1,2,4-trioxanes in order to verify the above finding and to determine which bands comprise the 'fingerprint'.
The first set of trioxanes chosen are cis-fused derivatives consisting of two isotopically different pairs (2,3 and 4,5). Isotopic 0-substitution is the best means of unambiguously identifying the vibrations of the peroxide linkage, since the aforementioned criteria are M e 0 -0 AMe M e 2 R 1 2 R = Me. O(1). O(2). O(4) = ' 6 0 3 R = M e ; 0 ( 1 ) , 0 ( 2 ) = ' 8 0 . 0 ( 4 ) = ~6 0 4 R = H : O(1). O(2). o(4) = ' 6 0 5 R = H, 0(1), O(2) =l80. o(4) = ' 6 0