CALICHEAMICINS, A NOVEL FAMILY OF ANTITUMOR ANTIBIOTICS .4. STRUCTURE ELUCIDATION OF CALICHEAMICINS BETA-1BR, GAMMA-1BR, ALPHA-2I, ALPHA-3I, BETA-1I, GAMMA-1I, AND DELTA-1I

The details of the structural assignment of the potent antitumor antibiotic, calicheamicin gamma-1I (6, C55H74IN3O21S4), is reported. Methanolysis studies on 6 and N-acetylcalicheamicin gamma-1I (8, C57H76IN3022S4) permitted the structural assignment of the glycosidic chain. Details of the spectral analysis supporting the assignments of the 3-O-methyl-alpha-L-rhamnopyranoside (D-ring) and the methyl 2,4-dideoxy-3-O-methyl-4-(N-acetyl-N-ethylamino)-a-L-xylopyranoside (E-ring) is reported. The structure of calicheamicinone (32, C18H17NO5S3), containing a bicyclo[7.3.1 ] tridec-9-ene-2,6-diyne system and a methyl trisulfide, was elucidated by a series of chemical degradation studies, which included an unexpected free radical cycloaromatization reaction. The presence of 4,6-dideoxy-4-(hydroxyamino)-beta-D-glucopyranoside (A-ring) and its N-O glycosidic linkage to the thio sugar (B-ring) was ascertained by X-ray crystallography of 24 (C36H40INO13S2), a degradation product of 6. The chemical structures of calicheamicins beta-1Br (1), gamma-1Br (2), alpha-2I (3), alpha-3I (4), beta-1I (5), and delta-1I (7) were assigned by correlating their H-1 and C-13 NMR data with that of calicheamicin gamma-1I. By tracking the biological activities of the degradation products, the enediyne system of calicheamicinone was shown to be essential for the DNA-damaging abilities of the calicheamicins. A mechanism whereby the enediyne could be triggered to cyclize via a 1,4-diyl, the putative DNA cleaving species, is proposed.