Esters of 1‐O‐Demethylthiocolchicines: Formation of Isomers in Chloroform Solution

Abstract

1-0-Acetyl-1-0-demethylcolchicine, and acylated l-0,2-0-didemethylthiocolchicines, in contrast to 2-0-acetyl-, 2-0,3-0-diacetyl-and 3-0 -acetyl analogs, showed after standing in CHC1, solution significant changes in optical rotation, a duplication of 'H-NMR signals, and the formation of new isomers on TLC. Solid-state X-ray diffraction of 0 -acetylated colchinoids and thio analogs, showed out of planar arrangements of the aromatic substituents, but the analysis could not help to explain the structures of the newly formed isomers in CHCI, solution.Introduction. -Colchicine, the major antimitotic alkaloid of the meadow saffron Colchicum autumnale, binds specifically and with high affinity to the tubulin dimer, the major protein subunit of microtubules [I]. 1-0 -Demethylcolchicine (1) [2] binds substantially less to tubulin protein in vitro than colchicine, or its 2-0-and 3-0-demethyl congener, but binding was somewhat restored upon 0-acetylation of 1 to 2 [I]. It is interesting to note that the rotational strength of 1 in CHCl, solution was considerably reduced after standing compared to that of colchicine, or its 2 -0 -and 3-0-demethylIt seemed interesting to study whether similar observations could be made in the isosteric series of 10-(methy1thio)-10-demethoxycolchicine (3), which is easier amenable to chemical modification because of the increased stability of the tropolonic ring towards acids [3]. We hoped that such an investigation would substantiate the importance of aromatic substitution in ring A of colchinoids with regard to tubulin binding, and possibly gives clues on how inactivation of 1-0 -demethylcolchicine in the tubulin-binding experiment could be explained.We now report results of such a study, carried out with several phenolic thiocolchicines 4-8, acetates 9-13 and the dibenzoate 14, showing drastic changes in CHC1, solution in 'H-NMR spectra, by optical rotations and chromatographic behavior, when the Me0 group(s) was replaced by an AcO or BzO group (s) (2,12, 14).

New Tetracyclic Colchicinoids from the Reaction of N‐Deacetylthiocolchicine and N‐Deacetylcolchicine with Nitrous Acid and tert‐Butyl Nitrite

“…As expected, the troponoid ring of 5 is almost planar, with a total puckering amplitude [11] of 0.073(3) ä and a maximum distance from the mean plane of 0.04 ä for C (10). The geometrical parameters of this ring and of its methylthio and oxo substituents ( Table 1) are very similar to those observed in other thiocolchicine derivatives [12] [13]: C(15)ÀS(1)ÀC(10) lies approximately in the plane of the ring, and the C(10)ÀS(1) bond length is close to that found in conjugated systems (1.751 ä) [14]. The troponoid ring exhibits a clear alternation of long and short bonds (see Table 1) similarly to colchicine [15] and thiocolchicine (CCD refcode TCOLCH).…”

“…The troponoid ring exhibits a clear alternation of long and short bonds (see Table 1) similarly to colchicine [15] and thiocolchicine (CCD refcode TCOLCH). Only the C(9)ÀC(10) bond distance, 1.485(3) ä, is significantly lengthened compared with thiocolchicine (1.458 ä) or related compounds [12] but lies not too far from those in pseudothiocolchicine [13] (1.480 ä) and demethylthiocolchicine [16] (1.475 ä).…”

The importance of the phenyl‐tropolone ‘aS’ configuration in colchicine's binding to tubulin

“…Thus, the large negative rotation observed for natural (+)-deacetamidocolchicine (having only axial chirality) exhibited a net positive CD spectrum similar to that of unnatural (7R)-colchicine (having atomic and axial chiralities) in the presence of tubulin (as will be discussed later), the optical rotation exhibited by these colchicinoids therefore is attributed to the axial chirality originating from the phenyl-tropolonic moiety rather than the C-7 chiral center. This conclusion is further supported by optical measurements of a CDC13 solution of (7S)-l-acetyl-l-demethylcolchicine in which a reduction of negative rotation values ([t~]D) from --185 at 22°C to -76 at 50°C [13] was accompanied, respectively, by a change in molar ratio of aS to aR conformers from about 1:0.4 to 1:0.8 as measured by NMR. The CD spectrum of tubulin is characterized by a large negative band at about 272 nm and a small positive band at 295 nm ( fig.l, curve E), presumably contributed by aromatic residues and disulfides of the protein.…”

Anticancer Drug Design Based on Plant-Derived Natural Products¹

“…However, although both the tridemethylated thiocolchicine (29) and colchicine (30) derivatives were totally inactive towards tubulin, they still exhibited potent inhibition of topo II [3,4]. When the mono-demethylated compounds were acetylated (e.g., 27 → 31), tubulin binding activity was restored [34]. (2) The seven-membered B ring and the C(7) side chain are not crucial for tubulin binding, but may affect the conformation and binding properties.…”

“…A Demfanov rearrangement reduces the colchicinoid B ring from seven to six carbons [56], leaving a hydroxymethyl substituent. Compound 32, several acylated derivatives (33)(34)(35)(36)(37)(38), and the dehydrated methylene analog (39) inhibited tubulin polymerization and radiolabelled colchicine binding to tubulin. Except for the butyrate ester (34), these 6-membered B ring compounds were, however, less cytotoxic than thiocolchicine [56].…”