Photochemical isomerization of colchicine and thiocolchicine

The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiation of colchicine gave a well-known transposition reaction to beta- and gamma-lumicolchicines, thiocolchicine did not react. Femtosecond transient spectroscopy of colchicine showed a strong band with maximum at 510 nm appearing at tau = 0. It disappeared within few hundred femtoseconds, leaving a broad structureless band with a maximum around 470 urn. A second band is observed around 410 nm. The analysis in time showed that the 510-nm component appeared instantaneously and decayed following a biexponential low with time constants of 300 +/- 100 fs and 40 ps. The kinetics at 420 nm has a measurable rise lime of 300 +/- 150 fs. Quantum mechanical calculations on colchicine showed that this absorption is due to a S-1 --> S-11 transition. In thiocolchicine, the instantaneous formation of a structure with maxima out of the investigated spectral region was observed. A strong absorption around 650 nm indicated the presence of a band with a maximum at longer wavelengths (> 700 nm) and a peak around 380 nm, which partially coincides with the ground-state absorption and therefore strongly affected absorption around 650 nm and its rapid (similar to500 fs) decay by its bleaching. The instantaneous formation of an absorption was observed. At shorter wavelengths (400 nm), the t decay was fitted with a biexponential curve with the first time constant of about 80 ps. The second part of,the decay had a very long tail up to 500 ps. Transient spectroscopy and configuration interaction calculations are in agreement with a mechanism involving a disrotatory cyclization of colchicine in its first excited singlet state. The lack of reactivity observed in thiocolchicine was explained by considering the presence of efficient ISC to the triplet state.