The photochemical behavior of colchicone and thiocolchicone

The irradiation of colchicone 5 led to the formation of lumicolchicone 7. T he same reaction cannot be obtained by using thiocolchicone 6 as substrate. Transient absorption spectroscopy of colchicone and beta-lumicolchicone sh owed that probably the photoisomerization occurred on colchicone in its fir st excited singlet state. The spectroscopic data are in agreement with the hypothesis that lumicolchicone was generated in the ground state from the S -1 state of colchicone without the presence of any intermediate. Semiempiri cal calculations on colchicone and thiocolchicone showed that the highest s ingle occuped molecular orbital and the lowest unoccupied molecular orbital of the singlet excited colchicone can give a disrotatory ring closure to 7 , while thiocolchicone cannot give the same type of process.