Direct access to 3-substituted 1,4-oxathiepino[5,6-b]pyridine-5-one through one-pot substitution cyclization reaction of 2-mercapto-3-nicotinic acid with α-bromo ketones

Synthesis of [1,4]Oxathiepino[5,6-b]quinolines via Base-Mediated Intramolecular Hydroalkoxylation

A base-mediated intramolecular hydroalkoxylation that was used to prepare a series of seven-membered S,O-heterocycles is described. 2-Thiopropargyl-3-hydroxymethyl quinolines were prepared starting from 2-mercaptoquinoline-3-carbaldehydes, via S-propargylation and reduction of a formyl group. Interestingly, 2-mercaptopropargyl-3-hydroxymethyl quinolines were converted into the corresponding oxathiepinoquinolines in the presence of t-BuOK. It is proposed that the S-propargyl moiety, in the presence of base, is converted into its allenyl isomer; subsequent addition of a hydroxyl group to the terminal double bond yields the 3-methyl-5H-[1,4]oxathiepino[5,6-b]quinoline in good to high yield. Notably, the procedure is adaptable to the conversion of N-propargyl indole-2-methanol into the corresponding intramolecular hydroalkoxylation product.

Total synthesis of protosappanin A and its derivatives via palladium catalyzed ortho C–H activation/C–C cyclization under microwave irradiation

Protosappanin A, a complex natural product with high bioactivity, and 25 of its derivatives were synthesized through an intramolecular ortho C–H activation/C–C cyclization, ring-enlargement and deprotection reaction. C–H activation as the key step was investigated and optimized.