CYCLOAROMATIZATION OF α-OXOKETENE DITHIOACETALS WITH LITHIOACETONITRILE AND LITHIOPROPIONITRILE: A FACILE ROUTE TO SUBSTITUTED AND ANNELATED PYRIDINES

Article abstract of DOI:10.1016/S0040-4020(01)82036-6

The enolacetals 2 obtained by 1,2-addition of lithioacetonitrile or lithiopropionitrile to α-oxoketene dithioacetals 1 undergo intramolecular Ritter reaction accompanied with 1,3-MeS shift in the presence of phosphoric acid to affort a variety of substituted and annelated 2,6-bis(methylthio)pyridines 3 and 4 respectively in good yields.Cyclization of 2 in the presence of bromine and acetic acid yielded the corresponding 2-bromo-6-methylthio-4,5-substituted and annelated pyridines 9 in good yields.The bis(methylthio) groups in 3 could either be removed with RaneyNickel or replaced by methyl group in the presence of triphenylphosphine-Nickel chloride complex to afford the corresponding desulfurized (8) or methylated (15) pyridines in good yields.Attempted cyclization of enolacetals 2a, 2p and 2s in the presence of borontrifluoride etherate and methanol yielded only the dienes 14a-b and β-cyanomethyl-α,β-unsaturated ester 14c respectively.