Cycloaromatization of α-oxoketene dithioacetals with lithioacetonitrile and lithiopropionitrile: a facile route to substituted and annelated pyridines

Abstract

The enolacetals 1 obtained by 1,2-addition of lithioacetonitrile or lithioprop- ionitrile to α-Oxoketene dithioacetals O undergo intramolecular Ritter reaction accompanied with 1,3-MeS shift in the presence of phosphoric acid to afford a variety of substituted and annelated 2,6-bis(methylthio)pyridines 6 and 1 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 O could either be removed with Raney Nickel or replaced by methyl group in the presence of triphenylphosphine-Nickel chloride complex to afford the corresponding desulphurized (6) or methylated (5) pyridines in good yields. Attempted cyclization of enolacetals a , P and s in the presence of borontrifluoride etherate and methanol yielded only the dienes 4a-2 and β-cyanomethyl-α,β -unsaturated ester 4c respectively