The solution of 3-methoxy-5-nitrosalicylaldehyde (0.5 g,
1.45 mmol) in ethanol (10ml) was added drop wise to a solution
of 1-(3-carboxypropyl)-3,3-dimethyl-2-methyleneindolinium
bromide (0.28 g, 1.45 mmol) in ethanol (10ml) over 30 min. The
reaction mixture was refluxed over 1 day. The resulting brown
solution was cooled down and the product was precipitated out
by adding ethyl acetate. The yellow precipitate was filtered and
washed with cold ethanol. The crude product was purified by
column chromatography, silica/ethy