[141707-79-1] · C5H5NOS · N-Methylthionomaleimide · (MW 127.18)
Physical Data: mp 44-44.5 °C.
Solubility: sol most organic solvents.
Form Supplied in: volatile purple crystals; not commercially available.
Preparative Method: reaction of N-methylmaleimide with Lawesson's reagent (2,4-Bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4-Disulfide) under carefully controlled conditions.1
Handling, Storage, and Precautions: stable for several months in the freezer; decomposes to yellow crystals at ambient temperature in the presence of light and air.
This is the first thiocarbonyl analog of maleimide with no substituents on the ring carbons. Its use as a dienophile in [4 + 2] reactions has been studied.
Previously reported thiocarbonyl analogs of maleimide undergo [4 + 2] reactions with a variety of dienes to give reaction at the thiocarbonyl function.2 N-Methylthionomaleimide (1) reacts predominantly at the ring unsaturation. Reaction with 2,3-dimethylbutadiene in various solvents leads almost exclusively to the adduct (2) (Table 1). Only when reacted in 5 M ethereal Lithium Perchlorate3 was a significant amount of the adduct (3) produced (eq 1).
Some degree of regioselectivity can be expected in using (1) as a dienophile. Reaction of (1) with 1-methoxy-1,3-cyclohexadiene (4) in toluene at 70 °C gave a 2:1 mixture of the endo adducts (5) and (6) (eq 2). Using ethereal LiClO4 as solvent improved the ratio to 85:15. Reaction with Danishefsky's diene (7) gave adduct (8) as the only isolated product in 56% yield (eq 3).
Paul M. Herrinton
The Upjohn Company, Kalamazoo, MI, USA