Synthesis of Aminonaphthalenes from Morita–Baylis–Hillman Carbonates via 6π‐Electrocyclization of Ketenimine Intermediates

Abstract

Morita-Baylis-Hillman (MBH) adducts have been used extensively for the synthesis of various cyclic and acyclic compounds. 1 One of the important chemical transformations of MBH adducts starts from a conversion of electrophilic β-carbon of acrylate moiety to a nucleophilic center by forming MBH adduct-derived phosphorous ylide I, as shown in Scheme 1. 2,3 The MBH ylide I could be formed by the reaction of MBH bromide or acetate with tertiary phosphine in the presence of base, 2 and the ylide has been used for the synthesis of Ramirez ylides, 2a spirooxindoles, 2b,c poly-substituted aromatics, 2e-h and 1,3dienes. 2i-k The ylide I could also be generated more efficiently from MBH carbonate 1a and tertiary phosphine without an external base, 3 and the ylide has been used extensively for the synthesis of thiophenes, 3a pyrazoles, 3b isoquinolines, 3c butenolides, 3e and spirooxindoles. 3o Very recently, we reported a stereoselective synthetic method of 3,4-dibenzylidenepyrrolidine-2,5-diones by the reaction of ylide I and phenyl isocyanate (2a) to form carbamoyl stabilized ylide 3a, and a following Wittig reaction with benzaldehyde (Scheme 1). 4 The amide proton of 3a was eliminated during the acid-catalyzed cyclization as MeOH. As a continuous work, we envisioned that ylide 3a could be converted to ketenimine intermediate II by liberation of triphenylphosphine oxide in the absence of an acid catalyst at elevated temperature. The ylide carbon atom of 3a could abstract the amide proton and a following elimination of triphenylphosphine oxide would generate II (vide infra). Then, the ketenimine could be transformed to aminonaphthalene 4a 5 via 6π-electrocyclic ring closure (ERC) process. [6][7][8] In order to check the feasibility of our assumption, a toluene solution of 3a 4 was heated to reflux for 3 h. To our delight, methyl 4-phenylaminonaphthalene-2-carboxylate (4a) was obtained in good yield (72%). When we used benzene as a solvent, a long reaction time (20 h) was required and the yield was moderate (51%). In addition, 4a could be synthesized directly from MBH carbonate 1a in one-pot procedure in good yield (63%) by the reaction of 1a and PPh 3 (1.1 equiv) in toluene at room temperature (5 min) to form I, addition of phenyl isocyanate (2a, 1.1 equiv) to form 3a (80 C, 1 h), and a final heating to reflux of the reaction mixture (3 h). 9 The yield of one-pot reaction (63%) was better than the two-pot procedure (68/72% = 49%). The structure of 4a was confirmed unequivocally by its crystal structure, and the structure of 4a is shown in Scheme 1 along with the crystal structure of 3a. 4,10 Encouraged by the successful result various MBH carbonates 1b-1h were prepared, 3,4 and the reactions with some representative isocyanates 2a-2d have been carried out in one-pot manner. The results are summarized in Table 1. The reactions of 1b and 1c afforded 4b (52%) and 4c (54%) in moderate yields. The reaction of 4-nitrophenyl derivative 1d afforded 4d in somewhat lower yield (44%). The reactions of 1e-1g afforded 4e-4g in moder...

An Expedient Synthesis of Arene‐fused Phthalimides from Morita–Baylis–Hillman Carbonates

“…A dipolar interaction between the sulfur atom and phosphorous atom in 2c might facilitate an abstraction of amide proton and subsequent elimination of Ph 3 PO to form a ketenimine intermediate. The ketenimine underwent 6π‐electrocyclization reaction to produce 5 , as already reported in our previous report …”

“…However, the reaction of amide ylide 2c , bearing a 2‐thienyl moiety, showed somewhat different result. The reaction of 2c and 3b afforded the desired product 4j in low yield (42%) along with benzo[ b ]thiophene 5 in appreciable amount (14%), as shown in Scheme . A dipolar interaction between the sulfur atom and phosphorous atom in 2c might facilitate an abstraction of amide proton and subsequent elimination of Ph 3 PO to form a ketenimine intermediate.…”

“…The compounds have been synthesized from MBH carbonates via the formation of MBH ylides, reaction with phenyl isocyanate to form amide ylides, and a subsequent acid‐catalyzed stereoselective Wittig reaction with various aldehydes . More recently, aminonaphthalene has been synthesized from the amide ylide 2a via 6π‐electrocyclic ring closure of the ketenimine intermediate, as shown in Scheme . As a continuing study, we reasoned out that 6π‐electrocyclization reaction of 3,4‐diarylidene‐ N ‐phenylpyrrolidine‐2,5‐dione I and a subsequent aerobic oxidation of intermediate II would produce furan‐fused phthalimide derivative 4a , as also shown in Scheme .…”