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

Recently, we reported synthesis of 3,4-diarylidene-N-phenylpyrrolidine-2,5-dione derivatives from Morita–Baylis– Hillman (MBH) carbonates. 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 1. 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 furanfused phthalimide derivative 4a, as also shown in Scheme 1. Various arene-fused phthalimides have been synthesized and found their usefulness in the areas of dual fluorescent dyes as a probe for ratiometric detection of DNA, organic light-emitting devices, and for the synthesis of conjugated polymers. Thermal 6π-electrocyclization reaction of conjugated triene has been studied extensively. Double bond of aromatic compounds could also be involved in 6πelectrocyclization reactions. Recently, we also reported 6π-electrocyclization reactions of conjugated trienes bearing an aromatic double bond. The electrocyclization could be conducted at relatively lower temperature when the resonance energy of the arene ring is small, such as furan, thiophene, or naphthalene. Thus, at the outset of our experiment, the reaction of 2a and 2-furaldehyde (3a) was carried out in refluxing toluene in the presence of a catalytic amount of AcOH under O2 balloon atmosphere. However, 4,6-diphenyl-1-oxa-6-azaindacene-5,7-dione (4a) was not formed at all. Instead, the formation of diarylidene intermediate I was observed, as in our previous paper. When the reaction was conducted at higher temperature (160 C) in 1,2-dichlorobenzene (ODCB) for 4 h, 4a was obtained in good yield (61%) to our delight. Compound 4a might be formed via an acid-catalyzed cyclization of 2a to form imide ring, a subsequent Wittig reaction with 3a to form I, thermal 6π-electrocyclization to II, and a final aerobic oxidation to 4a. Encouraged by the successful result, the reactions of 2a with various aromatic aldehydes were examined. We selected five-membered heteroaromatic aldehydes and naphthaldehydes that have small resonance energy, in order to overcome the high activation energy during the 6πelectrocyclization. The results are summarized in Table 1. The reaction of 2a with 2-thiophenecarboxaldehyde (3b) afforded 4b in a similar yield (59%). The reactions with 5-methylfurfural (3c) and 5-methyl-2thiophenecarboxaldehyde (3d) afforded 4c and 4d, respectively, in moderate yields (59 and 57%). The reaction of 3-furaldehyde (3e) gave 4e in a similar yield (62%). 1-Naphthaldehyde (3f) and 2-naphthaldehyde (3g) also afforded 4f and 4g in moderate yields (56 and 57%), respectively. In addition, other amide ylides 2b and 2c were prepared according to the reported method, and the reactions were also examined. The reactions of 4-chlorophenyl derivative 2b with 3c and 3d also afforded 4h (57%) and 4i (59%) in moderate yields, respectively. However, the reaction of amide ylide 2c, bearing a 2thienyl 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 2. A dipolar interaction between the sulfur atom and phosphorous atom in 2c might facilitate an abstraction of amide proton and subsequent elimination of Ph3P O to form a ketenimine intermediate. The ketenimine underwent 6πelectrocyclization reaction to produce 5, as already reported in our previous report. It is also interesting to note that the reaction of 2c and 3a afforded a mixture of 4k and 4k0 (1:2, 35%) along with 5 (8%), as shown in Scheme 3. Although the stereochemistry of both double bonds of 3,4-diarylidene-N-phenylpyrrolidine-2,5-dione intermediate I (see Scheme 1) could be controlled under mild conditions (benzene, 80 C, 30 min) in our previous paper, the double bonds might be isomerized in part under the drastic conditions (ODCB, 160 C, 4 h), as shown in Scheme 3. Thus, both double bonds of furan and thiophene could be involved in a following 6πelectrocyclization reaction to produce 4k and 4k0. Note