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Diversely substituted benzo [a]carbazoles and related derivatives are important because of their interesting biological activities such as neuroprotective property, antitumor activity, and kinase inhibitory activity. 1 Thus, various synthetic methods have been developed 2 including Lewis acidcatalyzed annulation of 2-arylindoles with propargylic alcohol derivatives, 2a,b double cascade cyclization of diynes catalyzed by gold, 2c-e iodine, 2f or palladium, 2g and coppercatalyzed synthesis via α-C-arylation of ketones. 2h Recently, we also reported the synthesis of benzo[a]carbazoles from 2-arylindoles via sequential propargylation, propargyl-allenyl isomerization, and 6π-electrocyclization approach, as shown in Scheme 1. 3 Various 5-benzyl-and 5-methylbenzo[a]carbazoles have been synthesized; however, 5-unsubstituted benzo[a]carbazoles could not be synthesized by the approach. 3 In order to develop a synthetic method of 5-unsubstituted benzo[a]carbazoles, we decided to examine the reaction of β-(2-phenylindolyl) nitroalkane 3a by Mn(OAc) 3 -mediated cyclization and subsequent elimination of nitrous acid, 4,5 as shown in Scheme 1. A similar synthesis of a naphthalene ring via Mn(OAc) 3mediated intramolecular oxidative coupling reaction between arene and nitroalkane moieties has already been reported in the previous studies. 5a The required starting materials 3a-3h were prepared from corresponding 2-arylindoles and β-nitrostyrenes according to the reported methods (see Appendix S1, Supporting information). 6a-g As reported in our previous paper, 5a the reaction of 3a was examined in EtOH (reflux, 24 h) in the presence of Mn(OAc) 3 (4.0 equiv). To our delight, 4a was obtained in a reasonable yield (56%) along with a low yield of 5-nitro derivative 5a (13%). The reactions in other solvents such as MeOH, n-propanol, n-butanol, and polyethylene glycol (PEG)-3400/ t BuOH were less effective when compared with EtOH. The reaction in AcOH showed the formation of many intractable side products. The use of CAN (cerium ammonium nitrate) in EtOH was completely ineffective. When we used lesser amount of Mn(OAc) 3 (3.0 equiv) in EtOH (reflux, 24 h), the yields of 4a (48%) and 5a (8%) decreased a little.The synthesis of various benzo[a]carbazoles was carried out under the typical reaction condition, and the results

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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...