Synthesis of Spirocyclohexadieneyl‐2‐Oxindoles by 6π‐Electrocyclization of Trienes Derived from Wittig Reaction of Morita‐Baylis‐Hillman Carbonates and α,β‐Unsaturated Aldehydes

Abstract

Spirooxindoles exist in a huge number of natural substances and pharmaceutically interesting compounds.1 Thus, tremendous efforts have been devoted to efficient protocols to access these important motifs over the past years. [1][2][3][4][5] Recently, we reported the synthesis of spirocyclohexadienyl-2-oxindoles by 6π-electrocyclic ring closure (6π-ERC) of the trienes, derived from Morita-Baylis-Hillman (MBH) adducts of cinnamaldehyde and isatin (Scheme 1).2e Very recently, we also reported the synthesis of dispirobisoxindole from MBH carbonates of N-methylisatin (Scheme 1).3 During the course of our study, we decided to examine the synthesis of structurally related spirocyclohexadienyl-2-oxindoles 4,5 with different substitution pattern by 6π-ERC of the trienes, derived from MBH carbonates of N-methylsatin and cinnamaldehyde, as shown in Scheme 1.At the outset of our experiment, the reaction of MBH carbonate 1a and trans-cinnamaldenyde (2a, 1.1 equiv) in 1,2-dichlorobenzene (ODCB) was examined at 150 C in the presence of PPh 3 (1.0 equiv).3 To our delight, spirocyclohexa-2,4-dienyl-2-oxindole 3a was obtained in good yield (69%) in short time (2 h). The formation of the other diastereomer (3a 0 , vide infra, Scheme 2) was not observed. The structure of 3a was unequivocally confirmed by its crystal structure (vide infra, Scheme 2).6 However, two diastereomeric dispirobisoxindoles (see, Scheme 1) have been formed as side products in appreciable yield (23%), as in our previous paper.3 When we carried out the reaction in refluxing p-xylene (5 h), the yield of 3a increased to 74%, and the yield of dispirobisoxindoles decreased (18%). The yield of 3a increased slightly (77%) when we carried out the reaction in refluxing toluene; however, a long reaction time (24 h) was required. When we used an excess amount of 2a (2.0 equiv) in refluxing xylene (5 h), the yield of 3a increased up to 85%. In the reaction mixture, the formation of only a trace amount (<5%) of dispirobisoxindole was observed. 7Encouraged by the results we synthesized various spirooxindoles 3b-3l, and the results are summarized in Table 1. Various spirooxindoles 3b-3e were synthesized in good to moderate yields (67-89%) by the reactions of substituted isatin derivatives 1b-1e with 2a. The reaction of 1a and trans-3-(2-furyl)acrolein (2b) afforded 3f in moderate yield (60%). The reaction of crotonaldehyde (2c) gave 3g in good yield (78%). In the reaction, we used an excess amount of 2c (3.0 equiv) because the boiling point of 2c (104 C) was lower than the reaction temperature (138 C). The reactions of 1a with α-methyl-trans-cinnamaldehyde (2d) and α-bromocinnamaldehyde (2e) afforded 3h and 3i in good yields (68% and 84%, respectively). However, the reactions with α-substituted aliphatic aldehydes, trans-2-methyl-2-butenal (2f) and 1-cyclohexene-1-carboxaldehyde (2g), produced the corresponding spirooxindoles 3j (23%) and 3k (33%) in low yields with 3.0 equiv. of 2f or 2g. The result might be due to sluggish reactivity in the Wittig reaction of α-branched α...

Synthesis of Spirooxindoles Bearing Benzopyrano[2,3‐b]indole Moiety by NBS‐Mediated Oxidative Coupling Reaction

“…Spirooxindoles exist in a large number of natural substances and pharmaceutically interesting compounds . Thus, tremendous efforts have been devoted to development of efficient protocols to access these important motifs over the past years . Recently, we reported the synthesis of spiroindeno[1,2‐ b ]indolyloxindoles from isatins and 2‐arylindoles in the presence of TiCl 4 via sequential inter‐ and intramolecular Friedel–Crafts reactions, a as shown in Scheme .…”