Pd(II)-mediated cyclodehydrogenation of formyldiarylamines—total synthesis of natural carbazoles clauraila A, clausenal and 6-methoxymurrayanine

Synthesis of Clausenal, 1,5‐Dimethoxycarbazole‐3‐carbaldehyde and 2,5‐Dimethoxycarbazole‐3‐carbaldehyde

“…A complete comparison of the 1 H and 13 C NMR data for our synthetic clausenal (1) with the corresponding data for the other synthetic samples of 1 shows that the data in CDCl 3 are virtually identical with those reported by Das [7] and Sperry [10] ( Table 2 and Table 3). The 1 H and 13 C NMR data of our synthetic 1 obtained in [D 6 ]DMSO are also in excellent agreement with those reported by da Silva Emery [6] and Sperry, [10] whereas there are some significant differences to the corresponding values published by Lokhande [8] (Tables S2 and S4). In [D 6 ]acetone as solvent, the 1 H and 13 C NMR data of our synthetic sample of 1 are also in good agreement with those reported by Tamariz [9] (Tables S3 and S4).…”

“…[c] The original assignment for these signals reported by Sperry et al [6] has been revised based on our assignment. [d] For the signal of the CHO group, a second value (194.4) was reported in the same publication by Chowdhury et al [4] thetic clausenal (1) with those reported by Chowdhury for the natural product shows that except for the signal for C-3, all the signals are in relatively good agreement (Table 3).…”

“…described by the groups of da Silva Emery in 2015, [6] Das in 2016, [7] Lokhande in 2017, [8] and Tamariz in 2017. [9] In another synthesis of clausenal (1) also reported in 2017, Sperry et al cast some doubts on the structural assignment of the natural product.…”

“…[4] However, the chemical shift of the low-field proton H-4 at δ = 8.60 ppm for the natural product does not match with the corresponding signal of Sperry's synthetic compound (δ = 8.17 ppm). [6] We hypothesized that the comparatively high value for the more deshielded proton 4-H of the natural product might be caused by the second methoxy group which if located at C-5, in close proximity to 4-H, would lead to a downfield shift of the proton signal at C-4. Thus, considering the spectroscopic data reported for the isolated natural product, we envisaged the 1,5-dioxygenated carbazole 2 and the 2,5-dioxygenated carbazole 3 as potential structural alternatives for clausenal (1).…”

“…Moreover, we give a complete comparison of the NMR data for the three products 1-3 with those data reported for the natural product [4] and for the synthetic samples of 1 obtained by previous synthetic routes. [6][7][8][9][10]…”