Carbazomycins total synthesis

The total synthesis of the carbazomycins emphasizes the utility of the iron-mediated synthesis for the construction of highly substituted carbazole derivatives. The reaction of the complex salts 6a and 6b with the arylamine 20 leads to the iron complexes 21, which prior to oxidative cyclization have to be protected by chemoselective 0-acetylation to 22 (Scheme 13). Oxidation with very active manganese dioxide followed by ester cleavage provides carbazomycin B 23a [93] and carbazomycin C 23b [94]. The regioselectivity of the cyclization of complex 22b to a 6-methoxycarbazole is rationalized by previous results from deuterium labeling studies [87] and the regiodirecting effect of the 2-methoxy substituent of the intermediate tricarbonyliron-coordinated cyclo-hexadienylium ion [79c, 79d]. Starting from the appropriate arylamine, the same sequence of reactions has been applied to the total synthesis of carbazomycin E (carbazomycinal) [95]. 

The arylamine 780a required for the total synthesis of carbazomycin A (260) was prepared from commercially available 2,3-dimethylphenol (781). The regioselective... 

The arylamine 780b required for the total synthesis of carbazomycin B (261) was obtained by catalytic hydrogenation, using 10% palladium on activated carbon, of the nitroaryl derivative 784 which was obtained in six steps and 33% overall yield starting from 2,3-dimethylphenol 781 (see Scheme 5.85). Electrophilic substitution of the arylamine 780b with the iron-complex salt 602 provided the iron complex 787 in quantitative yield. The direct, one-pot transformation of the iron complex 787 to carbazomycin B 261 by an iron-mediated arylamine cyclization was unsuccessful, probably because the unprotected hydroxyarylamine moiety is too sensitive towards the oxidizing reaction conditions. However, the corresponding 0-acetyl derivative... 

Six years later, we described a considerably improved total synthesis of the carbazomycins A (260) and B (261) using highly efficient synthetic routes to the arylamines 780a and 794 (610). Moreover, this methodology uses air as an oxidant for the construction of the carbazole framework by oxidative coupling of the iron-complexed cation 602 with the arylamines 780a and 794. 

The arylamine 794 required for the improved total synthesis of carbazomycin B (261) was prepared in quantitative yield by hydrogenation of the nitroaryl derivative 793 (see Scheme 5.85). Oxidative coupling of the iron complex salt 602 and the arylamine 794 in air afforded the tricarbonyl(ri -4b,8a-dihydro-9H-carbazole)iron complex (795). Demetalation of 795, followed by aromatization, led to O-acetylcarbazomycin B (796). 

The total synthesis of carbazomycin C (262) was achieved by executing similar reaction sequences as in the iron-mediated arylamine cyclization route described for the synthesis of carbazomycin B (261) (see Scheme 5.87). The electrophilic substitution of the arylamine 780b using the complex salt 779 afforded the iron complex 797, which was transformed to the corresponding acetate 798. Using very active manganese dioxide, compound 798 was cyclized to O-acetylcarbazomycin C (799). Finally, saponification of the ester afforded carbazomycin C (262) (four steps and 25% overall yield based on 779) (611) (Scheme 5.90). 

The total synthesis of carbazomycin D (263) was completed using the quinone imine cyclization route as described for the total synthesis of carbazomycin A (261) (see Scheme 5.86). Electrophilic substitution of the arylamine 780a by reaction with the complex salt 779 provided the iron complex 800. Using different grades of manganese dioxide, the oxidative cyclization of complex 800 was achieved in a two-step sequence to afford the tricarbonyliron complexes 801 (38%) and 802 (4%). By a subsequent proton-catalyzed isomerization, the 8-methoxy isomer 802 could be quantitatively transformed to the 6-methoxy isomer 801 due to the regio-directing effect of the 2-methoxy substituent of the intermediate cyclohexadienyl cation. Demetalation of complex 801 with trimethylamine N-oxide, followed by O-methylation of the intermediate 3-hydroxycarbazole derivative, provided carbazomycin D (263) (five steps and 23% overall yield based on 779) (611) (Scheme 5.91). 

The arylamine 780c required for the total synthesis of carbazomycin E (264) was prepared in seven steps starting from vanillyl alcohol (803). Vanillyl alcohol (803) was transformed to the tetrasubstituted aryl derivative 804 via generation of the benzyl methyl ether followed by ortho-directed lithiation and subsequent... 

The total synthesis of the carbazomycins G (269) and H (270) based on our iron-mediated approach uses the O-acetylcarbazoles 971 and 972 as precursors, which are derived from the the iron-complex salts 602 and 779 and the arylamine 973 (650,651) (Scheme 5.134). 

The required arylamine 973 was prepared starting from the aryl acetate 937. The compound 937 was also used for the palladium(II)-catalyzed total synthesis of carbazoquinocin C (274) (545) (see Scheme 5.124) and carbazomycin G (269) (652). The acetate 937 was transformed to the corresponding 5-nitro derivative 974 by reacting with fuming nitric acid in a mixture of acetic anhydride and glacial acetic... 

One of the carbazole-l,4-quinones, 3-methoxy-2-methylcarbazole-l,4-quinone (941), required for the total synthesis of carbazomycin G (269), was already used as a key intermediate for the total synthesis of carbazoquinocin C, and was obtained by the addition of aniline (839) to 2-methoxy-3-methyl-l,4-benzoquinone (939), followed by oxidative cyclization with catalytic amounts of palladium(II) acetate (545,645) (see Schemes 5.124 and 5.125). Similarly, in a two-pot operation, 4-meth-oxyaniline (984) was transformed to 3,6-dimethoxy-2-methylcarbazole-l,4-quinone... 

Hibino et al. reported the total synthesis of carbazomycin G (269) by the regioselective addition of methyllithium onto 3-methoxy-2-methylcarbazole-l,4-quinone (941) (653). The required immediate precursor of carbazomycin G, carbazole-l,4-quinone 941, was obtained from 3-(2-methoxyethenyl)-N-(phenylsul-fonyDindole (986). The benzannulation involves an allene-mediated electrocyclic reaction of a 67t-electron system generated from the 2-propargylindole 989, which was derived from the 3-vinylindole 986 in three steps. 

Scheme 8. Total synthesis of carbazomycin B via a 5-exo-trig cyclization...

In 1989 we reported an iron-mediated route for the construction of the tricyclic carbazole skeleton [72, 73]. This convergent method was applied to the total synthesis of the naturally occurring alkaloid carbazomycin A [72]. Key steps of our iron-mediated approach are the consecutive C C bond formation and oxidative cyclization (formation of the C N bond) between an electrophilic tricarbonyl(ri -cyclohexadienyhum)iron complex salt 30 and an arylamine 31 (Scheme 10). Subsequent oxidation and demetalation provides the aromatized carbazole 32. 

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