Total synthesis imines

Deuterium labeling of C-18 has also been accomplished by an alternate procedure adapted from the Nagata steroid synthesis. During the course of the total synthesis of pregnanolone, thevC-18 function is introduced in the form of a nitrile group. Reduction of this function in intermediate (247) with lithium aluminum deuteride leads to a deuterated imine (248), which upon Wolff-Kishner reduction and acid-catalyzed hydrolysis... 

As a part of the total synthesis of (+ )-corydalic acid methyl ester (12), a reaction of a cyclic enolate with an imine has been applied. The 2-toluamide enolate 9, which in this case is substituted at the methyl group, adds stereospecifically to imine 10, affording mainly tram-iso-quinolone 11 with a d.r. (transjds) > 95 525. 

The use of chiral bis(oxazoline) copper catalysts has also been often reported as an efficient and economic way to perform asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines with conjugated dienes [81], with the main focus on the application of this methodology towards the preparation of biologically valuable synthons [82]. Only some representative examples are listed below. For example, the copper complex 54 (Scheme 26) has been successfully involved in the catalytic hetero Diels-Alder reaction of a substituted cyclohexadiene with ethyl glyoxylate [83], a key step in the total synthesis of (i )-dihydroactinidiolide (Scheme 30). 

Scheme 3.18. Domino radical cyclization reaction of W-aziridinyl imine 3-68 in the total synthesis of ( )-modhephene (3-70).

Allenylsilanes 159 (Eq. 13.53) and 161 (Eq. 13.54) differ in the axial stereochemistry of the allene function. In each case, formation of the benzyl imine, followed either by treatment with tin(IV) chloride in benzene at room temperature or heating in toluene, leads to diastereomeric products 160 and 162 [63], Significantly, there is no crossover, pointing to a concerted (or fast, stepwise) process. Since the absolute stereochemistry of the allenylsilanes is easily controlled, the methodology is ideal for applications in total synthesis. Weinreb and co-workers have used the reaction for his synthesis of the marine natural product (-)-papuamine. 

The first step in the total synthesis of the alkaloid ( )-ipalbidine 104 was the reaction of the diene 103 with A1-pyrrol ine (equation 60)53. The proportions of threo- and m // TO-dihydro- -pyridones, 106 and 107, respectively, produced in the diethylaluminium chloride-catalyzed reactions of the a-benzyloxyimines 105 (R = n-CsHn, i-Pr or i-Bu) with the diene 86 (equation 61), depend on the nature of R and the amount of imine used54. 

For the quinone imine cyclization of iron complexes to carbazoles the arylamine is chemoselectively oxidized to a quinone imine before the cyclodehydrogenation [99]. The basic strategy of this approach is demonstrated for the total synthesis of the 3-oxygenated tricyclic carbazole alkaloids 4-deoxycarbazomycin B, hyellazole, carazostatin, and 0-methylcarazostatin (Scheme 17). 

Over the past 15 years, we developed three procedures for the iron-mediated carbazole synthesis, which differ in the mode of oxidative cyclization arylamine cyclization, quinone imine cyclization, and oxidative cyclization by air (8,10,557,558). The one-pot transformation of the arylamine-substituted tricarbonyl(ri -cyclohexadiene) iron complexes 571 to the 9H-carbazoles 573 proceeds via a sequence of cyclization, aromatization, and demetalation. This iron-mediated arylamine cyclization has been widely applied to the total synthesis of a broad range of 1-oxygenated, 3-oxygenated, and 3,4-dioxygenated carbazole alkaloids (Scheme 5.24). 

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). 

Lebold and Kerr reported the total synthesis of the eustifolines-A (172), -B (173), -C (93),-D (227), and glycomaurrol (92) starting from the readily available quinine imine 1574 and diene 1575 (899). This methodology uses the Diels-Alder reaction of a quinone monoimine 1574 and subsequent Plieninger indolization of the adduct 1576 for the synthesis of the key tetrahydrocarbazole framework. 

Schkeryantz and Pearson (59) reported a total synthesis of ( )-crinane (298) using an intramolecular azide-alkene cycloaddition (Scheme 9.59). The allylic acetate 294 was first subjected to an Ireland-Claisen rearrangement followed by reduction to give alcohol 295, which was then converted into the azide 296 using Mitsunobu conditions. Intramolecular cycloaddition of the azide 296 in refluxing toluene followed by extrusion of nitrogen gave the imine 297 in quantitative yield. On reduction with sodium cyanoborohydride and subsequent reaction with... 

The functionalities in the previously described reaction may be reorganized so that the imine is part of the diene moiety, which is then reacted with an alkene. This particular arrangement of functionality has been used as one of the key steps in the total synthesis of streptonigrone (equation 189)684. 

E. COMPLETION OF THE TOTAL SYNTHESIS REDUCTION OF IMINE AND DESULFURIZATION... 

Double oxidation reactions of aminophenols (127) to the corresponding quinone-imines (128) using PIDA and PIFA were utilized for total synthesis of an enediyne antibiotic, ( )-dynemicin A (6), by Danishefsky and co-workers... 

A final approach to the c -3a-arylhydroindole skeleton that has resulted in a concise total synthesis of ( )-epielwesine (449) featured the acid-promoted cyclization of (Z)-vinylsilane imines such as 575 (Scheme 53) (2/9). Thus, sequential alkylation of 3,4-(methylenedioxyphenyl)acetonitrile (432) with (Z)-4-bromo-l-butenyltrimethylsilane and l-bromo-2-chloroethane provided... 

Such an example has been demonstrated by Johnson and Sames, who chose a platinum-mediated dehydrogenation as a key step in the synthesis of the antimitotic rhazinilam 33 (Scheme 6) [20], The key intermediate 27 was converted into the imine 28, which was allowed to react with Me Pt(//-SMe2)]2 to afford the platinum complex 29. Subsequent treatment with triflic acid resulted in elimination of methane and furnished the cationic complex 30. Upon thermolysis in trifluoroethanol, the complex lost a second methane molecule, which resulted in the activation of the ethyl group. A subsequent /1-hydride elimination gave the hydrido-Pt(n) complex 31. Treatment with aqueous KCN followed by hydrox-ylamine removed the platinum and yielded the liberated amine 32. Johnson and Sames added a homologization and a macrolactamization and completed the total synthesis of rhazinilam (33) by removal of the carboxyl group. 

The total synthesis of O-methylsterigmatocystin features the intermolecular imino-acylation of methyl 2,6-dihy-droxy-4-methoxybenzoate 804 with iV-alkylnitrilium salts 805 to form the benzophenone ketoimines 806. Treatment of the benzophenone ketoimines 806 with K2CC>3 under reflux in acetonitrile forms the corresponding imine xanthone, which is hydrolyzed to afford the xanthone (Scheme 225) <1999JOC4050>. 

Chiral Lewis acids have been employed by Yamamoto et al. [197 -199] in order to carry out enantioselective aza Diels-Alder reactions starting from achiral substrates however, these transformations required stoichiometric amounts of the chiral mediator 3-16 which was generated in situ from (fl)-binaphthol and triphenylborate. The best results were obtained with the pyridine derivative 3-14 which afforded the desired cycloadduct 3-15 in high optical purity (Fig. 3-5). Using chiral imines, the authors found a high level of double asymmetric induction, and this methodology could be applied to the enantioselective total synthesis of two piperidine alkaloids. 

The hetero-DielsAlder reaction (hDA) using iminium species has been commonly used for the synthesis of reduced pyridines and pyridones . The aza-1,3-butadiene 107 was employed in hDA reaction to synthesize the key intermediate 108 in the total synthesis of the pyridine-based natural product piericidin (Scheme 60) <2005JA15704>. The reaction of Danishefskys diene with N-functionalized imine 109 in the presence of (A)-BINOL zinc complex has been utilized to produce 4-piperidones 110 in moderate to high enantioselectivity (Scheme 61) <2004SL711>. 

For complexes of type 10 (with a hydrogen at the carbene carbon) a synthesis was worked out in which a formamide is first reacted with K2[Cr(CO)j] followed by reaction with TMSCI [7]. This way, the non-racemic formamide 12 leads to the chirally modified amino carbene complex 13, which serves as starting material for the diastereoselective synthesis of various optically active yff-lactams [8]. An example is the (formal) total synthesis of 1-carbacephalothin 16, a carbon analog of the cephalosporins (Scheme 5) [8b]. In this case, the complex 13 is irradiated in the presence of in situ prepared imine 14 to afford the /(-lactam with high dia-stereoselectivity but only in modest yield. The product (15) could (in principle) be converted in to the target compound 16. 

A base-induced cyclization involving the substituted nitrobenzene 423 and the imine 424 gave the fused pyrrole 425, a crucial intermediate in a total synthesis of ( )-rhazinilam (Equation 119) <2001T8647>. 

Primary amines have been transformed into imines which when metallated react with carbonyl compounds. Treatment with butyllithium, alkylation with allyl bromide and hydrolysis gives highly substituted aldehydes (Scheme 6). Thus, in this example, the carbon adjacent to the original amino nitrogen atom becomes the carbonyl carbon. The technique has been used several times in the course of total synthesis, as with a recent approach to crinine and as the key step in a recent highly regioselective preparation of a,3 unsaturated aldehydes (equation 33). ... 

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