4-Fluoro-3-nitrobenzoic acid

A variation of this method led to the generation of bis-benzimidazoles [81, 82], The versatile immobilized ortho-phenylenediamine template was prepared as described above in several microwave-mediated steps. Additional N-acylation exclusively at the primary aromatic amine moiety was achieved utilizing the initially used 4-fluoro-3-nitrobenzoic acid at room temperature (Scheme 7.72). Various amines were used to introduce diversity through nucleophilic aromatic substitution. Cyclization to the polymer-bound benzimidazole was achieved by refluxing for several hours in a mixture of trifluoroacetic acid and chloroform. Individual steps at ambient temperature for selective reduction, cyclization with several aldehydes, and final detachment from the polymer support were necessary in order to obtain the desired bis-benzimidazoles. A set of 13 examples was prepared in high yields and good purities [81]. 

A second side reaction suffered by Ar(4)-unfunctionalized quinoxali-nones leading to racemization at the a-carbon was revealed in the closely related studies of Morales and co-workers, who assembled the [6,6] ring system from 4-fluoro-3-nitrobenzoic acid coupled as an ester to Wang resin (lb) (Scheme 12).9 This undesirable side reaction could also be prevented by functionalization of iV(4) prior to cleavage, which in this instance was accomplished by acylation with chloroformates and thiochloroformates. Derivatization at (V(l) was again effected using the Ellman alkylation protocol and provided optically pure samples of the quinoxalinones (72). 

The same authors have also reported on the solid-phase synthesis of benzimidazolones 7 from resin-bound 4-fluoro-3-nitrobenzoic acid lb, amines, disuccinimidocarbonate (DSC), and alkyl halides (Scheme 14).10... 

In a model study on the effect of changing the position of the attachment point, we carried out the benzothiazepinone synthesis starting from resin-bound 3-fluoro-4-nitrobenzoic acid 79 instead of 4-fluoro-3-nitrobenzoic acid la (Scheme 15).5 Gratifyingly, we were able to obtain 8-carbamoyl-... 

Support-bound 4-fluoro-3-nitrobenzoic acid has become a widely used intermediate for the preparation of a variety of heterocycles (see Chapter 15). Aromatic nucleo-... 

Cheng-Ting et al. [13] developed a simple and efficient method for the synthesis of benzimidazole linked quinoxalinones (i) on soluble polymer support using microwave conditions. The key steps involved in the implemented linear synthesis are the acid catalyzed condensation of 4-fluoro-3-nitrobenzoic acid with polymer... 

C7H4Br202 3,5-dibromobenzoic acid 618-58-6 25.00 2.0408 2 9779 C7H4FN04 4-fluoro-3-nitrobenzoic acid 453-71-4 25.00 1.5071 2... 

As shown in Scheme 23, solid-phase-bound Fmoc-protected amino acids 82 are deprotected and coupled with 4-fluoro-3-nitrobenzoic acid. The acid moiety of 83 is converted to the amide 84, and reduction of the nitro group with SnCU 2112O initiates cyclative cleavage to 85 after aqueous work-up. This synthesis gives two points of diversification, which can be increased by an alkylation step in solution to give 86. Additionally, the authors oxidized the heterocychc core 85 with p-chloranil to give the corresponding quinoxalinones (not shovm) [38]. 

While initially the main synthetic interest resided in l,4-benzodiazepin-2-ones [1, 11-13] and l,4-benzodiazepin-2,4-diones [14—18] (and subsequently -2,3-diones ]19] and -2,5-diones ]20]), another l,5-benzodiazepin-2-one system was explored with the strategy outlined in Scheme 3 ]21]. Of particular interest is the role 4-fluoro-3-nitrobenzoic acid, a common fundamental building block central to aU of the the desired structures. Accessing diversity through one or more simple reactants common to aU members of a library has favorable practical consequences on the production phase of a combinatorial Hbrary. 

A 50-mL single-necked, round-bottomed flask was charged with the 4-fluoro-3-nitrobenzoic acid loaded resin 172, the appropriate amino add ester hydrochloride salt (2 equiv.), and 5% DIEA/DMF. The mixture was agitated at rt for 24 h, filtered, and then the resin was washed with DMF (three times), CH2CI2 (three times), iPrOH (three times) and then again in the same order, and dried to afford enantiomerically pure aniline intermediate 173. 

Combinatorial parallel synthesis of head-to-tail bisbenzimidazoles 41 has been performed using polymer-immobilized 1,2-diaminobenzenes (Fig. 2). The PEG-bound diamines were hf-acylated at the primary aromatic amino group with 4-fluoro-3-nitrobenzoic acid. The substituted amides were cy-clized to benzimidazoles under acidic conditions. Successive reduction and cyclization with various aldehydes yielded 5-(benzimidazol-2-yl)benzimid-azoles. Finally, the desired products 41 were released from the polymer support to afford the bisbenzimidazoles in good yields and with high purity [46]. 

Substituted tetrahydroquinoxalin-2-ones have been prepared on the RAM support 7c [303] by first coupling 4-fluoro-3-nitrobenzoic acid to the support in the presence of HATU using a procedure comparable to that described earlier followed by displacement of fluorine by an a-aminoester. Reduction of the nitro group useing tin (II) chloride lead to ring closure (Scheme 13) and further alkylation was followed by TFA-induced cleavage. 

The advantages of MW technology combined with liquid-phase combinatorial chemistry were also successfully applied to a rapid synthesis of quinoxalin-2-ones. PEG bound o-fluoronitrobenzene 716 was synthesized by coupling of 4-fluoro-3-nitrobenzoic acid (715) with PEG 714, in the presence of DCC and a catalytic... 

Another example of using carbonyldiimidazole derivatives for cyclization was reported by Houghten and coworkers (Scheme 9.30)." ° Starting with Boc-amino acid loaded MBHA resin 242 and after deprotection with TEA in DCM and neutralization with DIEA, 4-fluoro-3-nitrobenzoic acid 243 was attached to the resin-bound amino acid, giving resin 244. The fluoro group was substituted by different symmetrical diamines in DMF overnight to afford corresponding resin 245, the free amine of which reacted with Boc-amino acid 246 under... 

MBHA resin 493 was treated with 6 equiv of Boc amino acid 494 and 6 equiv of both DIC and HOBt in DMF at room temperature for 2h, followed by 55% TFA in DCM for 30 min, to give resin 495, which underwent exhaustive reduction with borane-THF complex, affording the diamine 496 (Scheme 9.59). Three equivalents of 4-fluoro-3-nitrobenzoic acid 497 and 3 equiv of HBTU as well as 6 equiv of DIEA in DMF at room... 

We next explored the applications of liquid-phase synthesis of benzofused heterocycles from a common building block PEG-bound nitro-activated aryl fluoride (Fig. 8). In these synthetic strategies, we created multiple accesses to the structurally diverse core molecules based on a fundamental scaffold 10. We focused on the construction of a variety of benzimidazole libraries starting from a versatile synthon 4-fluoro-3-nitrobenzoic acid via liquid-phase nucleo-... 

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