Solid-Phase Parallel Synthesis of Drug-Like Artificial 2H-Benzopyran Libraries

This review covers the construction of drug-like 2H-benzopyrans and related libraries using solid-phase parallel synthesis. In this context, the preparation of substituted benzopyrans such as mono-, di- and trisubstituted benzopyran derivatives and additional ring-fused benzopyrans such as benzopyranoisoxazoles, benzopyranopyrazoles, six-membered ring-fused benzopyrans, and polycyclic benzopyrans are highlighted.

This paper reviews the use of solid-phase parallel synthesis in the construction of 2H-benzopyran libraries that contain substituted 2H-benzopyrans (A, B, and C) and additional cycle-fused benzopyrans (D and E), except the derivatives of 1H-benzopyran, coumarin, and chromone moieties ( Figure 2). In addition, the solid-phase synthesis of modified benzopyran derivatives using the 2H-benzopyran moiety as an intermediate is also discussed. The sections have been divided according to the number of substituents in the benzopyran core and the kinds of fused-benzopyran cores. Publications cited herein are mostly refereed journals and not patents.

Solid-Phase Synthesis of 3-Substituted Benzopyran Compounds
Park and co-workers reported the solid-phase synthesis of 3-substituted 2H-benzopyran 1 (Scheme 1) and the fluorous-tag-based solution-phase synthesis of benzopyran derivatives with discrete core scaffolds to construct a 284-member library [50]. Scheme 1. Solid-phase synthesis of 3-substituted 2H-benzopyrans 1 by Park et al. [50].  The library construction was started from four different chromanones 2 (2a-d, see Figure 3), which were subjected to α-bromination and subsequent silyl protection at the phenolic hydroxyl group. α-Bromoketones 3 were reduced to α-bromoalcohols 4 by NaBH 4 , followed by acid-catalyzed dehydration and subsequent silyl deprotection, to yield four vinyl bromides containing 2H-benzopyran moiety 5. After the activation of (4-methoxyphenyl)-diisopropylsilylpropyl polystyrene resin 6 with TfOH, vinyl bromide intermediates 5 were immobilized on the activated resin 7 in the presence of 2,6-lutidine to afford polymer-bound intermediates 8.  The 3-substituted 2H-benzopyran resins 9 were introduced to various aryl and heteroaryl moieties via Suzuki coupling of boronic acids (24 commercially available aryl-and heteroaryl-boronic acids) with Pd(PPh 3 ) 4 and Na 2 CO 3 in aqueous 1,4-dioxane with high yields and purity. After the standard cleavage protocol of silyl linkers using HF/pyridine and subsequent quenching with TMSOEt, the desired 3-substituted 2H-benzopyrans 1 (96 examples) were successfully prepared on a scale of 5-10 mg with an average purity of 86%.
The various substituted aryl rings (R 3 ) were introduced via palladium-mediated Suzuki coupling of aryl boronic acids. Among the many conditions tested in the solid-phase, the reaction condition with Pd(PPh 3 ) 4 and Na 2 CO 3 in aqueous 1,4-dioxane displayed a robust chemical transformation of 3 with various substituted aryl boronic acids (18 different aryl boronic acids), resulting in high yields of the desired 3-substituted 2H-benzopyran resins 13. Finally, the privileged benzopyrans 10 were produced by cleavage of resins 13 by using HF/pyridine in tetrahydrofuran (THF) and subsequent quenching with TMSOEt, and the resulting 144-member small-molecule collection was synthesized on a scale of 10-20 mg and their average purity was 87% without any purification steps.

Scheme 4.
Solid-phase synthesis of 3-hydroxy-4-amino-substituted benzopyrans 18 by Gong and Yoo [55]. After various solvent systems and oxidizing agents were examined to avoid the formation of m-chlorobenzoic acid-added adduct resin in the case of oxidation of resin 22 with m-chloroperbenzoic acid (mCPBA), the two-phase solvent system comprised of chloroform and saturated aqueous NaHCO 3 with mCPBA afforded the epoxide resin 23. The regioselective ring opening of the polymer-bounded epoxide 23 with nine amines produced the 3-hydroxy-4-amino-substituted benzopyran resins 24 in good overall yields without significant contamination of the by-products. The desired benzopyran derivatives 18 (9 examples) were finally liberated from the resin 24 using trifluoroacetic acid (TFA).

Solid-phase Synthesis of 2,3-Disubstituted Benzopyran Compounds
Takahashi and co-workers described the efficient solid-phase synthesis of EGCG (see Figure 1) and the combinatorial synthesis of protected methylated epicatechin derivatives 35 ( The solid-phase synthetic strategy of 2,3-disubstituted benzopyrans 35 began with the treatment of the aldehyde 36 with bromo Wang resin 32 (1.6 mmol/g) to provide the aldehyde resins 37 [63]. The treatment of the aldehyde resins 37 with the methyl ketones 38 under NaOMe basic conditions provided the solid-supported enone 39, which underwent epoxidation with tBuOOH to give the solidsupported epoxide 40. The regioselective epoxide-ring opening [64] of 40 with 1-dodecanethiol in the presence of Zn(OTf) 2 proceeded without cleavage of the Wang linker to afford the solid-supported -hydroxyketone 41, the acylation of which with benzoic acids (42) then gave the precursor 43 for reductive cyclization. The exposure of 15%TFA in CH 2 Cl 2 in the presence of a PS-benzaldehyde resin followed by the addition of triethylsilane promoted the cleavage of the Wang linker, reduction of the sulfide and the bromide, and reductive etherification to provide the protected 2,3-disubstituted benzopyrans 44. Finally, the 2,3-disubstituted benzopyran derivatives 44 were deprotected by conventional hydrogenolysis in the solution-phase by using a palladium catalyst to provide EGCG derivatives 35 (64 examples).
The two aldehydes 36, six ketones 38, and five carboxylic acids 42 were used as building blocks for the synthesis of 60 members of 2,3-disubstituted benzopyran library. The purity of the library was estimated by LC-MS analysis (58-15% purities). The growth-inhibitory effects of the resulting library compounds were examined [62]. Most of the 7-OMe derivatives exhibited biological activity comparable to that of the naturally occurring EGCG.

Solid-Phase Synthesis of 2,6-Disubstituted Benzopyran Compounds
Gong and co-workers reported the construction of a 2,6-difunctionalized 2H-benzopyran library of 1,200 analogues by using the solid-phase protocols [65]. An alternative linker-based synthetic strategy was developed because of a restriction that a carbamate linker based solid-phase synthetic pathway to generate a substituted benzopyran library (18 and 25) could not be introduced at the 2-position of the benzopyran system by using strong bases. In the strategy, acid sensitive methoxy benzaldehyde (AMEBA) resin 45 [2-(4-formyl-3-methoxyphenoxy)ethyl polystyrene from Merrifield resin] [66] was selected as the polymer support since the secondary amino group, resulting from reductive amination, should be highly reactive towards various alkyl halides, acid halides, isocyanates, and sulfonyl chlorides (Scheme 8). Moreover, the final products should be readily cleaved from the support by using dilute TFA solutions [67,68]. In the first step of the sequence, 6-aminobenzopyran resin 46 was prepared by reaction of AMEBA resin 45 with 6-aminobenzopyran 47 [69] under reductive amination conditions [70] [NaBH(OAc) 3 in DMF containing 1% acetic acid]. Scheme 8. Solid-phase synthesis of 2,6-difunctionalized benzopyrans 49, 53, and 547 by Gong et al. [65].
In the first-generation diversification step, the secondary amine group in 46 was transformed into the amide, sulfonamide, or urea groups in resin 48 by respective reactions with acid chlorides, sulfonyl chlorides, and isocyanates in the presence of triethylamine in DMF. To confirm the product formation, resin 48 was treated with 20% TFA in CH 2 Cl 2 to give 6-amino-substuted 2H-benzopyran 49 (25 examples, 85-65% yields, and 99-72% purities).
For the purpose of second-generation diversity, resins 50 containing a free primary hydroxyl group were prepared by reaction of resins 48 with NaOMe in MeOH/THF at room temperature [71]. Functionalization of the hydroxyl groups in resins 50 was promoted by reactions with alkyl halides and acid chlorides to generate respective 2,6-difuctionalized 2H-benzopyran resins 51 with an ether-substituent and 52 with an ester-substituent at position 2 in the 2H-benzopyran moiety. Alkylation reactions of 50 were carried out in the presence of lithium tert-butoxide in DMF and took place smoothly to yield the corresponding ethers. Subsequent treatment of the resins 51 with 20% TFA in CH 2 Cl 2 produced the desired 2,6-difuctionalized 2H-benzopyran derivatives 53 with an ether-substituent (the representative 22 examples, 73-26% yields, and 99-83% purities) in high four-step overall yields from resin 46. The ester-containing resins 52 were prepared by treatment of resins 50 with various acid chlorides in the presence of DBU and 4-dimethylaminopyridine (DMAP) in DMF. To confirm product formation, the resins 52 were treated with 20% TFA in CH 2 Cl 2 to yield the desired 2,6-difuctionalized 2H-benzopyrans 54 with an ester-substituent (the representative 21 examples, 73-29% yields, and 99-81% purities).
In the first-generation diversification step, resin 61, containing a secondary amine group, was reacted with alkyl halides in the presence of diisopropylethylamine (DIEA) in CH 2 Cl 2 . For second-generation diversification, resins 61 containing a secondary amino group were prepared by removal of Fmoc on resins 61 with 20% piperidine in DMF. Functionalization of the secondary amine groups on resins 62 is promoted by reaction with various electrophiles, including acid chlorides, sulfonyl chlorides, isocyanates, and isothiocyanates. This leads to the generation of the respective amide, sulfonamide, urea and thiourea derivatives. Further confirmation of product formation was accomplished by treatment of resins 63 and 64 with 20% TFA in CH 2 Cl 2 and by characterization of the liberated 2,6-difunctionalized 2H-benzopyrans 11 and 12 with 2-functionalized-aminomethyl group.
Lipinski's rule [76] and similar formulations [77,78] serve as guidelines to estimate the physicochemical properties of the 2,000-member library of 6-alkylamino-2-(functionalizedaminomethyl)-2H-1-benzopyran derivatives 55 and 56. Most of the key parameters for members of the library fall within the range of those predicted for reasonable oral bioavailable drugs by using the commonly known guidelines.
As shown in Scheme 10, resin-bounded spirobenzopyran 67 was prepared by reaction of BAL resin 60 with N-[ethylcarbamate-spiro(2H-1-benzopyran-2,4-piperidine)-6-yl]amine 68, which was synthesized by general manipulations, under reductive amination conditions. In the first generation diversification step, the secondary amine group on resin 67 was transformed into the tertiary amine and amide on benzopyran resins 68 and 69 at position 6 by the reactions with various acid chlorides and alkyl halides in the presence of bases, respectively.
After the carbamate deprotection in a spiro-ring of benzopyran resins 68 and 69 by hydrolysis reactions, the secondary piperidine amines on the 6-amino-or 6-amido-substituted 2H-benzopyran resins 70 and 71 were converted by reactions with various sulfonyl chlorides to generate respective 6-amino-substituted amide resins 72 and 6-amino-substituted resins 73 for the introduction of second generation diversity.

Scheme 10.
Solid-phase synthesis of 6-amido-and 6-amino-substituted-2-functionalized benzopyrans 65 and 66 by Gong et al. [80].   In general, the goal of a drug discovery process is to synthesize chemical entities which are orally bioavailable; i.e. they possess physiological properties that allow them to be absorbed into the gastrointestinal system. Lipinski's Rule [76] and similar formulations [77,78] served as guidelines to estimate the physicochemical properties of the synthesized 222-member library of 6-amido-and 6-amino-substituted-2-functionalized benzopyrans 65 and 66, respectively [80].

Solid-Phase Synthesis of 3,4,6-Trisubstituted Benzopyran Compounds
Gong and co-workers described the construction of a 3,4,6-trisubstituted benzopyran library of 2,000 analogues using consecutive nucleophilic addition via m-CPBA epoxidation on solid support [99,100]. Various reaction conditions were examined to find a condition of the nucleophilic alcohol addition at an epoxide on resin and completion of 3-hydroxy-4-alkoxy-6-amino-substituted benzopyrans 81 on solid support. The desired 3-hydroxy-4-alkoxy-benzopyran resins 82 were obtained by the consecutive nucleophilic alcohol addition reactions of resins 26 with nucleophiles, immediately followed by m-CPBA epoxidation. Finally, the cleavage of resins 82 with 25% TFA in CH 2 Cl 2 produced the target 3-hydroxy-4-alkoxy-6-amino-substituted benzopyrans 81 (26 examples) without significant contamination of by-products (Scheme 12).

Scheme 12.
Solid-phase synthesis of 3,4,6-trisubstituted benzopyrans 84 and 86 by Gong et al. [99].   For the preparation of esters at position 4 in the benzopyran moiety, the resins 82 were treated with various acid chlorides with pyridine and DMAP as bases in CH 2 Cl 2 , to produce the benzopyran resins 85 with an ester-substituent, which were again treated with 25% TFA in CH 2 Cl 2 h to give the desired 3,4,6-trisubstituted benzopyrans 86 with ester-substituent (26 representative examples) in good four-step overall yields. The obtained 3,4,6-trisubstituted benzopyrans 84 with an ether-substituent were identified as prolyl 4-hydroxylase inhibitors via a screening process using HSC-T6 and LI 90 cells that express an immortalized rat hepatic stellate cell line and as part of a test of the type I collagen contents employing the ELISA method [100].

Scheme 14.
Solid-phase synthesis of pyrazole-fused benzopyrans 104 by Park et al. [107]. The regioselective synthesis of benzopyranopyrazole derivatives 100 was achieved by the condensation of a -keto aldehyde with mono-substituted hydrazine (R 1 BocNNH 2 ) in AcOH [108,109]. The Cbz protection group on the piperazine moiety was removed from benzopyranopyrazole 96 by 40% KOH or dimethyl sulfate and BF 3 ·OEt 2 [118] for the immobilization of the piperazinyl secondary amine 96 on a solid support.
As shown in Scheme 14, Wang resin was activated with p-nitrophenylchloroformate in the presence of DIPEA, followed by the loading of the piperazinyl secondary amine 96 on the solid support. The nitro group on polymer-bounded intermediates 101 was reduced with tin chloride dihydrate in DMF. The resulting aniline resins 102 were subsequently diversified with a set of 12 building blocks (six carboxylic acids, one isothiocyanate, two isocyanates, and three sulfonyl chlorides) identical to that used for the modification at the R 2 position. The final cleavage step with resins 103 was performed under 50% TFA in dichloromethane to liberate various benzopyranopyrazoles 104 (96 examples). Overall, the average purity of the final 96 benzopyranopyrazoles 104 with R 1 and R 2 diversification was 84%.

Solid-Phase Synthesis of Polycyclic Benzopyran Compounds
Novel polycyclic scaffolds 128-130 containing the benzopyran moiety with variable substituents were described by Park et al. [51]. The excellent endo-selective Diels-Alder reaction with dienophiles 131 (17 substituted maleimides) and solid-supported dienes 132 (8 resins from 2, see Scheme 2 and Figure 3), which were derived from palladium-mediated Stille-type vinylation on vinyl triflate intermediate 12, gave benzopyran-containing polycycloheterocyclic resins 133 (Scheme 17). After HF/pyridine cleavage of resins 133 and subsequent quenching with TMSOEt, diastereomerically enriched novel tricyclic benzopyran derivatives 128 were obtained on a scale of 10 mg each (136 examples). Their average purity measured by LC-MS analysis of the crude products, was around 85%.  To expand the molecular diversity of the small-molecule library, novel polycyclic benzopyran derivatives 128 were transformed to discrete core skeletons 129 and 130, using chemical transformations such as Pd/C-based diastereoselective hydrogenation of 128 by the library-to-library approach and the sequence reaction of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)-mediated aromatization of resins 133 and liberation of resins 134, respectively. The average purity of 129 (54 examples), which obtained from 12 without further purification, was about 81%, and that of 130 (36 examples) measured by LC-MS analysis of the crude products, was 84%.
The shape of the resulting core skeleton 129 is structurally discrete and more concave than that of its precursor 128 because of the conversion at the monoene site of sp 2 carbon to sp 3 carbon. Compared to heterogeneous hydrogenation, which introduces the sp 3 carbon center in an asymmetric fashion, the aromatization using DDQ can remove the existing stereogenic carbon centers of the monoene precursor 128 and provide a new flatter core skeleton 130.

Summary
The combinatorial synthesis of drug-like small organic molecules plays a significant role in the area of drug discovery. Especially, the various natural and artificial benzopyran compounds as bioactive molecules have proven to be broadly useful as therapeutic agents because of their high degree of structural diversity. In this respect, many synthetic methods have been developed for fabricating the privileged benzopyran structures with drug-like properties by using solid-phase synthetic strategies. In this article, we have introduced the preparation of diverse and drug-like benzopyrans as substituted benzopyrans, additional cycle-fused benzopyrans, and their related compounds. Further studies in this area are underway and the various strategies for syntheses of benzopyran derivatives on solid support will be reported for medicinal chemistry and drug discovery.