Diversity-Oriented Synthesis of a Library of Substituted Tetrahydropyrones Using Oxidative Carbon-Hydrogen Bond Activation and Click Chemistry

Eighteen (2RS,6RS)-2-(4-methoxyphenyl)-6-(substituted ethyl)dihydro-2H-pyran-4(3H)ones were synthesized via a DDQ-mediated oxidative carbon-hydrogen bond activation reaction. Fourteen of these tetrahydropyrans were substituted with triazoles readily assembled via azide-alkyne click-chemistry reactions. Examples of a linked benzotriazole and pyrazole motif were also prepared. To complement the structural diversity, the alcohol substrates were obtained from stereoselective reductions of the tetrahydropyrone. This library provides rapid access to structurally diverse non-natural compounds to be screened against a variety of biological targets.


Introduction
Natural products and their derivatives continue to provide innovative sources for drug discovery [1][2][3][4]. However, many naturally occurring substrates appear to selectively target more highly connected networks associated with essential biological pathways [5,6]. The aims of the NIH supported Centers for Methodology and Library Design (CMLDs) are to produce diverse chemical libraries using novel synthetic methodologies [7]. These non-natural compound collections are OPEN ACCESS delivered to High Throughput Screening (HTS) centers for biological evaluation. Novel chemical scaffolds (i.e., chemotypes) can provide opportunities to investigate biological targets or pathways that may be inaccessible using only the array of currently known natural products, clinically used compounds, or traditional (hetero)aromatic building blocks. In this communication, we describe our diversity-oriented synthesis (DOS) approach to generate a structurally diverse set of triazole-substituted tetrahydropyrans [8,9]. The chemical diversity of these compounds was evaluated against a 5 million "drug-like" compound database (vide infra).

Results and Discussion
The p-methoxybenzyl (PMB) ether 1 was converted into the substituted tetrahydropyrone 2 using our DDQ-mediated oxidative cyclization protocol [21,22]. The corresponding azide intermediate 3 was synthesized using standard conversions. A stepwise copper (I)-catalyzed Huisgen cycloaddition process was used to couple 3 with various terminal acetylenes to afford the 1,4-disubstituted 1,2,3triazoles 4-17 (Scheme 1 and Table 1) [29,30]. The alkynes were selected to produce a series of structurally diverse triazoles, all of which gave acceptable calculated physiochemical properties including clogP values [31,32]. Protodesilylation of analog 6 with TBAF and acetic acid furnished the unsubstituted triazole 6 in 29% yield over two steps.

Scheme 2.
Additional heterocyclic substitutions of the core THP system.
A final stereochemical diversity element was incorporated through the preparation of the syn-and anti-tetrahydropyranols, 20 and 21 (Scheme 3). We have previously accomplished stereoselective ketone reductions of similar 2,6-disubstituted tetrahydropyrones through the use of L-Selectride or NaBH 4 . [27] Thus, the 4-phenyltriazole 4 was further elaborated to selectively afford the alcohols 20 and 21. With the completion of the triazole and the corresponding heterocycles library, a diversity analysis of the 18 THP products was performed against a set of 5 million commercially available, "drug-like" compounds [35] using the cheminformatics package Canvas [36] and MOLPRINT2D [37,38] hashed binary fingerprints (32-bit, no scaling) with the default Mol2 atom types. This combination of fingerprint and atom type was chosen because it provides the best overall results for virtual screening enrichment across a wide range of targets [39,40]. Using the Tanimoto similarity metric, the tetrahydropyrans described herein were found to be highly diverse compaired to the 5 million drug-like compounds ( Figure 2). The average maximum similarity for the 18 products to any member of the 5 million compound database is 0.41, which is remarkably low considering the large number of compounds in the latter repository. The highest similarity is 0.55 (observed for 4, which along with 18 shows the highest similarity to the 5 million compound set). The least similar compound is 14, which has a maximum similarity to any one of the 5 million compounds of only 0.27. Figure 2 shows the distribution of similarities between each of the 5 million compounds and the one compound of the 18 THPs that in each case is most similar. The figure shows that a large majority of the 5 million "drug-like" compounds is highly dissimilar to the 18 THPs.

General
All reactions were performed under an argon atmosphere and all glassware was flame dried prior to use. CH 2 Cl 2 and THF were dried by passing through a column of activated alumina. Reactions carried out at −78 °C employed a CO 2 /acetone bath. Reactions were monitored by TLC analysis (pre-coated silica gel 60 F254 plates, 250 μm layer thickness) and visualization was accomplished with a 254 nm UV light and by staining with a KMnO 4 solution (1.5 g of KMnO 4 and 1.5 g of K 2 CO 3 in 100 mL of a 0.1% NaOH solution), CAM solution (5 g of cerium sulfate, 25 g of ammonium molybdate, 50 mL of conc. H 2 SO 4 and 450 mL of H 2 O, p-anisaldehyde solution (2.5 mL of p-anisaldehyde, 2 mL of AcOH, and 3.5 mL of conc. H 2 SO 4 in 100 mL of 95% EtOH). Purifications by chromatography were performed using SiO 2 (SiliaFlash® F60, Silicycle). 1 H-NMR spectra were recorded on Bruker Avance 300/400/600 MHz instruments in CDCl 3 . Chemical shifts were reported in parts per million with the residual solvent peak used as an internal standard. Chemical shifts are tabulated as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, bs = broad singlet, dd = doublet of doublet, dt = doublet of triplet, dq = doublet of quartet, sp = septet), coupling constant(s), and integration. 13 C NMR spectra were run at 75 or 100 MHz using a proton-decoupled pulse sequence with a d1 of 3 sec, and are tabulated by observed peak. Mass spectra were obtained on a Micromass Autospec double focusing instrument. IR spectra were obtained on a Identify IR-ATR spectrometer.  (4). To a suspension of 3 (0.114 g, 0.414 mmol) and phenylacetylene (0.045 mL, 0.410 mmol) in a mixture of water and tert-butyl alcohol (1:1, 1.7 mL) was added sodium ascorbate (0.032 mL, 1 M solution in water), followed by copper(II) sulfate pentahydrate (0.001 g, 0.004 mmol). The heterogeneous mixture was stirred at r.t. for 33 h, recharged with phenylacetylene (0.090 mL, 0.820 mmol) and stirred at r.t. for 21 h. The reaction mixture was diluted with water (7 mL), cooled in ice bath, and filtered. The precipitate was dissolved in ethyl acetate and chloroform, dried (Na 2 SO 4 ), filtered, and concentrated to afford the corresponding triazole 4 (0.110 g, 71%) as an off-white solid: m.  (5). To a suspension of 3 (0.072 g, 0.262 mmol) and trimethylsilylacetylene (0.075 mL, 0.529 mmol) in a mixture of water and tert-butyl alcohol (1:1, 1.1 mL) was added sodium ascorbate (0.025 mL, 1 M solution in water) followed by copper(II) sulfate pentahydrate (0.001 g, 0.004 mmol). The heterogeneous mixture was stirred at r.t. for 14 h, recharged with trimethylsilylacetylene (0.075 mL, 0.529 mmol) and stirred at r.t. for an additional 6 h. The reaction mixture was diluted with water, extracted with diethyl ether, dried (Na 2 SO 4 ), filtered, concentrated and purified by chromatography (SiO 2 ) eluting with hexane-ethyl acetate (1:1) to obtain the triazole 5 (0.053 g, 54%) as a colorless oil: IR (

Conclusions
In conclusion, a structurally diverse library of 2,6-disubstituted tetrahydropyrans featuring the DDQ-mediated C-H activation reaction has been developed. Most of the tetrahydropyrans were modified at the side chains via a click chemistry reaction to introduce substituted triazoles 4-17. Several additional heterocyclic derivatives were also prepared. Diversity analysis showed that this library occupied unique chemical space compared to a 5 million "drug-like" compound collection as measured by the Tanimoto metric. The biological evaluation of this tetrahydropyran-based screening library is currently in progress.