Synthesis of triazolopyridines and triazolopyrimidines using a modified Mitsunobu reaction

Triazolopyridines and triazolopyrimidines were synthesized using a modified Mitsunobu reaction starting from acylated 2-hydrazinopyridines and acylated-hydrazinopyrimidines under mild conditions


Introduction
Triazolopyridines and fused triazoles are useful scaffolds for the generation of biologically interesting molecules. 11a These relatively harsh reaction conditions are often incompatible with many acid or base labile functional and protecting groups such as esters or carbamates [t-butyloxycarbonyl (Boc), 9H-fluoren-9-ylmethyloxycarbonyl (Fmoc)], limiting the types of precursors that can be used in forming fused triazoles via these methods.Mild conditions for the formation of triazoles have been described in the literature, 3 however, significant variations in yields and reaction times limit the choice of substrates that can be utilized.For example, the use of Wamhoff and Zahran's conditions (2 eq. of triphenylphosphine dichloride, 2.8 eq.triethylamine in refluxing acetonitrile) to convert 1a to 2a gave less than 30% yield.Due to an interest in synthesizing fused [1,2,4]triazolo [4,3-a]pyridine heterocyclic systems for structure-activity relationship (SAR) studies, an effort was initiated to develop a general, mild and efficient method for the synthesis of triazolopyridines and other fused triazoles that would be compatible with amino acid substrates and other polar functional groups.As shown conceptually in Scheme 1, it was postulated that a suitably mild dehydrating agent which could activate the carbonyl group of an acylated hydrazinopyridine under conditions that also maintained the nucleophilicity of the pyridine nitrogen could lead to intramolecular cyclization and provide the desired triazolopyridine.

Results and Discussion
The Mitsunobu reaction has been one of the most extensively studied and applied reactions in organic synthesis, particularly in the formation of carbon-heteroatom bonds such as saturated heterocyclic rings, ethers, esters, azides and amines. 4The Mitsunobu reaction has also been widely utilized in the formation of tetrazoles. 5Activation of amides, including various cyclic amides such as pyridones and pyrimidinones, under Mitsunobu conditions has been well documented. 6In addition, the Mitsunobu reaction has been used for the formation of benzoxazoles, 7 but its general utility in forming heteroaromatic rings, such as triazolopyridines, has not been explored. 8Since the formation of triazolopyridines starting from acylated hydrazinopyridines, such as 1a (Scheme 2), requires the loss of one water molecule, any appropriate dehydrating reagent should be able to mediate the cyclization if the reagent is suitably activating and if the pyridine nitrogen has sufficient nucleophilicity.The dehydrating nature of the Mitsunobu reaction suggested it might be effective in mediating the transformation outlined in Scheme 1.As shown in Scheme 2, subjecting acylated hydrazinopyridines 1a and 1b to the standard Mitsunobu reaction using triphenylphosphine (Ph 3 P) and diethyl azodicarboxylate (DEAD) in THF afforded the desired products 2a (5%) and 2b (20%).Although these initial attempts proceeded in low yields, the formation of the desired triazolopyridines under these mild reaction conditions provided sufficient justification for optimizing the reaction.Upon consideration of the proposed mechanism of the Mitsunobu reaction, it seemed reasonable to suggest a reaction mechanism proceeding through intermediate 5 (Scheme 3). 9,4d Addition of transient protecting groups might stabilize the intermediate and allow for the intramolecular attack of the pyridine nitrogen while preventing the decomposition of the starting material.To test this hypothesis, the effect on the product yield of incorporating an in situ transient protecting group like trimethylsilyl (TMS) was assessed.Duncia et al 5b reported the use TMS-N 3 under Mitsunobu conditions to efficiently form tetrazoles. Based on this precedent, TMS-N 3 was the first additive that was tried, as shown in Scheme 4 proceeding to the triazolopyridine 7 via the putative intermediate 8.The addition of TMS-N 3 and triphenylphosphine to the reaction mixture followed by DEAD enhanced the yield of triazolopyridine 2a from ~5% to 90%, and of triazolopyridine 2b from 20% to 95%. 10 In general, the reactions were complete in less than 10 minutes at room temperature.The formation of tetrazoles using Duncia's conditions is reported to require more than 24 h and excess azide to complete while in our case the cyclization is rapid and requires only one equivalent of the azide.This favorable outcome is probably due to the intramolecular nature of our reaction.
To understand the reaction further, other silylating agents and azide sources were tried, and it was found that TMS-N 3 is the optimal reagent (Table 1). 11When no additive was used, less than 21% of the desired product was observed by LCMS (Entry 1).No starting material remained, as determined by TLC and LCMS, but no other major peaks other than triphenylphosphine oxide and the hydrazine dicarboxylate were observed.Multiple small peaks were observed over the length of the HPLC trace.When TMS-Cl was used, no desired product was observed and the starting material was not consumed (Entry 3).Similar results were also obtained with TMS-I (Entry 4).With TMS-OTf, a modest amount of product was observed (Entry 5, 24% yield), with the remainder of the mass balance being starting material.With TMS-CN, N,O-bis trimethylsilylacetimidate or N,O-bis trimethylsilyltrifluoroacetimidate, low levels of product were observed with complete consumption of the starting material (Entry 6-8). 12The sequence of reagent addition is crucial for the success of the cyclization reaction.DEAD has to be added last, otherwise no difference in product yield was observed relative to those in Scheme 2 (37% product observed in the LCMS after 72h compared to 69%). 31P NMR provided evidence that when 1a was mixed with Ph 3 P and TMS-N 3 at room temperature, the Ph 3 P only reacted slowly in THF as evidenced by the disappearance of the 31 P signal of Ph 3 P at -4.3 ppm and the appearance of the 31 P signal of Ph 3 P=NTMS at δ -0.66 ppm over a period of 6 h.Furthermore, use of commercially available Ph 3 P=NTMS (Entry 9) instead of Ph 3 P and TMS-N 3 , failed to effect the cyclization of 1a and 1b, presumably excluding the formation of Ph 3 P=NTMS as a possible intermediate.
Addition of excess Bu 4 NN 3 did not improve the reaction (Entry 10) but when sodium azide or the more THF-soluble cesium azide was added (Entry 11) a significant amount of triazolopyridine was formed.This suggests that the azide plays an important role in this reaction and the impact of the TMS group to the reaction might be only to provide a stable, soluble form of the azide for activation of the active species. 11,13 DEAD (1.5 eq.) was added to a solution of the hydrazide (50 mg), Ph 3 P (1.2 eq.) and the additive (1 eq.) in THF (0.2 M).All reactions were allowed to proceed for 72h.The reactions were monitored after 1, 6, 20, and 72h.No changes were observed between 6 and 72h.b The percentage of 1d and 2d was determined using reverse phase HPLC by integration of the area of the signal by ultraviolet detection at 220 nm.The major peak was always Ph 3 PO and was subtracted from the total integration before calculating the percentage of product.c Isolated yield.When diisopropylazodicarboxylate (DIAD) was used as an alternative to DEAD, it was found that in the absence of the acylated 2-hydrazinopyridines, a white precipitate formed upon addition of 1 eq. of DIAD to an equimolar mixture of Ph 3 P and TMS-N 3 at 0.1 M, or in more concentrated solutions.No precipitate was formed with any of the other pairs of reagents at 0.1 M.This white solid also formed in ether, and was soluble in DCM or chloroform.The solid was found to be moisture sensitive and thermally unstable.NMR studies showed the presence of the phosphine, DIAD but no TMS group.When cesium azide was used instead of TMSN 3 , a white precipitate also formed.The IR of the solid showed the presence of a band at 2100 cm -1 .This solid was able to perform the triazolopyridine cyclization but excess reagent was required to complete the reaction, indicating that this species might be involved in the cyclization reaction but another component is needed to make the conversion efficient, possibly the TMS group.Attempts to obtain crystals for an X-ray structure determination have been unsuccessful to date.14 Other possible mechanisms are shown in Scheme 5. Pathway A shows the formation of a DEAD, triphenylphosphine and azide complex 9 in which the TMS group (or a metal) is associated with the negative charge on the DEAD.Displacement of the azide by nucleophilic attack of the carbonyl of the acyl hydrazine 4 results in a complex 10 that proceeds onto product 7.Alternatively, as shown in Pathway B, attack of the acyl hydrazine 4 can displace the DEAD from the complex 9.By this pathway, the triazolopyridine 7 is then formed through loss of triphenylphosphine oxide and azide.The potential scope and application of the TMS-N 3 assisted Mitsunobu reaction to form triazolopyridines is shown in Table 2. Amino acid protecting groups such as Boc and Fmoc are compatible with the reaction conditions.Similarly, methyl ethers and benzyl, t-butyl, methyl and ethyl esters can also be used.Furthermore, Table 2 includes several examples of triazolopyridines derived from amino acids.These products have potential use as peptide mimetics as the triazolo ring can be envisioned to be an amide isostere.Triazolopyridines formed from amino acid precursors show a high retention of stereochemistry (ee = 99%) (examples 2a, 2k, 2l, Table 2, as determined by using chiral HPLC or SFC, see experimental section for details).Acyl hydrazinopyridines in which R 1 is a secondary carbon were also found to undergo the modified Mitsunobu reaction (2d, 2g, 2h, 2i, 2j, Table 2).Table 3 shows that triazolopyrimidines (2r-2u) can be synthesized using this method, although higher reaction temperatures (50 o C) and longer times are needed.

Conclusions
In summary, a general application of the Mitsunobu reaction for the synthesis of triazolopyridines and triazolopyrimidines (Tables 2 and 3) has been developed using TMS-N 3 as an additive.The described methodology has general application for the formation of derivatized triazolopyridines under mild conditions, and for the generation of peptidomimetics when amino acids are used as precursors.

Experimental Section
General Procedures.Proton and carbon NMR spectra were recorded on one of the following instruments: Bruker DRX-400 MHz , Jeol ECL-400 or 500 MHz instrument.Analytical high pressure liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS) analyses were conducted using Shimadzu LC-10AS pumps and a SPD-10AV UV-vis detector.MS detection was performed with a Micromass Platform LC spectrometer.HPLC and LC/MS methods are detailed below.Preparative Reverse Phase (RP) HPLC was performed using two Shimadzu LC-8A pumps and a SPD-10AV UV-vis detector set at 220 nm on C18 RP columns (YMC Pack ODSA S5 20 × 100 mm or 30 × 250 mm) using methanol/water mixtures buffered with 0.1% trifluoroacetic acid.Enantiomeric excess was determined using supercritical fluid chromatography (SFC, Berger) using a ChiralPak AD column (4.6 x 250 mm, 10 µm particles) at 2 mL/min flow rate of a 17% MeOH in CO with UV detection at 260 nm or by HPLC with a Daicel Chiralcel AD column (4.6 x 250 mm) using an isocratic mixture of 6% isopropanol in hexanes at a flow of 1 mL/min with UV detection at 220 nm.High-resolution mass spectra (HMRS) were recorded on a JEOL SX102 mass spectrometer.Elemental analyses were performed by Robertson Microlit Laboratories and the results obtained were within ±0.4% of the theoretical values, unless otherwise indicated.All non-aqueous reactions were carried out under an atmosphere of nitrogen or argon at ambient temperature unless otherwise noted.Commercial reagents and solvents were used without further purification.2-Hydrazopyridines (5-Br and 5-CF 3 ) were commercially availlable.Acylations of the hydrazopyridines were done using the mixed anhydride carbonate method or by using polymer supported IIDQ 17 .

Table 1 .
Additives for the dehydrative cyclization

of 1u is representative for the preparation of the pyrimidines (a) Benzylamine addition
. A solution of benzylamine (3.02 mL, 27.6 mmol) and triethylamine (4.40 mL, 31.6 mmol) in ethanol (4 mL) was added drop-wise to a solution of 5-bromo-2,4dichloropyrimidine (6.00 g, 26.3 mmol) in ethanol (13 mL) and the mixture was stirred for 1 h.A white precipitate formed.The reaction mixture was poured into water (100 mL) and the white solid was filtered washed with water and dried under reduced pressure to give 7.