A Novel Access to Pyrido [ 4 , 3-d ] pyrimidine Scaffold

A general four-step approach to 1,2,3,7,8,8a-hexahydropyrido[4,3-d]pyrimidin-2-ones via Staudinger/intramolecular aza-Wittig reaction of 5-acyl-4-(β-azidoalkyl)-1,2,3,4-tetrahydropyrimidin2-ones promoted by PPh3 was developed. Synthesis of the starting pyrimidinones included preparation of 3-azidoaldehydes by the addition of hydrazoic acid to α,β-unsaturated aldehydes, transformation of 3-azidoaldehydes into N-[(3-azido-1-tosyl)alkyl]ureas followed by the reaction with 1,3-diketone enolates and dehydration of the resulting products under acidic conditions.


Introduction
Pyridopyrimidines are of current interest due to their multifaceted pharmacological profiles. 1Among them, pyrido [4,3-d]pyrimidines remain relatively less explored in spite of their interesting applications.
For example, they manifest remarkable inhibitory properties against epidermal growth factor receptor tyrosine kinase 2 and dihydrofolate reductase. 3These compounds possess antioxidant, 4 antitumor, 5 antiulcer, 6 antibacterial, 7 and pesticidal activities. 8The described syntheses of pyrido [4,3-d]   pyrimidines mainly start with either pyridine or pyrimidine precursor which is modified to annulate the other ring. 1 However, to the best of our knowledge, Staudinger/intramolecular aza-Wittig reaction, 9 a powerful strategy for nitrogen heterocycles construction has never been applied to pyrido [4,3-d]   pyrimidines synthesis.

Results and Discussion
3-Azidoaldehydes served as starting compounds for the synthesis of pyrido [4,3-d]pyrimidines.The described methods of their preparation include oxidation of 3-azido alkohols, 11 reduction of 3-azido esters, 12 reaction of 3-tosyloxy aldehydes with sodium azide, 13 and addition of hydrazoic acid to α,βunsaturated aldehydes. 14However, 3-azidoaldehydes were prepared only on a small scale (<1 g) and usually used in further reactions without purification.Our initial task focused on preparation of pure 3azidoaldehydes on a multigram scale.We used the method based on the reaction of sodium azide with α,β-unsaturated aldehydes 1a-e in aqueous acetic acid which seems to be the most promising.
3-Azidopropanal (2a) was prepared by the addition of aqueous solution of NaN 3 (1.5 equiv) to a cooled (-12 C) solution of acrolein in acetic acid (Scheme 2).The product was isolated from the reaction mixture as a yellowish oil in 62% yield after extraction with diethyl ether followed by neutralization of the ether extracts with aqueous Na 2 CO 3 , drying, and evaporation of the solvent under reduced pressure.We failed to remove diethyl ether completely and achieve constant mass of the residue due to high volatility of azide 2a.According to 1 H NMR data, the crude 2a always contained a small quantity of diethyl ether.We attempted to purify the crude 2a by fast vacuum distillation at 48-59 °C/15-20 mmHg collecting the main fraction in an ice-cooled flask.As a result, azide 2a was obtained as a colorless transparent liquid (53% yield from acrolein).However, the distilled 2a was unstable and decomposed in the receiving flask already during distillation.After some time, we observed slow gas evolution (probably HN 3 ) from the main fraction and a minor decrease in vacuum.Therefore, 3-azidopropanal (2a) was used immediately after distillation.
We extended this method to the preparation of other 3-azidoaldehydes 2b-e.In contrast to acrolein, crotonaldehyde (1b) reacted with NaN 3 (1.5 equiv) in aqueous acetic acid more slowly affording only 72% conversion after 4.25 h at -12 °C according to 1 H NMR spectroscopic analysis of a sample of the reaction mixture dissolved in D 2 O. Increase in the reaction temperature improved conversion which changed to 83% after additional 55 min at 0 °C, and then to 93% after 1.5 h at 25 °C.The work-up of the reaction mixture as described above for 2a gave practically pure azide 2b containing only 3% of the starting material.The obtained results prompted us to examine the addition of HN 3 to aldehydes 2b-e more thoroughly using 1 H NMR spectroscopy.The selected data are given in Table 1.--------------------------Entry 1 1:NaN Table 1 shows that the addition of HN 3 to crotonaldehyde (1b) proceeded rapidly (<1 h) at room temperature to give a 92:8 equilibrium mixture of 2b and 1b, respectively (entry 1).A greater excess of NaN 3 only slightly shifted this equilibrium to 2b (entry 2).Compared with 1b, the rate of the addition decreased insignificantly when pent-2-enal (1c) was used (entry 1 vs entry 3).In contrast to aldehydes 1a-c, α-alkyl substituted aldehydes 1d,e reacted much more slowly even if 2.5 equivalents of NaN 3 were used (entries 4, 5).
Based on 1 H NMR experiments, we developed a simple medium-scale procedure for preparation of azidoaldehydes 2b-e.According to this procedure, an aqueous solution of NaN 3 (1.5-2.5 equiv) was added to a solution of aldehyde 1b-e in AcOH followed by stirring of the resulting reaction mixture for 3-4 h at room temperature.Azidoaldehydes 2b-e were obtained in 51-71% yields after extractive workup, neutralization, drying, and distillation of crude products.Compared with 2a, compounds 2b-e were stable upon distillation but gradually decomposed during storage at room temperature (slowly in CDCl 3 solutions, faster in liquid phase) (NMR spectroscopy data).Stability of azides 2b-e, especially in

Scheme 3. Synthesis of ureidoalkylation reagents, N-[(3-azido-1-tosyl)alk-1-yl]ureas 5a-f.
Optimized reaction conditions for preparation of ureas 5a-f and their yields are summarized in Table 2.  Three-component condensation of 3-azidopropanal (2a), sulfinic acid 3, and urea (5 equiv) or Nmethylurea (1.5 equiv) smoothly proceeded in water at room temperature for 24 h to give substituted ureas 5a,b as white solids in 84 and 90% yields, respectively (entries 1 and 2).In contrast, the reactions of other azidoaldehydes 2b-e with acid 3 and urea in water at room temperature afforded only gummy materials containing the expected azidoalkyl ureas 5c-f along with considerable amount of various byproducts (NMR spectroscopy data).Compound 5c was successfully prepared in a yield of 92% by sequential addition of acid 3 and a fivefold excess of urea to a solution of 3-azidobutanal (2b) in 25% aqueous HCOOH (entry 3).However, only complex mixtures formed in the reactions of aldehydes 2c-e with 3 and 4a when aqueous HCOOH was used in various concentrations.Condensation of these aldehydes with 3 and 4a cleanly proceeded in 30% aqueous EtOH to give the expected products 5d-f in 79-92% yields (entries 4-6).Under optimal conditions (Table 2), sulfones 5a-f precipitated from the reaction mixtures formed after the addition of all reagents as white solids.They were isolated by filtration with >95% purity according to 1 H NMR spectra of the crude products and used in the ureidoalkylation step without additional purification.Compounds 5c-f were obtained as mixtures of two diastereomers (Table 2).
According to the retrosynthetic plan (Scheme 1), the next step of the pyrido [4,3-d]pyrimidin-2-one scaffold synthesis involved two-step transformation of sulfones 5 into the corresponding 5-acyl-4-(βazidoalkyl)-1,2,3,4-tetrahydropyrimidin-2-ones using our previously developed methodology for pyrimidine ring construction. 10First, we studied ureidoalkylation of sodium enolates of acetylacetone, benzoylacetone, and dibenzoylmethane with sulfones 5a-c in dry MeCN or THF (Scheme 4, Table 3).The reaction of sulfone 5a with the Na-enolate of 6a readily proceeded at room temperature in 7 h 45 min to give a product of nucleophilic substitution of the tosyl group, the corresponding ureido ketone 7a which spontaneously and completely cyclized into hydroxypyrimidinone 8a under the reaction conditions.Pyrimidine 8a was isolated in 75% yield as a single diastereomer (Table 3, entry 1).
The products 8a, 7b-g were readily isolated after removal of solvent followed by aqueous NaHCO 3 work-up and filtration with >95% purity ( 1 H NMR spectroscopy data) and were used in further syntheses without additional purification.Their yields were good, except compound 7d (54%).The moderate yield of 7d can be explained by partial loss of the product during aqueous work-up because of enhanced solubility of 7d in water.Our attempt to improve yield of 7d using extractive work-up of a mother liquor with CHCl 3 failed.Compounds 7b,e,g were obtained as diastereomeric mixtures (Table 3).
We also attempted to react sulfone 5e with the Na-enolate of 6b in MeCN (rt, 8 h) and sulfone 5a with the Na-enolate of 6d (Scheme 4; R 3 = Ph, R 4 = COOEt) in THF (rt, 8 h).However, after removal of solvent and addition of saturated aqueous NaHCO 3 to the resulting residues, only gummy materials were obtained that did not solidify even upon prolonged manipulations.Therefore, it became evident that in these and similar cases the synthesis of 5-acyl-4-(β-azidoalkyl)-1,2,3,4-tetrahydropyrimidin-2ones should be performed using an one-pot procedure directly from sulfones 5 without isolation of the ureidoalkylation products 7, 8 from the reaction mixtures.Previously, we demonstrated that this onepot procedure is often very effective for the pyrimidine synthesis.10f,g,i Thus, we developed two different synthetic methods for preparation of tetrahydropyrimidines 9a-p (Scheme 5).First, we examined the transformation of hydroxypyrimidine 8a and ureido ketones 7b,c,g into the corresponding tetrahydropyrimidines 9a,f,g,k.It was found that dehydration of 8a cleanly proceeded in refluxing EtOH for 1 h in the presence of TsOH (0.19 equiv) to give pyrimidine 9a in 77% yield (Table 4, entry 1).The yield of 9a decreased to 63% when this reaction was carried out under similar conditions but in refluxing MeCN.Ureido ketones 7b and 7g smoothly underwent cyclization-dehydration in refluxing EtOH in the presence of TsOH to give the corresponding pyrimidines 9f and 9g in high yields (entries 7 and 8).In contrast, the reduced electrophilicity of the benzoyl carbonyl groups in dibenzoylmethane derivative 7c extremely hampered the cyclization-dehydration of this compound to afford pyrimidine 9k.In this case greater amounts of TsOH (>0.5 equiv) and longer reaction times were required for completion of conversion of the starting material in refluxing EtOH or MeCN.These conditions led to formation of a significant amount of various byproducts that complicated isolation of 9k and sharply decreased its yield.Compound 9k was obtained in pure form only in 21% yield by refluxing 7c in EtOH in the presense of 1.01 equiv of TsOH for 2 h 15 min followed by isolation of 9k using silica gel column chromatography (entry 12).In this experiment dibenzoylmethane (6c) was isolated in a 30% yield as one of the byproducts.
Next, we developed a convenient one-pot synthesis of tetrahydropyrimidines 9a-e,h-j,l-p based on the reaction of sulfones 5a,c-f with Na-enolates of 6a,b,d in THF (rt, 8-8.17 h) followed by the addition of 1.30-1.43equiv of TsOH and heating at reflux for 1.5-3.17h (Table 4, entries 2-6, 9-11, 13-17).The completion of the second step was monitored by TLC.Tetrahydropyrimidines 9 were isolated from the reaction mixtures after removal of the solvent, aqueous NaHCO 3 work-up of the resulting residues, and filtration of the obtained solids.Generally, the yields of pyrimidines 9 varied from moderate to high (47-82%) with the exception of compounds 9o,p.The latters were isolated by silica gel column chromatography in only 19 and 14% yield, respectively (entries 16, 17).Notably, the yield of pyrimidine 9a obtained from 5a and 6a in the one-pot procedure was slightly higher (61%) than the overall yield in two steps (58%) (entry 2 vs entry 1).
The final step of the synthesis of pyrido [4,3-d] Initially, we studied the reaction of 9a with PPh 3 (1.1 equiv) in various solvents (THF, MeCN, and 1,4-dioxane) at reflux for 1.5 h.The obtained reaction mixtures were evaporated in vacuo to dryness, and the composition of 5-acetyl substituted pyrimidine residues dissolved in DMSO-d 6 was determined using 1 H NMR spectroscopy.The starting material disappeared in all cases, and the expected pyridopyrimidine 10a formed as the main heterocyclic product.However, the reaction in THF, besides 45% of 10a, gave two other compounds in a ratio of 30:25 that seem to be intermediates of incomplete conversion of 9a into 10a.According to 1 H NMR spectrum, one of them (30%) was iminophosphorane 11a.These intermediates were absent in refluxing 1,4-dioxane, but significant amount of side products formed along with 10a.Refluxing MeCN gave the better result furnishing pyridopyrimidine 10a plus the above intermediates in a ratio of 83:13:4, respectively.An increase in the reaction time to 6 h was necessary to achieve complete conversion of 9a into 10a in MeCN at reflux ( 1 H NMR spectroscopy data).
The reaction of 5-benzoyl substituted pyrimidine 9f with PPh 3 (1.1 equiv) in refluxing THF for 6 h was studied.Although the starting material was consumed, no traces of the bicyclic product 10e were detected in the 1 H NMR spectrum of the crude reaction mixture.
Therefore, the results obtained show that the transformation of pyrimidines 9 into bicycles 10 is controlled predominantly by the intramolecular aza-Wittig reaction.Specifically, the rate of this step depends on electrophilicity of carbonyl group and steric factors in iminophosphoranes 11.Based on these data, further we carried out all the pyrido [4,3-d]pyrimidines syntheses in refluxing MeCN for 5.5-8 h (Table 5).Since compounds 10a-c were slightly soluble in MeCN, they precipitated from the reaction mixtures and were isolated in pure form in 84-94% yields by filtration.Compounds 10d-h were isolated in up to 96% yield using silica gel column chromatography of the residues obtained after evaporation of the reaction mixtures.Low yield of ethyl carboxylate 10h (26%) is caused by formation of a huge amount of various byproducts ( 1 H NMR spectroscopy data).
According to NMR data, pyrido [4,3-d]pyrimidines 10b-d,f,g formed as mixtures of two diastereomers in ratios that are close to isomer ratios in the starting pyrimidines 9b,d,e,g,i (Table 5).
Only compound 10h was obtained as a single diastereomer indicating that the second isomer of 10h did not form in the intramolecular aza-Wittig reaction of intermediate 11h.
Medium-scale synthesis of 3-azidoaldehydes based on the reaction of α,β-unsaturated aldehydes with hydrazoic acid generated from sodium azide and aqueous acetic acid was also developed.High

a
Room temperature; 1:1:5 molar ratio of 2:3:4 for the synthesis of 5a,c-f and 1:1:1.5 molar ratio of 2:3:4 for the synthesis of 5b.b  Isolated yields.c According to 1 H NMR spectra of the crude products.

Table 3 .
Reaction of azidoalkyl ureas 5a-c with 1,3-diketones 6a-c in the presense of NaH at room temperature.The amount of the corresponding sulfone 5 is 1.00 equivalent.According to 1 H NMR spectra of the crude products.d A single diastereomer with (4R*,5R*,6R*)-configuration.
a b Isolated yields.c
a Isolated yields.b

Table 5 .
Synthesis Reactions were carried out in refluxing MeCN in the presence of 1.13-1.18equiv of PPh 3 .
a bCrude starting materials were used.c Isolated yields.