1.1. Cyclization with Carbonyl Reactants
The most efficient chemical approach to access 2- and 3-unsubstituted thieno[2,3-
d]pyrimidin-4(3
H)-ones involved a condensation reaction between an aminothiophene substrate and formamide. Thus, compounds
1a–e treated with an excess of formamide at high temperature led to compounds
2a–e with good yields (76 to 97%), except for compound
1e for which the methoxy group in R
3 decreased the reaction yields compared to the ethoxy group (
1a) (
Scheme 2)
[1][2][3][4][5].
Scheme 2. Access to 2- and 3-unsubstituted thieno[2,3-d]pyrimidin-4-one derivatives (Me = methyl, Et = ethyl, and Ph = phenyl).
In contrast, mild conditions were sufficient to perform cyclization reaction with formamide to synthesize the thieno[3,2-
d]pyrimidin-4(3
H)-one isomers
4a–b with good yields (60 to 65%,
Scheme 3)
[6].
Scheme 3. Synthesis of 3-unsubstituted thieno[3,2-d]pyrimidines 4a–b.
Woodring et al. presented a variant of this process that also involved formamide in combination with ammonium formate
[5]. Cyclization of the thiophene intermediate
5 at 150 °C led to the unsubstituted thieno[3,2-
d]pyrimidin-4-one
6 with a 56% yield (
Scheme 4).
Scheme 4. Synthetic route to unsubstituted thieno[3,2-d]pyrimidin-4-one 6.
In addition, reaction of 2-amino-3-cyanothiophene derivatives with formic acid could also be considered to access 2- and 3-unsubstituted thieno[2,3-
d]pyrimidin-4-ones
[4]. In such approach, the cyano group is firstly converted into its corresponding primary amide, which could then be cyclized in the presence of formic acid. Kanawade et al. used such an approach to prepare thienopyrimidinone
8a from 2-amino-3,5-dicyanothiophene
7a (
Scheme 5). Replacing formic acid by formamide led to the formation of the 4-amino analogue, as reported by Aly et al.
[7]. Thus, cyclocondensation involving
7b and formamide occurred under reflux to afford the expected
8b with a 83% yield (
Scheme 5).
Scheme 5. Access to thieno[2,3-d]pyrimidine derivatives from 2-amino-3-cyanothiophene derivatives.
Cyclocondensation of thiophene carboxamide
9 in the presence of sodium hydroxide was used to synthesize thieno[3,4-
d]pyrimidin-4(3
H)-one
10 (
Scheme 6). The expected molecule was isolated with a moderate yield (40%) after a 1 h reaction in refluxing methanol.
Scheme 6. Synthesis of 2-methyl-thieno[3,4-d]pyrimidin-4(3H)-one 10.
Using a similar approach, but with a nitrile group as the precursor of the primary amide, Desroches et al. synthesized 2-methyl- and 2-trichloromethyl-thieno[2,3-
d]pyrimidin-4(3
H)-ones
12 and
13, respectively (
Scheme 7)
[8]. Thus, treatment of 3-cyanothiophene acetamide
11a with hydrogen peroxide in alkaline medium (NaOH) afforded 2-methyl-thieno[2,3-
d]pyrimidin-4(3
H)-one
12 with a 72% yield. Using 3-cyanothiophene trichloroacetamide as a substrate and phosphoric acid in polyphosphoric acid triggered the cyclocondensation reaction and the formation of the 2-trichloromethyl-thieno[2,3-
d]pyrimidin-4(3
H)-one
13 with good yields (90%).
Scheme 7. Synthesis of thieno[2,3-d]pyrimidin-4(3H)-ones substituted in position 2.
1.2. Cyclization with Nitrile Reactants
Various pathways exploiting nitrile condensation were reported in the literature to produce thieno-fused analogues. De Schutter et al. used a synthetic route involving a thiophene amino ester treated in strongly acidic conditions by a cyanoalkyl derivative at 90 °C (
Scheme 8)
[9]. Thieno[2,3-
d]pyrimidin-4(3
H)-ones
16c, substituted in positions 2, 5, and 6 were then obtained in 1,4-dioxane in moderate to good yields (50 to 90%). In addition, Mavrora et al. used the same synthetic pathway and obtained chloroethyl derivatives
16a–b with good yields (
Scheme 8) after nitrile cyclocondensation at room temperature
[10]. Likewise, thieno[3,2-
d]pyrimidinones
15 substituted at position 2 were prepared from cyclization of the starting thiophene with the appropriate cyanoalkyl in acidic conditions at 90 °C in 1,4-dioxane (
Scheme 8)
[9]. To introduce a trichloromethyl group at position 2 of the thieno[3,2-
d]pyrimidine core, Desroches et al. used trichloroacetonitrile in acetic acid, saturated with HCl gas, to afford 2-trichloromethyl-thieno[3,2-
d]pyrimidine
17 with a 63% yield (
Scheme 8)
[8].
Scheme 8. Synthesis of 2-substituted thienopyrimidin-4-ones using nitrile reactants.
Using the same strategy, Kim et al. introduced a chloromethyl group at position 2 of thieno[3,2-
d]pyrimidinones after slight modifications of the reaction conditions
[11]. Formation of the thieno-fused core occurred with the cyclocondensation of malononitrile with 2-methyl-3-aminothiophene carboxylate under acidic conditions and mild heating to offer
18 with high yields (
Scheme 9).
Scheme 9. Synthesis of 2-chloromethyl-thieno[3,2-d]pyrimidinone 18.
Slavinski et al. presented another synthetic pathway to introduce a sulfonamide group at position 2, using sulfonyl cyanamide potassium salts
19 [12]. Acidification of the reaction with boiling glacial acetic acid led to cyclization and afforded 2-sulfonamide-thieno[3,2-
d]pyrimidinone derivatives
20 with low yields (20–34%,
Scheme 10).
Scheme 10. Formation of 2-sulfonamide-thieno[3,2-d]pyrimidinones 20.
1.3. Synthesis from (Thio)urea Reagents, Iso(Thio)cyanate or (Thio)cyanate Derivatives
An easy way to access thienopyrimidin-2,4-dione or 2-thioxo-thienopyrimidin-4-one derivatives consisted of cyclocondensation of the appropriate ethyl aminothiophene-carboxylate with potassium (thio)cyanate in an acidic medium. Patel et al. obtained 2-thioxo-thieno[2,3-
d]pyrimidin-4-one
22a with a 58% yield, using hydrochloric acid in refluxing 1,4-dioxane (
Scheme 11)
[13], whereas Temburkinar et al. and other groups
[14][15][16] used potassium cyanate in acetic acid to obtain thieno[3,2-
d]pyrimidin-2,4-dione
21a with 71 to a 88% yield.
Scheme 11. Synthesis of 2-thioxo-thieno[2,3-d]pyrimidin-4-one 22a and thieno[3,2-d]pyrimidin-2,4-dione 21a.
Another way to access such compounds was to condensate the starting aminothiophene with urea or thiourea, followed by cyclization to afford thienopyrimidinone compounds
21 or
22. Ortikov and Prabhakar teams used such conditions to synthesize 2-thioxo-thieno[2,3-
d]pyrimidin-4-one
22b and thieno[2,3-
d]pyrimidine-2,4-diones
22c (
Scheme 12) with good yields (72–91%)
[2][17][18]. Condensation and cyclization only occurred at very high temperatures after 2 or 3 h of heating without solvent. Thieno[3,2-
d]pyrimidin-2,4-one
21b could be synthesized under these conditions, whereas the synthesis of 2-thioxo-thieno[3,2-
d]pyrimidin-4-ones
21c required the use of
N,
N-dimethylformamide (DMF) as a solvent (
Scheme 12)
[19][20].
Scheme 12. Formation of 2-thioxo-thienopyrimidin-4-ones and thienopyrimidine-2,4-diones using (thio)urea.
Kankanala et al. used a common synthetic pathway to access 3-hydroxythieno[2,3-
d]pyrimidin-2,4-diones and thieno[3,2-
d]pyrimidin-2,4-diones
[21] bearing various groups in α and β positions of the sulfur atom. Firstly, the aminothiophene reacted with 1,1′-carbonyldiimidazole (CDI) to afford the imidazole-carboxamide intermediate after 2 h in refluxing toluene (
Scheme 13). Secondly, the substitution of the imidazole group by protected hydroxylamine generated the hydroxyurea intermediate. Then, a basic treatment deprotonated hydroxyurea to allow cyclization. Afterward, deprotection of the hydroxyurea led to the final compounds
23 with correct to good yields (40–85%).
Scheme 13. Synthetic pathway to afford 3-hydroxythienopyrimidin-2,4-diones 23.
To introduce more chemical diversity at position 3, a convenient synthetic route described by Abu-Hashem et al. involved nucleophilic attack of an aminothiophene derivative on an isocyanate or thioisocyanate in the presence of a catalytic amount of triethylamine in refluxing 1,4-dioxane (
Scheme 14A)
[22]. The (thio)ureidothiophene intermediate
24 or
25 was then isolated on average with good yields (60 and 70%). Thereafter, basic treatment of
24 or
25 with sodium ethoxide in refluxing ethanol led to thieno-fused derivatives
26 and
27 with good yields (70% and 75%) after 8 h. Dewal et al. obtained similar results using sodium methoxide under refluxing methanol to prepare trisubstituted thieno[2,3-
d]pyrimidin-2,4-dione derivatives
26 with 88–90% yields
[23]. In addition, Abu-Hashem et al. reported a one-pot reaction with phenylisothiocyanate and sodium hydroxide as a base, in refluxing ethanol for 6 h
[22]. Both the two-step procedure and the one-pot reaction offered
27a with a 70% yield (
Scheme 14A). Furthermore, the use of potassium carbonate in refluxing acetonitrile led to the 2-mercapto-thieno[2,3-
d]pyrimidin-4-one analogues
28b–c in even higher yields (78%)
[3][24]. In a similar way, 3-ethyl-2-thioxo-thieno[3,2-
d]pyrimidin-4-one
27b was also accessible via the cyclization of 2-methyl-3-aminothiophene carboxylate with ethylisothiocyanate in refluxing pyridine
[25]. In addition, 6-bromothieno[3,2-
d]pyrimidin-2,4-diol
30 was synthesized in milder conditions with potassium
tert-butoxide in DMF at room temperature and obtained it with a quantitative yield (
Scheme 14B)
[26]. It was then possible to introduce further chemical diversity in positions 2, 4, and 6, starting from this bicyclic product.
Scheme 14. (A). Synthesis of 3-substituted 2-thioxo-thienopyrimidin-4-ones or thienopyrimidine-2,4-diones 26–28. (B). Synthesis of 6-bromothieno[3,2-d]pyrimidine-2,4-diol 30.
Alternately, Cohen et al. suggested an original synthetic pathway to obtain thieno[3,2-
d]pyrimidin-4(3
H)-one derivatives
34, substituted in position 2 by an amino group
[27]. This one-pot procedure involved first the condensation of the starting material with ethoxycarbonyl isothiocyanate in DMF to generate the thiourea carbamate intermediate
32, that was not isolated (
Scheme 15). Afterward, a primary alkylamine reacted with this species, previously mixed with 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide (EDCI.HCl) and triethylamine. Guanidine intermediate
33 was observed but was not isolated. Then, this intermediate cyclized at 170 °C to afford thieno-fused derivatives
34 with 42 to 70% yields depending on the substituents.
Scheme 15. Synthetic pathway purposed by Cohen et al.
[27].
1.5. Cyclization with Amine/Hydrazine Derivatives
A more common way to access 3-amino-thieno[2,3-
d]pyrimidin-4-ones consisted of the condensation and cyclization between a thiophene derivative and hydrazine monohydrate in refluxing ethanol. Using this strategy, several groups reported the synthesis of compounds
42a–b with moderate to good yields (
Scheme 17)
[3][28]. Aly et al. employed the same reaction conditions to generate 3-amino-thieno[2,3-
d]pyrimidin-4-one
42c. Only the starting thiophene was different and achieved cyclocondensation with good yields (80%).
Scheme 17. Synthesis of 3-amino-thienopyrimidin-4-ones 42.
To introduce chemical diversity at position 3, a similar route was followed by Habib et al. using various primary amines to synthesize a set of 3-substituted thieno[2,3-
d]pyrimidinone derivatives
43 [3]. Firstly, the 2-aminothiophene
1c reacted with triethyl orthoformate under reflux to prepare the imino intermediate, which was not isolated (
Scheme 18). Then, the appropriate amine was added to allow cyclization and obtain 3-substituted thienopyrimidinone derivatives
43 with good yields (79–85%).
Scheme 18. Access route to synthesize 3-substituted thieno[2,3-d]pyrimidin-4-ones 43.
Finally, condensation of ammonia with
N-acylaminothiophenes
44 allowed access to 3-unsubstituted thieno[2,3-
d]pyrimidin-4-ones
45 [6][8]. The first synthetic route involved 25% ammonia heated at 105 °C in a sealed vial to obtain thieno[3,2-
d]pyrimidin-4-one
46a after 3 h, with a 63% yield (
Scheme 19). In contrast, using milder conditions with 30% ammonia at room temperature for 6 to 8 h led generally to lower yields (28–60%). Moreover, it has been observed by Desroches et al. that this method was not efficient when R = CCl
3 (compound
45e)
[8]. Indeed, with this substrate, cyclization in the presence of 25% ammonium hydroxide in a sealed vial failed.
Scheme 19. Synthesis of 3-unsubstituted-thienopyrimidin-4-ones 45 (Pr = propyl).