A facile synthesis and fungicidal activities of 2-(alkylamino)-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-ones

The aza-Wittig reactions of iminophosphorane 3 with aromatic isocyanates generated carbodiimides 4, which were reacted with alkylamines under mild conditions to give a series of 2-(alkylamino)-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-ones 6 and 8 in satisfactory yield. Their structures were confirmed by 1H NMR, EI-MS, IR and elementary analysis, and compound 8c was further analyzed by single-crystal X-ray diffraction. The preliminary bioassays indicated that these compounds showed excellent fungicidal activities against six kinds of fungi.


Introduction
Over the past ten years, aza-Wittig reactions of functionalized iminophosphoranes with isocyanates have been applied to produce carbodiimides, functional groups consisting of the formula N=C=N, able to undergo a plethora of heterocyclization reactions [1][2][3][4][5][6]. At the same time, many heterocycles containing thienopyrimidine system are of great importance for use as potential drugs because of their remarkable biological activity. For example, some 2-alkylthio-or 2-alkyl-substituted thienopyrimidinones show significant antifungal and antibacterial activities [7,8], whereas others exhibit good anticon-vulsant and angiotensin or H 1 receptor antagonistic activities [9]. The chemistry of thienopyrimidinones has also received attention because their starting materials, 2-amino-3-carboxythiophenes, can be conveniently synthesized by Gewald reaction [10]. Synthetically useful approaches to thienopyrimidinones starting from easily accessible 2-amino-3carboxythiophenes are therefore of great importance. Recently we have been interested in the synthesis of a series of new heterocyclic compounds via aza-Wittig reaction of α-or β-(ethoxycarbonyl)-substituted iminophosphoranes with Scheme 1: Preparation of iminophosphorane 3.
The reaction of carbodiimides 4 with primary amine RNH 2 in the presence of EtONa provided only 2-(alkylamino)-5,6dimethylthieno[2,3-d]pyrimidin-4(3H)-ones 8 (Scheme 3), one of the possible regioisomers. We obtained only 8 from the reaction mixture after recrystallization; the other isomer 9 was not found by 1 H NMR analysis of the reaction mixture.
The structure of 8 was deduced from its 1 H NMR data. For example, the 1 H NMR spectrum in 8b (R = n-C 4 H 9 ) shows the signals of NH at 4.01 ppm as a broad absorption and NCH 2 at 3.38-3.31 ppm as a multiple absorption, which strongly suggests the existence of a NHCH 2 CH 2 CH 2 CH 3 group in 8b. Furthermore a single crystal of 8c was obtained from a methylene dichloride solution of 8c and X-ray structure analysis verified again the proposed structure [15]. The solitary formation of 8 can be rationalized in terms of a base catalyzed cyclization of the guanidine intermediate 7 to give 8 across the arylamino group rather than the alkylamino one. This may prob- ably be due to the preferential generation of -N − Ar from more acidic -NHAr. The results are also listed in Table 1.

Fungicidal activity
The fungicidal activities of compounds 6 and 8 were screened against six kinds of fungi, Fusarium oxysporum, Rhizoctonia solani, Botrytis cinerea, Gibberella zeae, Dothiorella gregaria, Colletotrichum gossypii at a concentration of 50 mg/L according to the reported method [16]. Bioassays indicated that these compounds showed good to excellent fungicidal activities against six kinds of fungi. For example, 6b, 6d, 8a, 8c, 8d showed 100% inhibition of Botrytis cinerea. See Table 2.
In conclusion, we have developed an efficient synthesis of 2-(alkylamino)-5,6-dimethyl-thieno[2,3-d]pyrimidin-4(3H)ones via base-catalyzed reaction of functionalized carbodiimides with various amines. Due to the mild reaction conditions, good yields, easily accessible starting material and straightforward product isolation, we think that the versatile synthetic approach discussed here in many cases compares favorably with other existing methods. The preliminary bioassay of the compounds indicated that the 2-amino-5,6-dimethylthieno[2,3d]pyrimidin-4(3H)-ones can be used as lead structure for developing novel fungicides. Further bioassay, optimization and structure-activity relationships of the title compounds are underway.

Experimental
Melting points were uncorrected. MS were measured on a Finnigan Trace MS spectrometer. IR were recorded on a  To a solution of iminophosphorane 3 (0.92 g, 2 mmol) in dry DCM (15 mL) was added the aromatic isocyanate (2 mmol) under nitrogen at room temperature. After the reaction mixture was stirred for 6-12 h at 0-5 °C, the solvent was removed under reduced pressure and ether/petroleum ether (1:2, 20 mL) was added to precipitate triphenylphosphine oxide. After filtration, the solvent was removed to give carbodiimide 4, which was used directly without further purification.