1.3- thiazoles, microwave-assisted

Li and co-workers introduced a rapid and efficient microwave-assisted method to prepare new disubstituted 1,3,4-thiazoles from 1,4-disubtituted thiosemicarbazides with the objective to obtain biologically active molecules. The intermediate l-aryloxyacetyl-4-(4-methoxybenzoyl)thiosemicarbazide was irradiated in an excess of glacial acetic acid in a domestic microwave oven and led to the formation of 2-(methoxybenzoyl-5-aryloxymethyl)-l,3,4-dithiazoles in good yields [30] (Scheme 20). 

J ,3J ,4J ,5J )-2,5-bis(benzyloxy)-3,4-dihydroxy-Nd -bis (lS)-2-methyl-l-[(methylamino)carbonyl]propyl hexanediamide is a C2-symmetric HIV-1 protease inhibitor [29]. Derivatization in the para positions of the benzyl-oxy groups via microwave-assisted Stille reaction on the corresponding di-brominated inhibitor smoothly yielded the desired heteroarylated derivatives (Scheme 10). Interestingly, the 1,3-thiazole derivative showed a higher antiviral activity on the wild type virus than the lead compound. The activity remained at the same level in the presence of seriun. Unfortimately, a low activity was observed on mutants. 

Alternative methods for the microwave-assisted preparation of thiazole derivatives are summarized in Scheme 6.219. 

Frere, S., Thiery, V. and Besson T., Eco-friendly microwave-assisted scaleable synthesis of 2-cyanobenzo-thiazoles via N arylimino 1,2,3 dithiazoles, Synth. Commun., 2003, 33, 3789-3798. 

The synthesis of oxygen-heterocycles by intermolecular reactions of a-TK with nucleophiles has received only limited attention. However, a-tosyloxy derivatives of several acetophenones have been prepared by brief exposure of neat HTIB/ketone mixtures to microwave (MW) radiation, and employed on mineral oxide surfaces for microwave-assisted syntheses of thiazoles and 2-aroylbenzofurans 40 (Scheme 11) (98JCS(P1)4093, 99JHC1565). 

Khan et al. [9] reported the environment friendly microwave-assisted synthesis of substituted steroidal[6,7-d]thiazoles (iv). The key step involved the reaction of a-haloketones (iii) and thiourea/substituted thiourea via hantzsch protocol. 

The microwave-assisted asymmetric Heck reaction has also been used with success for intermolecular bond formation (Scheme 2.28). Using 2,3-dihydrofuran and phenyl triflate as model substrates, the most active catalyst screened was a combination of Pd2dba3 and a phosphine-thiazole supporting ligand. Heating a THF/DIPEA... 

Hydroxy-3-methoxyphenyl)-3-[4-(coumarin-3-yl)-thiazole-2-yl]-2-isoxa-zoline was synthesized using 2-N-(4-hydroxy-3-methoxyphenylchalconyl)-amino-4-(coumarin-3-yl)-thiazole and hydroxylamine hydrochloride by both the conventional and microwave-assisted method. The conventional method requires about 6-9 h, while microwave irradiation method requires only 2.5-3.5 min and yield is also high (Desai and Desai, 2008). 

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