Urea-like binucleophiles

Another representative of urea-like binucleophiles is guanidine and its derivatives. There are dozens of publications devoted to their reactions with a,(3-unsaturated carbonyls, which are usually very simple processes. For example, routine magnetic stirring of guanidine 61 with chalcone 5 at room temperature in ethanol led to the formation of 1,6-dihydropyrimidin-2-ylamine 76 [78] with good yields (Scheme 3.24). But in several publications it was mentioned that such reactions may be complicated by side reactions and isolation of the target dihydropyrimidines is difficult. Very often among... 

Amidines and thioamidines can also be viewed as urea-like binucleophilic compounds. There are a series of publications about their treatment with unsaturated carbonyls [7, 86, 87, 88, 89, 90, 91, 92]. The most general reaction products in this case are 1,4-dihydropyrimidines or 1,6-dihydropyr-imidines. Interaction of acetamidine 88 with mesityl oxide 52 [86] and benzamidine 90 with arylidenetetraline 91 [91] led to the corresponding 1,6-dihydropyrimidines 89 and 92, while the reaction of bezamidine and benzylideneacetone 24 yielded 1,4-dihydropyrimidine 93 [87] (Scheme 3.27). 

There are publications devoted to a study of the mechanism of the treatment of unsaturated carbonyls with cyanoacetic acid derivatives [134, 153]. According to these communications, the sequences of such reactions are similar to those of urea-like binucleophiles. Using piperidine as the catalyst, Al-Hajjar and Jarrar [134] and Otto [153] isolated a series of known intermediates. 

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