Die Reaktion von Benzylidenmalononitrilen mit Cyanthioacetamid (1) ergibt je nach den Reaktionsbedingungen entweder Thiopyrane oder Pyridine. Die Strukturen der Reaktionsprodukte wurden mittels l3C-NMR aufgeklart.
In the last decade we have been involved in a program aiming at efficient syntheses of substituted azoles, azines, and their condensed derivatives utilising functionally substituted nitriles as starting materials'-'). As a part of this program the utility of cyanothioacetamide (1) as starting material for the synthcsis of otherwisc difficultly accessible heterocycles was investigated. In conjunction to this work we investigated the reaction of 1 with the cinnamonitrile derivatives 2a -f. The literature contains plenty of reactions of active methylene reagents with 2, reactions which lead to 1,2-dihydropyridines or p y r a n ~~-~) .
We treated 1 with 2a-c in cold ethanolic triethylamine to yield 1 : 1 adducts. The same products were obtained from the reaction of 2g -i with malononitrile. Several possible structures can be excluded since the reaction occurs through addition to the activated double bond to yield either acyclic derivatives (resulting from the addition of the active methylene, amide, or thioimidic form of 1) or their cyclisation products. The formation of dihydropyridines seems to be unlikcly to us as one would expect that the products readily oxidise under the reaction conditions in a way similar to that reported for their 0x0 analogues"). I3C NMR of the reaction products reveals in addition to the aromatic multiplet at 6 = 126.6-128.6 and one CN signal at 6 = 118.6 signals at 6 = 72.4 (C-3 and C-5). 43.5 (C-4), and 151.1 (C-2 and C-6). These data can only be rationalised in terms of structure 3 in which carbons C-2, C-6 and C-3, C-5 are identical as the molecule exhibits a plane of symmetry. Complete interpretation of '3C NMR is reported in Table 2.
Attempts to effect reaction of 2d-f with 1 under the same conditions failed to produce isolable products.
In contrast to the behaviour of 2a-c towards 1 in cold ethanolic triethylamine solutions, compounds 2a -c react with 1 in refluxing ethanolic triethylamine solutions to yield 4a-c which were also obtained in the reaction of 2d-f with 1. The molecular formulae of these products are in accord with the addition of 2d-f to 1 followcd by elimination of ethanol and hydrogen. The formation of 4a-c from 2a-c is assumed to proceed via rearrangement of thc thiopyrans 3a -c which arc thc kinctically controllcd products. In agrccmcnt with this view is the ready conversion of 3a-c into 4a-c on long reflux in ethanolic-aqueous