1.4- Dihydropyrazine, conformation

The electron-rich and cyclically conjugated 8 rr electron system of 1,4-dihydropyrazine can be stabilized in essentially planar conformation by organosilicon substitution at the enamine nitrogen centers [1], In addition to electron transfer [1] and triorganosilyl exchange reactions [2], we have explored the possibility of inserting heterocumulenes X=C = Y into one or both of the N-Si bonds of the compounds 1 in order to functionalize this unusual ring structure [3],... 

Scheme 7 Synthesis of 1,6 dihydropyrazine derivatives and two representative 3D conformations...

For aromatic aldehydes or ketones, the (3R)-configuration can be derived from the1 H-nmr-spectrum. Like the benzyl-substituted dihydropyrazines 8a-e the hydroxy-benzyl-substituted compounds 13 c-f adopt the folded conformation. Hence, in the minor isomer with (3S)-configuration and conformation 14 the C-6-methyl signal suffers an upfield shift. 

Conformations. Calculations have been made of the preferred conformations for 1,4-dihydropyrazine,456, 1080 1,2,3,4-tetrahydropyrazine,100 piperazine (and several alkyl derivatives),1079 and the (reduced) pyrazine ring in several biologically important di- and tetrahydropteridines.100... 

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