Mechanism of formation and stabilities of the new dioxadiazadecalin systems. Ring-chain tautomerism

A linear free energy relationship study is presented for the reactions of threo- and erythro-1,4-diamino- and -2,3-diaminobutanediols (1, 2) with six p-substituted benzaldehydes, the end products of which are the novel cis- and trans-1,5-dioxa-3,7-diazadecalin (DODAD, 7ci, 7tr) and -1,5-diaza-3,7- dioxadecalin (DADOD, 8ci, 8tr) systems. The consecutive double 1,3-oxazane ring closures take place mostly via Schiff bases and are moderately polar ring-chain tautomeric reactions with low positive p-values (0.69) affected by steric strain, stereoelectronic effects, and intramolecular hydrogen bonds. These are relatively slow processes, which may occur in the solid as well, but are greatly enhanced by acid catalysis.