Transition metal‐ and acid‐induced transformations of 6‐siloxy‐substituted 5,6‐dihydro‐4 H ‐1,2‐oxazines: Preparation of pyrroles, nitrones, 1,4‐dicarbonyl compounds and derivatives thereof

A variety of 6‐siloxy‐substituted 5,6‐dihydro‐4 H ‐1,2‐oxazines (abbreviation: 1,2‐oxazines) 1 , 3 could be transformed into di‐ and trisubstituted pyrroles 2 , 4 by means of molybdenum hexacarbonyl. The mechanism of this deoxygenating ring contraction is discussed. With two bicyclic 1,2‐oxazines an acid‐catalyzed fragmentation affording α‐methylenecycloalkanones 7 has been observed, while other 1,2‐oxazines rearrange in methanolic acid to give nitrones 9, 10 . The desilylation of 6‐siloxy‐substituted 1,2‐oxazines 1 , 3 employing NEt 3 · 3 HF is a very general and smooth process providing 6‐hydroxy‐1,2‐oxazines 11 , 12 or their corresponding acyclic tautomers 13 , 14 in high yields. For two examples of 11 deoximations by use of formalin could be achieved with moderate efficiency giving 1,4‐dicarbonyl compounds 15 .