The 'tert-amino effect' in heterocyclic chemistry: Synthesis of 3,1-benzoxazines and 3,1-benzothiazines

Two different routes are described for the synthesis of the 2,2,2‐trifluoro‐l‐[2‐(dialkyl‐amino)phenyl]ethanones 2 and 9 and their hydrates 3 and 10, respectively, via trifluoroacetylation of the N,N‐dialkylanilines 1 and via a Barbier reaction of 2‐fluorobenzaldehyde. These compounds were thermally converted into a mixture of cis‐ and trans‐pyrrolo‐ and pyrido[1,2‐a] [3,1]benzoxazines, (11: cis; 12: trans). The structure of these compounds was proven by X‐ray analysis (11a) and 1H NOE difference spectroscopy. Cyclization of (R)‐9b (R1 = CH3) gave predominantly one enantiomer (12f, 70%) and in addition two diastereomers of 17a and two of 18a (total yield ≈ 17%), while cyclization of (S)‐9c (R1 = CH2OCH3) gave a mixture of 12g (18%), two diastereomers of 17b (36%) and two diastereomers of 18b (18%). The benzaldehyde 19a (R3 = H), acetophenone 19d (R3 = CH3), trifluoroacetophenones 19b,c (R3 = CF3) and benzophenone 19e (R3 = C6H5) reacted with Lawesson's reagent to yield exclusively the trans‐pyrrolo[1,2‐a] [3,1]benzothiazines 21a‐e in yields of 33‐77%. Reaction of 19f (R1 = CH3, R3 = H), 19g (R1 = CH3, R3 = CF3) and 19h (R1 = CH3, R3 = CH3) with Lawesson's reagent resulted in the formation of mixtures of isomers 21‐24.