2-Trifluoromethylperimidines with electron-withdrawing groups at the 6(7)-position: a case of extremely hindered annular prototropy

Heterocyclic analogs of pleiadiene. 75. Formylation of perimidines and 2,2-dimethyl-2,3-dihydroperimidine under vilsmeier conditions

“…In our view this indicates that the carbonyl oxygen is oriented towards the peri proton (Z-conformation). We have confirmed this conformation previously in the case of 6(7)-formyl-2-trifluoromethylperimidine by X-ray crystallography [14]. The signals of the protons of the 4-and 9-CHO group in aldehydes 8, 11, and 12 are shifted to weak field by ~0.7-0.8 ppm in comparison with the protons of the 6-and 7-CHO group ( aldehydes 3 and 5 are excluded from the comparison since hydrogen bonding fixes the formyl group in the plane of the ring).…”

“…Two more fractions were collected, the first of which contained predominantly a mixture of the 4-, 6-, and 7-carbaldehydes 8, 9, and 10 respectively, while second contained mainly the 6,9-and 7,9-dicarbaldehydes 11 and 12. They were finally separated by preparative TLC on Al 2 O 3 7.35 (1H, dd, J 8'7' = 8.12, J 8'9' = 7.30, H-8'); 7.63 (1H, d, J 6'5' = 8.81, H-6'); 7.80 (1H, s, H-2'); 8. Formylation of 2,2-Dimethyl-2,3-dihydroperimidine (14). POCl 3 (0.69 ml, 7.5 mmol) was added dropwise with cooling to 0°C to dried and freshly distilled DMF (5 ml) and the mixture was stirred for 20 min at -20°C.…”

“…In connection with the completely unusual results of formylation of 1,3-dimethyl-and 1,2,2,3-tetramethyl-2,3-dihydroperimidines [2], it seemed of interest to study their analog with free NH groups, 2,2-dimethyl-2,3-dihydroperimidine (14). We established that compound 14 reacted with the Vilsmeier reagent in a completely different manner: unexpectedly perimidine (1), 1-isopropenylperimidine (15), and the aldehyde 3 were isolated from the reaction mixture in yields of 52, 13, and 4 % respectively.…”

Heterocyclic analogs of pleiadiene 76. Synthesis and tautomeric conversions of mono- and disubstituted perimidines with electron-withdrawing substituents in the naphthalene fragment

“…1, c), in difference to their 2-CF 3 analogs [3], and also other derivatives of perimidine [4]. In this case the ratio of tautomers 2a and 2b in the equilibrium mixture was 38:62, i.e.…”

“…Their prototropic annular tautomerism has been investigated by 1 H NMR spectroscopy.We reported previously on the extremely slow annular prototropic tautomerism of derivatives of 2-trifluoromethylperimidine containing a formyl, acetyl, or p-toluenesulfonyl group in position 6(7) [2,3]. This phenomenon is expressed in the 1 H and 19 F NMR spectra of these compounds by the simultaneous observation of signals of both tautomers in low polarity solvents at room temperature and even on heating to 120-130 o C. It is assumed that the reason for this is the low basicity of these compounds in combination with spatial screening of both heteroatoms not only from the side of the 2-CF 3 substituent but also from the hydrogen atoms in positions 4 and 9.…”