Ring-chain tautomerism and other versatile behaviour of 1,4-diimino- and 1,2-phenylene derivatives of some C-substituted serinols

An Anomeric Effect Drives the Regiospecific Ring‐Opening of 1,3‐Oxazolidines under Acetylating Conditions

“…This situation is reflected best by the delocalization shown in 18 and not in 19. [13,14] Similar conclusions have been proposed for other 1,3-oxazolidines [15] and 1,3-oxazanes. [16,17] In summary, the anomeric effect is expected to result in a longer C2-O1 bond and a shorter C2-N bond relative to the regular C5-O1 and C4-N bonds that are not affected by the anomeric effect.…”

“…Maiereanu et al [15] have also reported an equilibrium for 4-dimethylaminobenzylidene derivatives of TRIS, although both tautomers are present in similar proportions (≈ 1:1). The spectroscopic data for such equilibria show the diagnostic resonances of the corresponding imines Scheme 2.…”

3,7‐Dioxa‐1‐azabicyclo[3.3.0]octanes Substituted at the C‐5 Position − From Local to Global Stereochemistry

“…Finally, the versatile double ringϪchain tautomerism manifested by the double oxazolidines 5aϪc, [25] which resulted from the condensation between the serinols 1aϪc and terephthalaldehyde, recently reported by us, [26] was exploited in the synthesis. Reaction of the mixture of epimers of 5aϪc with a slight excess of formaldehyde or benzaldehyde gave a separable mixture of three series of compounds (conversions mentioned in Scheme 4): the desired 7aϪe, the partial transaminalisation products 6aϪc (in reaction with formaldehyde only) and the known, total-transaminalisation compounds 2aϪc and 2dϪf-cis.…”

“…2,6-dichloropyrazine, THF, 60°C to room temp., 24 h; iv: 0.5 equiv. terephthalaldehyde; [ 26 ] v: 2 equiv. R 1 ϪCHϭO, toluene, pTsOH, DeanϪStark, reflux in a 4.90:1 ratio as deduced by a 1 H NMR spectrum of the crude material (90% global yield; ∆G ϭ Ϫ5.10 kJ/mol cis vs. trans); they were separated by simple fractional crystallisation.…”

“…In total agreement with the experimental data, we found that a diastereoisomer of type 8 is strongly disfavoured; indeed, in this class, only once did we pertinently confirm such a type of r-1Ϫt-2Ϫc-5Ϫt-8 linkage. [26] We extended our calculations to the O-methyl analogues 3a, 3e-cis and 3e-trans. Although, as normally expected, only out rotamers were revealed in all three cases, the results exhibited some stimulating differences (Schemes 8 and 9).…”