Stereoisomerism and Ring-Chain Tautomerism in 1-Hydroxy-2,3-dihydro-1H-pyrazolo[1,2-a]pyridazine-5,8-diones and 1-Hydroxy- and 1-Amino-2,3-dihydro-1H-pyrazolo[1,2-b]phthalazine-5,10-diones

Abstract

1‐Hydroxy‐2,3‐dihydro‐1H‐pyrazolo[1,2‐a]pyridazine‐5,8diones and 1‐hydroxy‐ and 1‐amino‐2,3‐dihydro‐1H‐pyrazolo[1,2‐b]phthalazine‐5,10‐diones were synthesized and their structures studied by NMR and MS methods. All compounds exist in similar cyclic forms in solution, except for 1hydroxy‐1‐methyl‐2,3‐dihydro‐1H‐pyrazolo[1,2‐a]pyridazine‐5,8‐dione, which displays ring‐chain tautomerism in CDCl3 solution. The corresponding 3‐methyl‐substituted derivatives exhibit a typical cis/trans isomerism as a result of latent ring‐chain‐ring tautomerism. The structures of stereoisomers concluded from NMR results were additionally confirmed by density functional calculations. Mass spectrometric results indicate open‐chain structures in the gas phase for the molecular ions of the 1‐hydroxy derivatives and cyclic structures for 1‐amino derivatives. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Axially chiral N‐(o‐aryl)‐4‐hydroxy‐2‐oxazolidinone derivatives from diastereoselective reduction of N‐(o‐aryl)‐2,4‐oxazolidinediones: Thermally interconvertible atropisomers via ring‐chain‐ring tautomerization

“…Ring-chain tautomerization has been shown to occur in a number of heterocyclic compounds. 10,[12][13][14][15][16][17][18][19][20] The tautomerism results from an interconversion of the chain and the ring isomers. For compounds 7-12, the two enantiomers (S-M) and (R-P) (see Fig.…”

“…23,24 However, such an intermediate was not observed in the NMR time scale in CDCl 3 . Taking the NMR spectra in d 6 -DMSO in the absence or presence of trifluoroacetic acid-d or pyridine-d 5 , which could stabilize the aldehyde intermediate 10,12 did not exhibit its existence as well. With the aim of trapping the aldehyde intermediate as an imine, the compound 10 was treated with benzyl amine in toluene under reflux for one day as has been described in Ref.…”

“…With the aim of trapping the aldehyde intermediate as an imine, the compound 10 was treated with benzyl amine in toluene under reflux for one day as has been described in Ref. 10. Analysis of the The interconversion was too slow at 303 K to determine the kinetics (3% interconversion in 100 min at 313 K).…”

“…For example, the N-(o-hydroxyphenyl) substituted N-(o-aryl)-4-hydroxy-5,5-dimethyl-2-oxazolidinones would be analogous to the well known BINOL. 9 This article describes the regio and diastereoselective NaBH 4 reduction of racemic N-(o-aryl)-5,5-dimethyl-2,4-oxazolidinediones (6)1-6 to yield axially chiral hemiaminal type compounds, N-(o-aryl)-4-hydroxy-5,5-dimethyl-2oxazolidinones 7-12, having also a chiral center at C-4 which were found to racemize by a latent 10 ring-chain-ring tautomerization rather than by rotation around the N sp2 -C aryl bond (see Fig. 1).…”

Organocatalytic cascade aza-Michael/hemiacetal reaction between disubstituted hydrazines and α,β-unsaturated aldehydes: Highly diastereo- and enantioselective synthesis of pyrazolidine derivatives

“…Compared with monosubstituted hydrazines in organocatalytic asymmetric synthesis, disubstituted hydrazines were also explored by several groups [72–73]. In 2007, Jørgensen et al reported that the organocatalyzed asymmetric aza-Michael addition of hydrazones to cyclic enones had been achieved in good yield and stereoselectivity [74].…”