N. A. Keiko

Configurational Assignment and Conformational Study of Methylglyoxal Bisdimethylhydrazones Derived from the 2-Ethoxypropenal Precursor

A configurational assignment of the isomeric methylglyoxal bisdimethylhydrazones derived from the 2-ethoxypropenal precursor has been performed based on experimental measurements and high-level ab initio calculations of 1J(C,C) and 1J(C,H) couplings. The results reveal the marked stereochemical dependence upon the orientation of the lone pairs of both nitrogen atoms in different isomers. Methylglyoxal bisdimethylhydrazone is shown to exist in a mixture of the EE and ZE isomers (ca. 75:25), both of which adopt predominant s-trans conformations with minor (up to 8°) out-of-plane deviations.

N,N-Disubstituted α-Amino-α,β-unsaturated Aldehydes and their Derivatives:1H and13C NMR Study

1H and 13C NMR spectra of N,N‐disubstituted α‐amino‐α,β‐unsaturated aldehydes and their azomethines and enammonium salts were studied. The spectra reflect the degree of p–π conjugation between the nitrogen lone pair and the π‐electrons of the carbon–carbon double bond. In this respect, the title 2‐aminoenals are shown to be in an intermediate position between α‐unsubstituted aminoalkenes and α,β‐unsaturated aldehydes. The degree of conjugation depends on the nature of the amine moiety and the activating group.

Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts

In the 13C NMR spectra of methylglyoxal bisdimethylhydrazone, the 13C‐5 signal is shifted to higher frequencies, while the 13C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the 1H‐6 chemical shift and 1J(C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Baders quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ(1H‐6) and 1J(C‐6,H‐6) parameters. The effect of the C–H⋯N hydrogen bond on the 1H shielding and one‐bond 13C–1H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The 1H, 13C and 15N chemical shifts of the 2‐ and 8‐(CH3)2N groups attached to the –C(CH3)═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH3)2N group conjugate effectively with the π‐framework, and the 2‐(CH3)2N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the 1H, 13C and 15N shieldings. Copyright

Synthesis of 2-(1-alkoxyvinyl)oxazolidines by condensation of 2-alkoxypropenals with 2-aminoalkanols and ring-chain tautomerism of the products

Reactions of 2-alkoxypropenals with 2-aminoalkanols afforded tautomeric mixtures of previously unknown 2-(1-alkoxyvinyl)oxazolidines and imino alcohols. The condensation takes 2 h at room temperature (89-100%) or 1-5 min under microwave irradiation. The tautomeric equilibrium shifts toward the open-chain structure with increase in the solvent polarity (CDCl3, CD2OD, DMSO-d6, D2O) and temperature. The presence of substituents in the oxazolidine ring raises the stability of the cyclic tautomer.

Reactions of 2-Alkoxypropenals with Thiols in Neutral and Acid Media

Addition of thiols to 2-alkoxypropenal in neutral medium at 20°C in the absence of a catalyst occurs regioselectively, following the Markownikoff pattern. The resulting 2-alkoxy-2-R-thiopropanals are capable of undergoing spontaneous isomerization to 1-alkoxy-1-R-thiopropanones. The addition and isomerization processes are accelerated by heating to 60°C or in the presence of acid catalysts (TsOH, HCl). The reaction is also accompanied by partial disproportionation of 2-oxopropanal O,S-acetals to give O,O- and S,S-acetals.

Reactions of α-halo-α, β-unsaturated aldehydes with secondary amines

Reactions of 2-halo-2-alkenals R″(R′)C=CX-CHO with secondary amines R2NH occur asipso-substitution of the halogen atom, along with fragmentation and condensation, yielding 1,2-diaminoethenes R2NCH=CHNR2, carbonyl compounds R″C(O)R′, 1,3-bis(amino)-2-haloolefins R″(R′)C(NR2)CX=CHNR2, and formamides R2NCHO. The ratio between the competing reactions depends on the structure of the starting compounds and the experimental conditions.

Reaction of 2-Alkoxypropenals with α-Hydroxyamino Oximes and 1,2-Bis(hydroxyamino)cyclohexane

Reactions of 2-alkoxypropenals with α-hydroxyamino oximes in neutral medium involve the aldehyde group of the former to afford both acyclic and cyclic azomethine oxides: N-(2-hydroxyiminoalkyl)-N-(2-alkoxy-2-propenylidene)amine oxides and 1-hydroxy-2,5-dihydroimidazole 3-oxides. The state of tautomeric equilibrium between the cyclic and acyclic products depends on the solvent nature and temperature. The reaction in acidic aqueous medium is accompanied by hydrolysis of the vinyl ether moiety in 2-alkoxy-propenals with formation of 2-oxopropionaldehyde which reacts with α-hydroxyamino oxime at the hydroxy-amino group to give substituted pyrazine 1,4-dioxides. The reaction of 2-alkoxypropenals with 1,2-bis-(hydroxyamino)cyclohexane leads to formation of 2-(1-alkoxyvinyl)-1,3-dihydroxyperhydrobenzimidazoles. The structure of the products was proved by IR, UV, and 1H and 13C NMR spectroscopy and X-ray analysis.

New Syntheses of the Bis-Guanylhydrazone and Bis-Thiosemicarbazone of Methylglyoxal

New methods of synthesis of methylglyoxal bis-guanylhydrazone (as bis-hydrochloride) and methylglyoxal bis-thiosemicarbazone are developed, which are based on the reactions of 2-ethoxypropenal with aminoguanidine and thiosemicarbazide in an acid medium.

Kinetics of dimerization of 2-alkylthiopropenals

The rate constants for cyclodimerization of α-alkylthioacroleins were determined. They are two orders of magnitude higher than those for dimerization of α-alkoxyacroleins.

Reaction of α-ethoxyarolein with diethyl malonate

The reaction of α-ethoxyacrolein with diethyl malonate in the presence of EtONa, lithium diisopropylamide, or the Na2CO3−benzene−Et3(PhCH2)NCl catalytic system proceeds as the Michael addition. In the presence of an equimolar amount of triethylamine, selective 1,2-addition followed by dehydration of the 1,2-adduct occurs. Owing to the strong +M effect of the EtO group, α-ethoxyacrolein is a substantially less active Michael acceptor than acrolein.