Krištof Kranjc

Diels–Alder reaction of highly substituted 2H-pyran-2-ones with alkynes: reactivity and regioselectivity

The Diels–Alder reactions of a variety of electron-rich 2H-pyran-2-ones 4 with alkynes 2 yielding aniline derivatives 6 under thermal conditions as well as at high pressures are presented. The effects of the substituents at the positions 3 and 5 in the 2H-pyran-2-one ring on the reactivity and the regioselectivity with different alkynes were qualitatively explained on the basis of electron demand and also by the formation of zwitterionic intermediates.

An efficient microwave-assisted green transformation of fused succinic anhydrides into N-aminosuccinimide derivatives of bicyclo[2.2.2]octene in water

A very efficient and clean green synthesis of highly substituted prochiral bicyclo[2.2.2]oct-7-enes, starting from bicyclo[2.2.2]oct-7-ene-2exo,3exo,5exo,6exo-tetracarboxylic acid 2,3∶5,6-dianhydrides and a variety of hydrazines, is described.

Effect of ring size on the exo/endo selectivity of a thermal double cycloaddition of fused pyran-2-ones.

A study of an unusual effect of the size of the ring fused to 2H-pyran-2-ones on the exo/endo selectivity of a thermal double cycloaddition of N-substituted maleimides or maleic anhydride yielding bicyclo[2.2.2]octene derivatives is presented. With subtle variations of starting compounds and reaction conditions exclusively exo,exo or exo,endo products can be prepared.

A way to avoid using precious metals: the application of high-surface activated carbon for the synthesis of isoindoles via the Diels-Alder reaction of 2H-pyran-2-ones.

The application of activated carbon (Darco KB) for the acceleration and direction of the transformation of various 2H-pyran-2-ones with N-substituted maleimides toward isoindole derivatives through the reaction sequence cycloaddition/elimination/dehydrogenation is described. In this reaction, the catalyst mainly influences the dehydrogenation step, which is essential to avoid the formation of bicyclo[2.2.2]octenes as the other possible products. We found that the combination of Darco KB, as the metal-free catalyst, and decalin, as the solvent in a closed vessel, represents the most successful conditions. A comparison of the effect of various dehydrogenation catalysts and reaction conditions is also presented. In addition, we have proven that the aromatization occurs via a hydrogen transfer from the cyclohexadiene intermediate to the maleimide derivative (therefore producing succinimides). This transfer is facilitated by the active surface of the heterogeneous carbon-based catalyst.

Microwave-Assisted Diels-Alder Reaction of 2H-Pyran-2-ones with Maleimides towards Fused Bicyclo[2.2.2]octenes

An efficient, green access to functionalized and highly constrained heteropolycyclic derivatives via a microwave-assisted cycloaddition reaction is reported. The double Diels-Alder reaction of a series of 2H-pyran-2-ones with maleimide and its N-substituted derivatives takes place in an aqueous mixture (and in some cases as neat reaction) to give a variety of fused bicyclo[2.2.2]oct-7-enes.

Bicyclo[2.2.2]oct-7-ene Derivatives: A Green Preparation of the Fused Succinimide Ring

The reaction of differently substituted bicyclo[2.2.2]oct-7-ene-2exo,3exo,5exo,6exo-tetracarboxylic acid 2,3:5,6-dianhydrides (la-g and 4) with various hydrazines was investigated. The starting la-g were transformed in an aqueous solution under microwave irradiation conditions to the corresponding fused N-aminosuccinimides with high yields. The derivative (4) also reacted at the additional carbonyl moiety to give 5a-c as the sole products.

Diels–Alder reaction of fused pyran-2-ones with ethyl vinyl ether

Ethyl vinyl ether was found to be an appropriate synthetic equivalent of acetylene for a set of Diels–Alder reactions with fused pyran-2-ones that yield fused carbocyclic systems. Transformations were conducted under microwave irradiation with DABCO (as a catalyst for the elimination of ethanol) and with n-butanol as the additive. A single-crystal X-ray diffraction structure is presented for N-(5,6,7,8-tetrahydro-6-methyl-8-oxonaphthalen-2-yl)benzamide.Graphical abstract

Methyl 2-benzamido-4-(3,4-dimethoxyphenyl)-5-methylbenzoate and N-{5-benzoyl-2-[(Z)-2-methoxyethenyl]-4-methylphenyl}benzamide

Methyl 2-benzamido-4-(3,4-dimethoxyphenyl)-5-methylbenzoate, C(24)H(23)NO(5), (Ia), and N-{5-benzoyl-2-[(Z)-2-methoxyethenyl]-4-methylphenyl}benzamide, C(24)H(21)NO(3), (IIa), were formed via a Diels-Alder reaction of appropriately substituted 2H-pyran-2-ones and methyl propiolate or (Z)-1-methoxybut-1-en-3-yne, respectively. Each of these cycloadditions might yield two different regioisomers, but just one was obtained in each case. In (Ia), an intramolecular N-H···O hydrogen bond closes a six-membered ring. A chain is formed due to aromatic π-π interactions, and a three-dimensional framework structure is formed by a combination of C-H···O and C-H···π(arene) hydrogen bonds. Compound (IIa) was formed not only regioselectively but also chemoselectively, with just the triple bond reacting and the double bond remaining unchanged. Compound (IIa) crystallizes as N-H···O hydrogen-bonded dimers stabilized by aromatic π-π interactions. Dimers of (IIa) are connected into a chain by weak C-H···π(arene) interactions.