Calculation-assisted regioselective functionalization of the imidazo[1,2-a]pyrazine scaffold via zinc and magnesium organometallic intermediates.

Straightforward calculations such as determinations of pKa values and N-basicities have allowed the development of a set of organometallic reactions for the regioselective functionalization of the underexplored fused N-heterocycle imidazo[1,2-a]pyrazine. Thus, regioselective metalations of 6-chloroimidazo[1,2-a]pyrazine using TMP-bases (TMP = 2,2,6,6-tetramethylpiperidyl) such as TMPMgCl·LiCl and TMP2Zn·2MgCl2·2LiCl provided Zn- and Mg-intermediates, that after quenching with various electrophiles gave access to polyfunctionalized imidazopyrazine heterocycles. Additionally, the use of TMP2Zn·2MgCl2·2LiCl as base for the first metalation allowed an alternative regioselective metalation. Nucleophilic additions at position 8 as well as selective Negishi cross-couplings complete the set of methods for selectively decorating this heterocycle of the future.

Pd-Catalyzed Regioselective Cyclopropanation of 2-Substituted 1,3-Dienes.

A Pd-catalyzed 3,4-regioselective cyclopropanation of 2-substituted 1,3-dienes by decomposition of diazo esters is reported. The vinylcyclopropanes generated are isolated in practical chemical yields with high levels of regioselectivity but low diastereoselectivity. The system operates under mild reaction conditions, is scalable, and tolerates various sensitive functional groups. A series of original postcatalytic derivatizations is presented to highlight the synthetic potential of the catalytic method.

Reliable Functionalization of 5,6-Fused Bicyclic N-Heterocycles Pyrazolopyrimidines and Imidazopyridazines via Zinc and Magnesium Organometallics.

DFT-calculations allow prediction of the reactivity of uncommon N-heterocyclic scaffolds of pyrazolo[1,5-a]pyrimidines and imidazo[1,2-b]pyridazines and considerably facilitate their functionalization. The derivatization of these N-heterocycles was realized using Grignard reagents for nucleophilic additions to 5-chloropyrazolo[1,5-a]pyrimidines and TMP2 Zn ⋅ 2 MgCl2 ⋅ 2 LiCl allowed regioselective zincations. In the case of 6-chloroimidazo[1,2-b]pyridazine, bases such as TMP2 Zn ⋅ MgCl2 ⋅ 2 LiCl, in the presence or absence of BF3 ⋅ OEt2 , led to regioselective metalations at positions 3 or 8. Subsequent functionalizations were achieved with TMPMgCl ⋅ LiCl, producing various polysubstituted derivatives (up to penta-substitution). X-ray analysis confirmed the regioselectivity for key functional heterocycles.

Photochemical Amplifier Based on Self-Immolative Dendritic Spacers.

A self-immolative dendritic structure was synthesized. It is based on phenol derivatives with three hydroxymethyl arms at both ortho and para positions of the core unit, potentially releasing up to 27 leaving groups in a third-generation dendrimer. The triggering event is the photolysis of a photosentive ortho-nitrobenzyl group. In doing so, we expected to transform a weak chemical or photochemical input into a large chemical output, which fulfills the definition of a molecular amplifier. Such dendrimers could find application as an indicator, a drug-delivery vector, or a solubilizing agent. The prepared dendrimer indeed released up to 27 leaving groups upon photolysis at 360 nm.