Synthesis of bis-(3,5)pyrazolophanes by sequential intermolecular-intramolecular nitrilimine cycloadditions

✍ Scribed by Gianluigi Broggini; Luisa Garanti; Giorgio Molteni; Gaetano Zecchi

• Book ID: 104209137
• Publisher: Elsevier Science
• Year: 1998
• Tongue: French
• Weight: 448 KB
• Volume: 54
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(98)83021-4

✦ Synopsis

A number of papers are available describing the construction of macrocyclic systems through intramolecular cycloadditions of 1,3-dipoles such as nitriioxides, I's nitrilimines, 6 azides, 7 nitmne~ s azomethinylides, ~'~° and carbonylylides, tl Our recent conm'butions in this field 6 are concerned with the use of suitably functionalised nitrilimines as precursors of (1,5)pyrazolophanes. We now wish to report a version of the same methodology leading to mono-and bis-(3,5)pyrazolophanes. Such kind of molecules are receiving attention as non-conventional ligands towards metal cationsJ 2

Results

We explored the behaviour of two types of nitri~nines, namely 5 and 15, which differ in the nature of the tether joining the dipole and dipolarophile groups (Schemes 1 and 2). All of them were generated in situ upon basic treatment of the corresponding hydrazonyl chlorides (i.e. 4 and 14, respectively), the synthesis of which was accomplished as outlined in Schemes 1 and 2.

In order to generate the desired nitfilimines, compounds 4 and 14 were treated with silver carbonate in dioxane at room temperature. Schemes 1 and 2 illustrate the complex reaction outcomes, while detailed data are collected in Tables I and2. The intermediate species 8a-e and 17e were really isolated when stopping the reaction at short times and were proven to be the precursors of the macrocyclic products 10a-e and 1Be, respectively. In the case of compounds 10 and 18 containing two stereocentres, we assigned a chiral rather than a rnesoform one whenever the NMR signal of the 4-pyrazolinic protons was splitted in the presence of Eu(hfc)3 [tris{ heptafluoropropylhydroxymethylene-(+)-camphorato } europium-(III)].