Xavier L. M. Despinoy

The Effect of 3-Substitution on the Structures of Pyrrole-2-carbaldehydes

In the title compounds, methyl 2-formylpyrrole-3-carboxylate, C 7 H 7 NO 3 , and 3-methoxypyrrole-2-carbaldehyde, C 6 H 7 NO 2 , the pyrrole rings show little distortion ascribable to the electronic properties of the substituents, whether they are electron donating or electron withdrawing.

An Unusual 3H-Pyrrole at 150K

Most pyrroles are observed to form the 1H-tautomer; here we report the structure of a rare example of a 3H-pyrrole, trimethyl 6-oxo-5,5a,10b,10c-tetrahydro-3H-pyrrolizino[1,2-e]indole-4,5, 10b-tricarboxylate, C19H18N2O7. The molecules pack as dimers linked via bifurcated hydrogen bonds.

Synthesis of pyrrolizin-3-ones by flash vacuum pyrolysis of pyrrol-2-ylmethylidene Meldrum’s acid derivatives and 3-(pyrrol-2-yl)propenoic esters

Monosubstituted pyrrolizin-3-ones 1 with substituents at the 1-, 5-, 6- or 7-positions are prepared in excellent yield by flash vacuum pyrolysis (FVP) of appropriate Meldrum’s acid derivatives 2. The mechanism involves formation of the pyrrol-2-ylmethylideneketene 29, which can also be generated thermally from 3-(pyrrol-2-yl)propenoate esters (e.g. 30). This alternative route has been used to make a range of 2-substituted pyrrolizin-3-ones, again in excellent yield. The 3-oxo-3H-pyrrolizine-2-carboxylic acid 42 could not be made in this way owing to facile decarboxylation to pyrrolizinone 1, and extension to the formation of the azaazulenone 48 was again unsuccessful.

Synthesis and unexpectedly facile dimerisation of 1-methoxycarbonylpyrrolizin-3-one

Flash vacuum pyrolysis of the diethyl esters 2 gives 2-ethynylpyrrole via the anhydride 7; the corresponding dimethyl esters 3 give, under similar conditions, the pyrrolizinone 5, which is remarkably unstable and spontaneously dimerises to give the [2 + 2] cycloadducts 8 and 9 whose structures are proved by X-ray crystallography.

Pyrrolizin-3-one and its 1,2-dihydro derivative: structures of the free molecules determined by electron diffraction and ab initio calculations and in the crystal by X-ray diffraction

The molecular structures of pyrrolizin-3-one 1 and 1,2-dihydropyrrolizin-3-one 2 have been investigated in the gas phase by ab initio calculations (both compounds) and electron diffraction using the SARACEN method of structural analysis (parent compound only), and in the solid phase at 150 K by low-temperature X-ray diffraction. Important structural parameters (rh1 structure) for a free molecule of 1 are: r(C–O) 1.215(4) and r[N–C(CO)] 1.437(4) A. For 2 these (re structure) are: r(C–O) 1.2095 and r[N–C(CO)] 1.4068 A. The corresponding values in the crystal for 1 are r(C–O) 1.207(2) and r[N–C(CO)] 1.408(2) A, and for 2r(C–O) 1.216(2) and r[N–C(CO)] 1.392(2) A.