W. P. Trompenaars

Synthesis of pyrrolizines by intramolecular capture of 1,4-dipolar intermediates in reactions of enamines with dimethyl acetylenedicarboxylate

Solvent polarity and reaction temperature strongly influence the reactions of dimethyl acetylenedicar-boxylate (DMAD) with 1-pyrrolidinyl enamines of acyclic and cyclic ketones. Whereas DMAD and 1-[1-phenyl-2-(phenylthio)ethenyl]pyrrolidine (3) give only a mixture of the isomeric 1,3-butadienes (5) in apolar solvents, in methanol the main product is the pyrrolizine 7, together with 5. Again in methanol, DMAD reacts at 0-5° with 8, 9 and 10 to give exclusively 1:1 adducts, the pyrrolizines 11,12 and 13, respectively, whereas at −50° 8 and 9 give 1:2 (enamine : DMAD) adducts, the pyrrolizines 14 and 15, respectively; a single crystal X-ray analysis of 14 gave the structure of the 1:2 adducts. In the same solvent methyl propiolate and 8 give only the linear Michael adduct 17. The enamine-ketone 18 reacts with DMAD in propylene carbonate at 0–5° to give, via (2 + 2)-cycloaddition and ring expansion, 19, and the linear Michael adduct 20. The mechanism of (2 + 2)-cycloaddition and pyrrolizine formation is discussed in terms of a common tied-ion pair intermediate formed in the first, rate-determining step, followed by a second solvent-dependent step.

Reactions of thiophenes with acetylenes in polar solvents; a novel annulation reaction

Although for a long time thiophenes have been considered to be inert in cycloaddition reactions, recently some have been reported to react with acetylenes via either a (2+2)or a (4+2)-cycloaddition reaction l-4 . Particularly some of the (2+2)-cycloadditions of thiopheneslr2, and of benzo[b]thiophenes5-7, proved to be versatile reactions in the synthesis of l-heterocycloheptatrienes. For example, reaction of 3-pyrrolidinothiophenes (1) with dimethyl acetylenedicarboxylate in apolar solvents at low temperature yielded the corresponding thiepins (A), a class of anti-aromatic compounds that could not be obtained by other routes. We now report that in polar solvents the reactions of A with dimethyl acetylenedicarboxylate take an entirely different course.

The madelung synthesis of dihydro-1H-pyrrolo- and tetrahydropyrido[1,2-a]- indoles under mild conditions

Benzeneacetonitriles substituted with lactam moieties in the ortho-position cyclize under the influence of a base, dependent on the ring-size of the lactam function, to dihydropyrrolo-, tetrahydropyrido[1,2-a]indole or dihydro-1-benzazepin derivatives, respectively.

On the mechanism of the reaction of enamines and dimethyl acetylenedicarboxylate (DMAD) in polar and apolar solvents

[2+2]-Cycloadducts of enamines and DMAD, formed in apolar solvents, isomerize to pyrrolizine derivatives under mild conditions in protic polar solvents like methanol and 1-butanol.

Reduction of nitro groups by ynamines; synthesis and x-ray crystal structure of n,n-diethyl-3,3a-dihydro-3-methylbenzofuro[3,2-c]isoxazole-3-carboxamide

3-Nitrobenzo[b]furan and 1-diethylaminopropyne react thermally at 5–10°C to give a 1:1 addition product ( ) in which one of the oxygen atoms of the nitro group is transferred to C-1 of the acetylene. The structure of the benzofuro[3,2-c]isoxazole ( ) has been determined by X-ray crystallography.

Reactions of nitroalkenes with ynamines; synthesis of stable four-membered cyclic nitrones and X-ray crystal structure of 2-(NN-diethylcarbamoyl)-2,4-dimethyl-3-phenyl-2,3-dihydroazete 1-oxide

2,3-Dihydroazete 1-oxides (3b–e), the first examples of crystalline, ‘stable’, four-membered cyclic nitrones, are formed by reaction of 1-nitroalkenes with ynamines, and other products are 3-nitrocyclobutenes; the structure of the cyclic nitrone (3b) has been determined by X-ray crystallography.