1,3-Dipolar cycloaddition reactions of 1-(3-nitrophenacyl)-1,10-phenanthrolinium N -ylide with activated alkynes

The 3+2 cycloaddition reaction of 1-(3-nitrophenacyl)-1,10-phenanthrolinium ylide 4 with activated alkynes gave pyrrolo[1,2-a][1,10]phenanthrolines 7a-e . The reaction of N -ylide 4 with DMAD, under controlled conditions, gave regioselectively the primary cycloadduct trans - 5a which, in the presence of triethylamine, rearranged regio-and stereoselectively to trans - 6a . Evidence for the helical chirality of pyrrolophenanthrolines 7b-e was obtained by NMR spectroscopy.


Introduction
0][11][12] The extended heteroaromatic system presents helical chirality, like that of the helicene-type compounds. 13erein we describe the reaction of 1-(3-nitrophenacyl)-1,10-phenanthrolinium N-ylide (4) with activated alkynes giving new pyrrolo [1,2-a] [1,10]phenanthrolines 7a-d.The structure of trans-5a was assigned on the basis of the coupling constants of the hydrogen atoms from the pyrroline and phenanthroline moieties, 1 H-NMR chemical shifts of CH and NCH groups, as well as 13 C-NMR chemical shifts of the carbonyl groups.
The most characteristic feature of trans-5a is the large homoallylic coupling, J 11,8a = 7.3 Hz.The vicinal and allylic coupling constants of the protons in the position 8a (δ = 6.38 ppm, dt), 8 (δ = 5.93 ppm, dd) and 7 (δ = 6.54 ppm, dd) of trans-5a were found to be 2.7 and 2.3 Hz, respectively.The close values of the chemical shifts in the 13 C-NMR spectrum (δ = 162.8and 163.5 ppm) of the two carbonyl ester groups represent a strong evidence that they are grafted on a double bond.
The chemical shift of the ketone group (δ = 193.3ppm) of trans-5a is deshielded by ca. 10 ppm compared to those of the corresponding aromatic analogues 7b,c (solvent CDCl 3 ).This indicated that the COAr group is linked to a Csp 3 .
The primary cycloadduct trans-5a rearranged regio-and stereoselectively, in solution, in the presence of triethylamine, to 1,2-dihydroderivative trans-6a.Most probably, by the action of triethylamine, the deprotonation occurred at H-11 and the resulting allylic carbanion isomerised and then was protonated to give trans-6a.
In solution, in the presence of air, the compound trans-6a aromatized to 7a.The structure of trans-6a was assigned by NMR spectroscopy.Thus, in the 1 H-NMR spectrum, a vicinal coupling constant of 4.5 Hz between H-10 (δ = 4.02 ppm, d) and H-11 (δ = 7.68 ppm, d) was observed.On the basis of the constant value, a trans configuration has been assigned.The different values of the chemical shifts of the carbonyl in the ester groups (δ = 165.8and δ = 173.3ppm) indicated that they are grafted on sp 2 and sp 3 carbon atoms, respectively.Supplementary evidence for the structures of trans-5a and trans-6a was given by COSY and HETCOR experiments.
The formation of stereoisomer trans-5a shows that the 1,3-dipolar cycloaddition is stereoselective and that the N-ylide participates in the reaction in the anti configuration.
The 1,3-dipolar cycloaddition between ylide 4 and activated unsymmetrical alkynes was regiospecific and the pyrrolophenanthroline derivatives 7d,e were obtained (Scheme 2).The structures of the pyrrolophenanthrolines 7a-e were assigned by elemental analysis and NMR spectroscopy.
In the 1 H-NMR spectra of compounds 7b,d, recorded in CDCl 3 , the methylenic protons of the ester group appeared as ABX 3 patterns.In the case of the compounds 7c and 7e the methyl groups in each isopropyl radical were found to be non-equivalent in the 1 H-NMR.
The behavior can be explained by non-coplanarity between pyrrolic and pyridinic moieties, rendering helical chirality to the molecules of 7b,c, at room temperature. 9This behavior renders the molecular framework chiral, thereby explaining the non-equivalence of the diastereotopic methylene and methyl (in the isopropyl group) protons in the 1 H-NMR spectra.Recently, this hypothesis was confirmed by X-ray analysis. 15ecause of the low solubility of compound 7a in CDCl 3 , the NMR spectrum was performed in a CDCl 3 + TFA mixture.We have observed that the coupling constant between protons H-2 and H-3 in CDCl 3 (compounds 7b-e) was 4.3 Hz, whereas in CDCl 3 + TFA (compound 7a) the respective constant is 6.2 Hz.The same coupling constant of ~ 6.2 Hz was observed when 1 H-NMR spectra of compounds 7b-e were recorded in CDCl 3 + TFA.Also, a strong deshielding was observed at protons H-2, H-3 and H-4, respectively (∆δ > 0.85 ppm).Both the coupling constant and the deshielding of protons H-2, H-3 and H-4 may be explained as due to protonation of the nitrogen atom (N-1) in the presence of trifluoracetic acid (TFA).
The reaction of N-ylide 4 with DMAD gave regioselectively the primary cycloadduct trans-5a which rearranged regio-and stereoselectively to trans-6a in the presence of triethylamine.

Experimental Section
General Procedures.Melting points were determined on a Boëtius hot plate and are uncorrected.Mass spectra were recorded using a VG-QMD-1000 Carlo Erba instrument.The IR spectra were recorded on a Nicolet Impact 410 spectrometer, in KBr pellets.The NMR spectra were recorded on a Varian Gemini 300 BB instrument, operating at 300 MHz for 1 H and 75 MHz for 13 C. Supplementary evidence was given by HETCOR and COSY experiments.The product trans-6a, obtained as yellow crystals, being too unstable to be analyzed by elemental analysis or melting point, was fully characterized by NMR spectroscopy.
).The product was recrystallized from nitromethane and yellow crystals were obtained.