Synthesis of chromeno[2,3-b ]carbazole and chromeno[3,2-f ]indazole derivatives. A new class of indole-and pyrazole-fused polycyclic compounds using o -quinodimethane chemistry. A reactivity and regioselectivity computational study

An efficient route to the new classes of derivatives tetrahydrochromeno[2,3-b ]carbazoles and tetrahydrochromeno[3,2-f ]indazoles has been developed. The cycloaddition reactions of chromones 3 and 4 with indole-o -quinodimethane 2 gave a diastereomeric mixture of Diels– Alder cycloadducts 5 – 8 in good yields, whereas the corresponding reactions of chromones 3 with pyrazole-o -quinodimethane 10 were more regioselective giving only cycloadducts 11 and 12 along with a small amount of the oxidation product 14 , which, however, was the main reaction product in the case of formylchromone. Frontier Molecular Orbital theory (FMO) predicted in all cases a HOMO (QDM) –LUMO (chromone) controlled reaction. The observed regioselectivity was explained on account of the ∆ c difference in absolute values of the p z orbital coefficients of the terminal reacting atoms in HOMO and LUMO, and also from the activation parameters calculated after locating the several transition states (TS) involved. The structure of all new compounds was confirmed by using 1D and 2D NMR data (COSY H-H, NOESY H-H, COSY C-H and COLOC C-H), ms and elemental analysis. Full assignment of all 1 H and 13 C NMR chemical shifts has been unambiguously achieved.


Introduction
Condensed heterocyclic systems are of considerable interest not only because of their potential biological activity but also because of their versatility as synthons in organic transformations.
Concerning the chromone moiety, besides forming the basic nucleus of an entire class of natural products, i.e. flavones, 1 it also forms the important component of pharmacophores of a large number of molecules of medicinal significance. 23][4][5] Moreover, chromone-fused heterocyclic derivatives have attracted a great deal of interest due to their wide applications in the field of pharmaceuticals. 68a 3-Formylchromone represents a very reactive system due to the presence of an α,β-unsaturated keto function, a conjugated second formyl group at C-3 and a center at C-2, which is very reactive towards Michael addition of nucleophiles with opening of the γ-pyrone ring followed by a new cyclization.Although 3-formylchromone has emerged as a valuable synthon for incorporation of the chromone moiety into a number of molecular frameworks, 8 its synthetic utility is limited due to the facile opening of the chromone ring 8,9 and strategies are being developed to circumvent it. 10][8][9] Recently, a synthetic approach involving the reaction of 3-formylchromone with obenzoquinodimethane, formed in situ by sulfur dioxide extraction from 1,3-dihydrobenzo-[c]thiophene and leading to benzo [b]xanthones appeared in the literature.11a-b These results in combination with our continuous interest in the chemistry of o-quinodimethanes 12 (o-QDMs) encouraged us to investigate further utilizations of formylchromones through reactions with heterocyclic o-QDMs, in order to obtain a variety of chromanoheterocycles, novel macrocycles having an intact chromone moiety at the periphery with potential biological applications.So, the possibility of incorporating the chromone moiety into the indole and also into the pyrazole nucleus was examined.Moreover, the study concerning the influence on the reaction regiochemistry by incorporating two different heterocyclic rings into the chromone moiety seemed of interest.In this work we wish to present a full report of our results. 13

Results and Discussion
Our experiments considered the Diels-Alder reactions of 3-formylchromones 3a-3e with two heterocyclic o-QDMs, namely indole o-QDM 2, containing a five-membered ring with one nitrogen heteroatom, and pyrazole o-QDM 10, a heterocycle with two nitrogen heteroatoms in the five-membered ring.
Compounds 2 and 10 were generated in situ through reaction of sodium iodide to the appropriate dibromo-derivatives 1 and 9, respectively, which were prepared by reaction sequences reported in the literature. 14,15Initially, the Diels-Alder reaction of 3a with o-QDM 2 was carried out by the usual procedure, namely in refluxing DMF for 45 min, whereupon from the reaction mixture after separation with column chromatography two fractions of products were isolated.The first fraction contained an inseparable on TLC mixture of the two diastereomers, which proved to be (see Structure Assignments section) the two possible trans diastereomers, 5a and 6a (12% overall yield) in a 10:1 ratio.From this mixture a pure sample of compound 5a was obtained by recrystallization from CH 2 Cl 2 -Et 2 O.The second fraction was shown to be a 2:1, inseparable on TLC mixture of the two possible cis diastereomers, 7a and 8a (7% overall yield, Scheme 1).Because the yields were considerably low, the reaction was repeated by modifying the reaction method.So, the reaction was repeated in boiling toluene for 10 h, using 18-crown-6 ether as a phase transfer catalyst, whereupon substantial improvement was achieved and the same products were isolated in good yields.The 5a-6a mixture was isolated in 31% and the 7a-8a mixture in 26% overall yield, respectively.Therefore, the reactions with substituted 3-formylchromones, 3b-3e, were performed under the same conditions and the results are presented in Table 1.The reaction was also carried out in boiling benzene, where no reaction was observed and also in boiling xylene, whereupon in this higher temperature polymerization of the quinodimethane took place and no addition products were isolated.Attention should also be drawn to the fact that under the reaction conditions deformylation of the expected cycloadducts always occurred 11,16 yielding the products 5-8.ARKAT USA, Inc.Moreover, experiments involving reactions of chromones 4 with indole o-QDM 2, generated in situ through the action of sodium iodide on compound 1 either in dry refluxing DMF for 30 min or in dry refluxing toluene for 20 h in the presence of 18-crown-6 ether were carried out.However, in all cases low yields (2-4%) of the corresponding mixture of cis and trans cycloaddition products, tetrahydrochromeno[2,3-b]carbazoles 5-8 were isolated, proving thus that the presence of the formyl group enhances considerably the dienophilicity of the chromone moiety (see Theoretical Calculations section).
ARKAT USA, Inc. Next, the reaction of 3-formylchromones 3a-3e with pyrazole-o-quinodimethane 10 were studied in boiling toluene for 10 h using 18-crown-6 ether.It is remarkable that in this case the reaction was highly regioselective and mixtures of only two diastereomers 11b-11e and 12b-12e, were isolated in good yields by column chromatography, with the benzoyl group being always on the same side with the pyrane oxygen (Scheme 2, Table 1).The possible regio isomers 13 were never isolated, although small amounts less than 2% were formed, as was observed from 1 H nmr spectra of the crude reaction mixtures.In most cases this mixture was accompanied with small amounts (2-5% yield) of the corresponding oxidation products 14.However, in the case of 3a the oxidation product 14a was isolated as the main reaction product (35% yield) together with 12a (20% yield).Compound 14a most probably is formed by the dehydrogenation of the trans bridgehead hydrogens (4a-H and 10a-H).All formed products were again prone to deformylation under the reaction conditions.It is also notable that in the reactions studied in this work opening of the pyran ring was never observed.However, upon purification of 11e on prep.TLC cleavage of the pyran ring occurred resulting to the hydroxy derivative 15.

Structure assignments
Concerning the stereochemistry of the cycloaddition products the structure of 5a will be analysed.This structure was deduced on account of the following data.For the saturated protons H A -H F (Figure 1a) one proton multiplets at 4.622, 3.499, 3.460, 3.306, 3.102 and 2.824 δ were observed in the 1 H nmr spectrum, correspondingly.The most downfield multiplet at 4.622 δ was attributed to the H A proton next to the chromone oxygen.This proton shows COSY correlations with the 3.102 δ proton and also with the 3.460 and 3.306 δ protons belonging to the 32.07 ppm carbon (C-6), whereas the 3.102 δ proton shows correlations with the 4.622 δ proton and also with the 3.499 and 2.824 δ protons belonging to the 19.82 ppm carbon (C-12).In this way the arrangement of the methylene groups was deduced.Moreover, from the coupling constants of J AD = 9.6 Hz, J AE = 13.3Hz and J EF = 10.8Hz the conclusion of an axial-axial (trans) configuration between these protons could be drawn, whereas the coupling constants of J AC = 6.1 Hz και J BE = 6.1 Hz indicated an axial-equatorial configuration of these protons. 17On the other hand, COLOC correlations between Η C with carbons at 45.67 (C-12a) and 115.87 ppm (C-11b), and between Η Β with carbons at 131.55 (C-6a) and 77.72 ppm (C-5a) indicated that these protons occupy equatorial positions in the cyclohexene ring justifying thus their favorable dihedral angles.The more downfield shift of H C compared to H D and of H B compared to H F can be attributed to the fact that equatorial protons in cyclohexane rings resonate at about 0.5 ppm more downfield than their axial counterparts and also to a small extent to their vicinity with the NCOPh and pyrane carbonyls, the NCOPh group causing also a slight broadening to the H C multiplet.Moreover, the homoallylic coupling constant J DF between the C-6 and C-12 axial protons varies between 2.5 to 3.3 Hz, whereas in the case of axial-equatorial protons is ~1.7 Hz.The minimized energy conformation of compound 5a calculated by AM1 is depicted in Figure 2a.To the minor component the 6a structure was deduced.
Concerning the two cis addition products 7 and 8 the structure of 7e was analysed based on the following data.The chemical shift of the carbon at 76.58 ppm with its proton H A resonating at 5.168 δ reveals its neighborhood to the ether oxygen.This proton shows a ddd multiplet with smaller coupling with its neighbors (J AB = 4.5 Hz, J AC = 4.2 Hz and J AD = 2.7 Hz, Figure 1b).The small coupling constants reveal a chair instead of a boat conformation of the cyclohexene ring, H A having an equatorial configuration.This configuration is also in agreement with the fact that H A shows COLOC correlation with the quaternary carbon C-6a at 131.33 ppm.On the other hand, H D shows a ddd multiplet at 3.222 δ with J DF = 9.4 Hz, J DE = 6.7 Hz and J AD = 2.7 Hz and no COLOC correlation with C-11b.This conformation is more flexible than that of the trans adduct 5; as a result, the methylene protons H B , H C at 6-and H E , H F at 12-positions are not diversified in their chemical shifts as it happens in 5.All COLOC correlations in the aromatic rings are depicted in Figure 1c.The structure which is in agreement with these data most probably has the chromone moiety almost perpendicular to the indole-cyclohexene moiety (Figure 2b).

Theoretical calculations
In order to investigate the reactivity of the reacting species and the regioselectivity of the products 5-8 and 11-12 we have studied the frontier molecular orbital (FMO) interactions of the reactants and the transition structures (TS) of the intermediates involved.Full geometry optimizations were carried out for o-QDMs 2 and 10 and chromones 3 and 4 as well as for the possible adducts and the corresponding transition structures (TS1-TS12) at the AM1 level of theory (Figures 3, 4 and 5).For each located TS after complete optimization with the keywords LET and PRECISE only one imaginary frequency was calculated assigned to the shorter new forming bond. 18The activation energy of almost any reaction is influenced by the polarity of the solvent used.In the present case, since in all studied reactions the conditions are similar, the solvent effects can be ignored, as long as the differences of activation parameters are calculated.In Table 2 the calculated HOMO-LUMO energies (eigenvalues) and the orbital coefficients (eigenvectors) for the atoms of all compounds involved in the [4+2] cycloaddition reaction are presented.According to FMO interactions the energy difference between the two orbitals involved is essential, the smaller the better, so the reaction of the indolo-o-QDMs 2 and pyrazolo-o-QDMs 10 with 3-formylchromones 3 and with chromones 4 are predicted to be HOMO (QDM) -LUMO (chromone) controlled (Figure 6a).The opposite attacking process is predicted to be energetically disfavored by 29 to 81 kcal/mol (Table 3).
Examining the MO coefficients in Table 2 the first attacking position of chromone can be predicted.In all cases the chromone position 5 (see Schemes 1 and 2 for the arbitrary numbering) has the larger coefficient and consequently the new forming bond begins there.In order to examine the regioselectivity of the reactions both the coefficients of the reacting MO and the energetically favored TS were examined.The reactions can proceed either by endo or by exo approach of the reacting species.So, all the possible TS were located and the heats of formation E TS # at the reaction temperature (110 o C, 383 K) were calculated.
According to FMO theory the reaction progress depends on the orbital overlapping in the TS, the larger overlap giving the better yield.In asymmetric π systems, as in our case, the regioselectivity depends on the difference of size of the mutual overlapping orbitals as well as on steric effects, which affect the easy proximity of the reacting species.As a measure for the first factor the difference of the vectors of p z orbitals can be used, whereas for the steric effects the energy of formation of the transition structure (or the free energy of activation ∆G # ) is a good tool.As a result, in the transition state complex the corresponding bond is always shorter.This means that even if the reaction is believed to be concerted, it is slightly asynchronous.In favor of this statement is the fact that both cis and trans isomeric products are present in the crude reaction mixture, exluding the isomerisation induced by the silica gel.In Table 2 ∆c represents the difference in absolute values of the p z orbital coefficients of terminal reacting atoms.The bigger the difference the better regioselectivity can be achieved.So, the fact that the ∆c value (Table 2) of the reacting HOMO of the pyrazole o-QDM 10 is almost double than the corresponding of indole o-QDM 2 is in agreement with the experimental results (Table 1), where in the case of 10 only one regio isomer was isolated.The activation energy factor is also very important.When two possible TS can be involved, the one with the lower activation energy is considered to be kinetically favored.In Tables 4 and  5 the calculated ratio of the intermediate adducts, before formyl elimination, resulting from endo/exo approach 19 of the reacting species is shown in the last column.Since these approaches afford the same products the comparison has only theoretical interest.The selected, from Tables 4 and 5, TS with the lower activation energies corresponding to the favored approach for each one of the studied reactions are given in Table 6.The comparison of these activation parameters showed that in all cases products 7 and 12 are predicted to be favored over 8 and 13, respectively.In Figure 6b the HOMO-LUMO interaction of compounds 2 and 3a in the endo approaching transition state TS1 is depicted.All isolated products resulted after deformylation of the primarily formed cycloaddition products.By adding up the yields given in Table 1 for 5+7, and 6+8 a ratio of approximately 13:2 (or 86:14 %) is obtained, a value very close to the theoretical prediction of 97:3 given in Table 6, calculated in vacuo without solvent effects.On the other hand, products 12 are predicted to be favored over 13 in a ratio of 99:1, which is practically the same with the one found experimentally (98:2) by 1 H nmr of the crude reaction mixture.
is the relative activation energy.h A negative value means a more stable TS for endo approach, the corresponding adduct being kinetically favored.i C endo /C exo is the relative calculated ratio of products 7 derived from endo versus that of exo approach by the Boltzmann equation for equilibrium distribution.j C endo /C exo is the corresponding calculated % ratio of products.The smaller reactivity of chromones 4 can be attributed mainly to the higher activation energy required to reach the TS.The computed activation energies E TS # of reactions between 2+4a and 2+4b for endo approach are ~33 kcal/mol higher than the corresponding ones for 2+3a and 2+3e.To overcome this additional energy the temperature of the reaction must be increased.However, as mentioned earlier, by using boiling xylene as solvent only resinous material was formed.

Conclusions
In conclusion, an efficient route for the synthesis of new classes of fused tetrahydrochromeno carbazoles and indazoles has been described by incorporating the chromone moiety into the indole and pyrazole nuclei, respectively.The new products are formed by the combination of two extremely active biological components and such polycyclic molecules are known to display substantial biological activities. 20In all cases in situ deformylation of chromenone Diels-Alder adducts was observed.Frontier Molecular Orbital theory (FMO) predicted the reaction in all cases to be HOMO (qdm) -LUMO (chromone) controlled.The observed regioselectivity is a result of the ∆c difererence in absolute values of the p z orbital coefficients of the terminal reacting atoms in HOMO and LUMO and of the activation parameters calculated after locating the several TS involved.In addition, the theoretical prediction for the product ratio is in excellent agreement with the experimental results.

Experimental Section
General Procedures.Melting points were measured on a Kofler hot-stage and are uncorrected.
Column chromatography was carried out using Merck silica gel.TLC was performed using precoated silica gel glass plates of 0.25 mm containing fluorescent indicator UV254 purchased from Macherey-Nagel using a 3:1 mixture of petroleum ether-ethyl acetate.Petroleum ether refers to the fraction boiling between 60 and 80 o C. NMR spectra were recorded on a Bruker AM 300 spectrometer at 300 MHz for 1 H and 75 MHz for 13 C, respectively, using CDCl 3 as solvent.
Coupling constants n J are reported in Hz and chemical shifts are expressed in δ values (ppm) relative to TMS as internal standard for 1 H and relative to TMS (0.00 ppm) or to CDCl 3 (77.05ppm) for 13 C NMR spectra.Second order 1 H NMR spectra were analysed by simulation. 17IR spectra were recorded on a Perkin-Elmer 1600 series FTIR spectrometer and are reported in wave numbers (cm -1 ).Low-resolution electron impact mass spectra (EIMS) were obtained either on a VG TS-250 instrument or on a 6890N GC/MS system (Agilent Technology) and elemental analyses performed with a Perkin-Elmer 2400-II CHN analyzer.The MO calculations for minimum energy conformation of compounds were computed with the AM1 method as implemented in the MOPAC package. 21All stationary points were refined by minimization of the gradient norm of the energy to at least 0.005 kcal/mol.Structural assignments of the derived compounds were established by analysis of their elemental analyses, IR, MS, and NMR spectra ( 1 H, 13 C, COSY, NOESY, HETCOR, and COLOC).

General procedure for the Diels-Alder reactions of 3-formylchromones (3a-3e) with indole o-quinodimethane 2
To a stirred solution of 3 (5.0mmol) in dry toluene (25 mL), 18-crown-6 ether was added (0.581 g, 2.2 mmol) followed by the addition of 1-benzoyl-2,3-bisbromomethylindole (1) (0.407 g, 1.0 mmol) and finally sodium iodide (0.33 g, 2.2 mmol).The reaction mixture was stirred at reflux under nitrogen for 10 h.The solvent was distilled off and the resulting residue was subjected to the following procedure in order to remove the excess of chromone, except in the case of nitro derivative.The residue was dissolved in 10 mL of dichoromethane and washed initially with 2×10 mL of 5% sodium hydroxide, then with 5% ammonium chloride and finally with water.
The organic phase was dried with anhydrous sodium sulphate, the solvent was distilled off and the residue was subjected to column chromatography on silica gel using petroleum ether/EtOAc (7:1) as eluent, to give in order of elution an inseparable mixture of 5 and 6 and a second inseparable mixture of 7 and 8.
bis(bromomethyl)pyrazole (9) (0.434 g, 1.0 mmol) and finally sodium iodide (0.33 g, 2.2 mmol) and the reaction mixture was stirred at reflux under nitrogen for 10 h.The solvent was distilled off and from the resulting residue the excess of chromone was removed, as described previously.
The residue was finally purified by column chromatography on silica gel using petroleum ether/EtOAc (7:1) as eluent, to give in order of elution the chromenoindazole 13, the trans tetrahydrochromenoindazole 11 followed by the cis isomer 12.
From compound 3a

Figure 1 .
Figure 1.NOESY and COLOC correlations between protons and carbons in the saturated part of fused rings of compounds 5a (a) and 7d (b).(c) COLOC correlations in the aromatic rings in compound 7d.

Figure 3 .
Figure 3. Transition structures optimized at AM1 level for the interaction of o-QDM 2 with chromone 3a for endo or exo approaching leading to product 7 (TS1, TS2) and 8 (TS3, TS4).The values of bond lengths involved in the new cyclohexene ring along with the heats of formation of the activated complex at TS and the vibrating frequency of the shorter new forming bond at 383 K are given.All bond lengths are in angstroms (Å) and energies in kcal/mol.For the numbering of reacting atoms see Scheme 1.

-8 and 11-12
b a Only the cis isomer 12a is present.b In all cases compound 14 is present in ~3%.

Table 2 .
Calculated HOMO-LUMO energies (eV) and orbital coefficients (eigenvectors) for the atoms involved in new bond formations a for the reaction of o-QDMs 2 and 10 with chromones 3 and 4 (in gas phase at 298 K, AM1) a For atom numbering of the diene moieties and chromones see Schemes 1 and 2. b HOMO-LUMO energies (eigenvalues, in eV).c Orbital coefficients (eigenvectors).d ∆c is the difference in absolute values of the p z orbital coefficients of the terminal reacting atoms 1-4 (HOMO).e As above, for 5-6 (LUMO).f N-LUMO.

Table 6 .
Calculated product ratio 7/8 and 12/13 according to their lower calculated activation parameters for endo or exo approach (in gas phase, 383 o K, AM1) a E TS is the lower transition state energy of the activated complex leading after deformylation to compound 7, 8, 12 or 13, correspondingly.