The synthesis of lin-benzoreumycin,lin-benzo-1-methylxanthine,and lin-benzo-1,9-dimethylxanthine

Commencing with 7-chloro-3-methylquinazoline-2,4(1H,3H)-dione ( 9a ), a five step synthesis of 7-methylpyrimido[5,4-g]-1,2,4-benzotriazine-6,8(7H,9H)-dione (lin-benzoreumycin, 6 ) has been accomplished. A synthesis of 1,7-dimethylpyrimido[5,4-g]-1,2,4-benzotriazine-6,8(1H,7H)-dione (lin-benzotoxoflavin, 5 ) employing an intermediate from the preparation of 6 (i.e., 7-chloro-3-methyl-6-nitroquinazoline-2,4(1H,3H)-dione, 9b ) was attempted but could not be accomplished beyond the dihydro precursor of 5 (i.e., 12 ). Compound 9b did lead to successful preparations of 7-methylimidazo[4,5-g]quinazoline-6,8(5H,7H)-dione (lin-benzo-1-methylxanthine, 7 ) and 3,7-dimethylimidazo[4,5-g]quinazoline-6,8(5H,7H)-dione (lin-benzo-1,9-dimethylxanthine, 8 ) by first reacting 9b with ammonia (for 7) or methylamine (for 8 ) followed by reductive cyclization in formic acid.     

Synthesis of 4,6-disubstituted-7-β-D-ribofuranosyl- and arabinofuranosylpyrazolo[3,4-d]pyrimidines and certain related ribonucleosides

Several disubstituted pyrazolo[3,4-d]pyrimidine, pyrazolo[1,5-a]pyrimidine and thiazolo[4,5-d]pyrimidine ribonucleosides have been prepared as congeners of uridine and cytidine. Glycosylation of the trimethylsilyl (TMS) derivative of pyrazolo[3,4-d]pyrimidine-4,6(1H,5H,7H)-dione ( 4 ) with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose ( 5 ) in the presence of TMS triflate afforded 7-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)pyrazolo-[3,4-d]pyrimidine-4,6(1H,5H)-dione ( 6 ). Debenzoylation of 6 gave the uridine analog 7-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidine-4,6(1H,5H)-dione ( 3 ), identical with 7-ribofuranosyloxoallopurinol reported earlier. Thiation of 6 gave 7 , which on debenzoylation afforded 7-β-D-ribofuranosyl-6-oxopyrazolo[3,4-d]pyrimidine-4(1H,5H)-thione ( 8 ). Ammonolysis of 7 at elevated temperature gave a low yield of the cytidine analog 4-amino-7-β-D-ribofuranosylpyrazolo[3,4-d]pyrimidin-6(1H)-one ( 11 ). Chlorination of 6 , followed by ammonolysis, furnished an alternate route to 11 . A similar glycosylation of TMS-4 with 2,3,5-tri-O-benzyl-α-D-arabinofuranosyl chloride ( 12 ) gave mainly the N7-glycosylated product 13 , which on debenzylation provided 7-β-D-arabinofuranosylpyrazolo[3,4-d]pyrimidine-4,6(1H,5H)-dione ( 14 ). 4-Amino-7-β-D-arabinofuranosyl-pyrazolo[3,4-d]pyrimidin-6(1H)-one ( 19 ) was prepared from 13 via the C4-pyridinium chloride intermediate 17 . Condensation of the TMS derivatives of 7-hydroxy- ( 20 ) or 7-aminopyrazolo[1,5-a]pyrimidin-5(4H)-one ( 23 ) with 5 in the presence of TMS triflate gave the corresponding blocked nucleosides 21 and 24 , respectively, which on deprotection afforded 7-hydroxy- 22 and 7-amino-4-β-D-ribofuranosylpyrazolo[1,5-a]pyrimidin-5-one ( 25 ), respectively. Similarly, starting either from 2-chloro ( 26 ) or 2-aminothiazolo[4,5-d]pyrimidine-5,7-(4H,6H)-dione ( 29 ), 2-amino-4-β-D-ribofuranosylthiazolo[4,5-d]pyrimidine-5,7(6H)-dione ( 28 ) has been prepared. The structure of 25 was confirmed by single crystal X-ray diffraction studies.     

Electron impact mass spectrometry of [C7H8N]+ and [C6H7]+ and [C6H7]+ fragment ions produced from o-toluidine and N-methylaniline

An energetic study of the production of [C₇H₈N]⁺ and [C₆H₇]⁺ fragment ions from o-toluidine and N-methylaniline is reported. The mechanisms for the formation of the ions are suggested. Metastable peaks associated with the formation and fragmentation of reactive [C₇H₈N]⁺ and [C₆H₇]⁺ ions were detected and kinetic energy released were determined. The results indicate that the [C₇H₈N]⁺ ion is formed at threshold from o-toluidine with an aminotropylium structure whereas for N-methylaniline the ion is formed with anN-phenylmethaniminium structure. [C₆H₇]⁺ ions are believed to be formed at threshold from the two precursors with a protonated benzene structure.     

Synthesis and Biological Activities of 7‐Arylazo‐7H‐pyrazolo[5,1‐c][1,2,4] Triazol‐6(5H)‐Ones and 7‐Arylhydrazono‐7H‐[1,2,4]triazolo[3,4‐b] [1,3,4]thiadiazines

Two series of 7‐arylazo‐7H‐3‐(2‐methyl‐1H‐indol‐3‐yl)pyrazolo[5,1‐c][1,2,4]triazol‐6(5H)‐ones 4 and 7‐arylhydrazono‐7H‐3‐(2‐methyl‐1H‐indol‐3‐yl)‐[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazines 7 were prepared via reactions of 4‐amino‐3‐mercapto‐5‐(2‐methyl‐1H‐indol‐3‐yl)‐1,2,4‐triazole 1 with ethyl arylhydrazono‐chloroacetate 2 and N‐aryl‐2‐oxoalkanehydrazonoyl halides 5 , respectively. A possible mechanism is proposed to account for the formation of the products. The biological activity of some of these products was also evaluated.     

Murexide reaction of caffeine with hydrogen peroxide and hydrochloric acid. II

In continuation of the study on the murexide reaction of caffeine with 3% hydrogen peroxide/hydrochloric acid and then with ammonia giving a purple coloration, we investigated the oxidation reaction of caffeine with 6% hydrogen peroxide/hydrochloric acid to isolate ten reaction products, 3-hydroxy-4,6-dimethyloxazolo[4,5-d]pyrimidine-2,5,7(3H,4H,6H)-trione 1 , 1,3-dimethylalloxan 2 , murexoin 3 , 1,3,7-trimethyl-2,6,8-trioxo-9-hydroxy-1H,3H,7H-xanthine 5 , 1,3,7-trimethyl-2,6,8-trioxo-1H,3H,7H-xanthine 6 , 1,3,7-trimethyl-2,6-dioxo-8-chloro-1H,3H,7H-xanthine 7, 5-(1,3-dimethyl-1,2,3,4,5,6-hexahydro-2,4,6-trioxopyrimidin-5-yl)-aminomethylene-1,3-dimethyl-1,2,3,4,5,6-hexahydro-2,4,6-trioxopyrimidine ammonium salt 9 , 1,3-dimethylpalabanic acid 10 , 1-methyl-2,4,5-trioxoimidazole 11 , 3-hydroxy-5,7-dimethyloxazolo[5,4-d]pyrimidine-2,4,6(3H,5H,7H)-trione 12 and 4,6,8-trimethyl-1,2,4-dioxazino[6,5-d]pyrimidine-3,5,7(4H,6H,8H)-trione 13 . The oxidation reaction using 6% hydrogen peroxide/hydrochloric acid was found to produce a similar purple coloration to that of the murexide reaction despite no subsequent addition of ammonia, indicating the liberation of ammonia by the oxidation of caffeine. Among the above compounds, the purple colored substance murexoin 3 and the yellow colored compound 9 were both ammonium salts, and compound 5 was the red colored substance. In the present investigation, these three compounds were found to contribute to the coloration.     

New heterocyclic structures. [1,2,5]Thiadiazolo[3′,4′:4,5]pyrimido-[2,1-b][1,3]thiazine and thiazolo[3,2a][1,2,5]thiadiazolo[3,4-d]pyrimidine

Derivatives of two new molecular structures, namely, 7,8-dihydro-6H,10H-[1,2,5]thiadiazolo[3′,4′:4,5]pyrimido[2,1-b][1,3]thiazin-10-one and 6,7-dihydro-9H-thiazolo[3,2-a][1,2,5]thiadiazolo[3,4-d][pyrimidin-9-one, and derivatives of N-substituted sulfamic acid, namely, (8-amino-3,4-dihydro-2H,6H-pyrimido[2,1-b][1,3]thiazin-6-on-7-yl)sulfamic acid and (7-amino-2,3-dihydro-5H-thiazolo[3,2-a]pyrimidin-5-on-6-yl)sulfamic acid, were separated out as by-products in the reduction reaction of 8-amino-3,4-dihydro-7-nitroso-2H,6H-pyrimido[2,1- b][1,3]thiazin-6-one and 7-amino-2,3-dihydro-6-nitroso-5H-thiazolo[3,2-a]pyrimidin-5-one derivatives, respectively, with sodium hydrosulfite. A mechanism of reaction, which hypothesizes the action of sodium hydrosulfite in an asymmetic form, is proposed. The results of single-crystal X-ray investigation on 7,8-dihydro-6H,10H-[1,2,5]thiadiazolo[3′,4′:4,5]pyrimido[2,1-b][1,3]thiazin-10-one (R = 0.032 for 863 reflections) and (8-amino-3,4-dihydro-2H,6H-pyrimido[2,1-b]- [1,3]thiazin-6-on-7-yl)sulfamic acid, sodium salt (R = 0.028 for 3507 reflections) are reported.     

Reactions of the cycloheptatrienyl complexes [MX(CO)2(η-C7H7)] (M=Mo, X=Br; M=W, X=I) with CNBu: X-ray crystal structure of [WI(CO)2(CNBu)2(η-C7H7)]

Reaction of [MX(CO)₂(η⁷-C₇H₇)] (M=Mo, X=Br; M=W, X=I) with two equivalents of CNBu^t in toluene affords the trihapto-bonded cycloheptatrienyl complexes [MX(CO)₂(CNBu^t)₂(η³-C₇H₇)] (1, M=Mo, X=Br; 2, M=W, X=I). The X-ray crystal structure of 2 reveals a pseudo-octahedral molecular geometry with an asymmetric ligand arrangement at tungsten in which one CNBu^t is located trans to the η³-C₇H₇ ring. Treatment of 2 with tetracyanoethene results in 1,4-cycloaddition at the η³-C₇H₇ ring to give [WI(CO)₂(CNBu^t)₂{η³-C₉H₇(CN)₄}], 3. The principal reaction type of the molybdenum complex 1 is loss of carbonyl and bromide ligands to afford substituted products [MoBr(CNBu^t)₂(η⁷-C₇H₇)] 4 or [Mo(CO)(CNBu^t)₂(η⁷-C₇H₇)]Br. Reaction of [MoBr(CO)₂(η⁷-C₇H₇)] with one equivalent of CNBu^t in toluene at 60°C affords [MoBr(CO)(CNBu^t)(η⁷-C₇H₇)], 5, which is a precursor to [Mo(CO)(CNBu^t)(NCMe)(η⁷-C₇H₇)][BF₄], 6, by reaction with Ag[BF₄] in acetonitrile. In contrast with the parent dicarbonyl systems [MoX(CO)₂(η⁷-C₇H₇)], complexes of the Mo(CO)(CNBu^t)(η⁷-C₇H₇) auxiliary, 5 and 6, do not afford observable η³-C₇H₇ products by ligand addition at the molybdenum centre.     

1, 3-dipolar cycloaddition of diazomethane to azolopyridazines. The synthesis of 8-methyl-8H- and 9-methyl-9H-pyrazolo[3, 4-d]-s-triazolo[4, 3-b]pyridazine and 1-methyl-1H- and 2-methyl-2H-imidazo[1, 2-b]pyrazolo[3, 4-b]pyridazine derivatives

The transformations of 7-methyl-7H- and 8-methyl-8H-pyrazolo[4, 3-d]tetrazolo[1, 5-b]pyridazines 1, 2, 9 and 10 into 8-methyl-8H- and 9-methyl-9H-pyrazolo[3, 4-H]-s-triazolo[4, 3-b]pyridazines 7 and 8 , and 1-methyl-1H-and 2-methyl-2H-imidazo[1, 2-b]pyrazolo[3, 4-d]pyridazines 13 and 14 are described.     

Synthesis of 3H-thieno[3,2-c]-1,2-dithiole-3-thione and its reaction with n-butylamine

A convenient synthesis of 3H-thieno[3,2-c]-1,2-dithiole-3-thione (7) is proposed. The reaction of 7 with n-butylamine afforded the N-butylthieno[3,2-c]isothiazole-3(2H)-thione (7a) in dynamically equilibrium [1] with its 3H-thieno[3,2-c]-1,2-dithiole-N-butyl-3-imino isomer 7b. Characterizations and antimicrobial activities of the synthesized products are reported.     

DMF acetals as alkylating and cyclizing agents: A facile route to substituted pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-diones

Summary Several 7-methyl-5-alkyl-2-vinylpyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-diones were prepared. The successful cyclization and alkylation of 6-(-methylbenzylidenehydrazino)-1-methyluracils2a–d using dimethylformamide acetals at high temperature provided6a–d,7a–d, and8a–d. Treatment of6a–d and7a–d with acid afforded 7-methyl-5-alkylpyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-diones9a,b; under the same conditions,3a–d reacted to 7-methylpyrazolo[3,4-d]-pyrimidine-4,6(5H)-dione (4) in good yield.

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DMF-Acetale als Alkylierungs- und Ringschlußreagentien: ein einfacher Weg zu substituierten Pyrazolo[3,4-d]pyrimidin-4,6(5H,7H)-dionen

Zusammenfassung Es wurden verschiedene 7-Methyl-5-alkyl-2-vinylpyrazolo[3,4-d]pyrimidin-4,6(5H,7H)-dione hergestellt. Cyclisierung und Alkylierung der 6-(-Methylbenzylidenhydrazino)-1-methyl-uracile2a–d mit Hilfe von Dimethylformamidacetalen bei hohen Temperaturen ergab6a–d,7a–d und8a–d. Behandlung von6a–d und7a–d mit Säure lieferte die 7-Methyl-5-alkylpyrazolo[3,4-d]pyrimidin-4,6(5H,7H)-dione9a,b; unter den gleichen Bedingungen reagierten3a–d in guter Ausbeute zu 7-Methylpyrazolo[3,4-d]pyrimidin-4,6(5H)-dion (4).

     

Complete 1H and 13C NMR spectral assignment of hydrogenated oxoisoaporphine derivatives

2,3,8,9,10,11‐Hexahydro‐7H‐dibenzo[de,h]quinolin‐7‐one, 5‐methoxy‐2,3,8,9,10,11‐hexahydro‐7H‐dibenzo[de,h]quinolin‐7‐one, 5‐methoxy‐6‐hydroxy‐1,2,3,7a,8,9,10,11,11a,11b‐decahydro‐7H‐dibenzo[de,h]quinolin‐7‐one, 5‐methoxy‐5,6,8,9,10,11‐hexahydro‐4H‐dibenzo[de,h]quinolin‐7‐ol, 5,6,8,9,10,11‐hexahydro‐4H‐dibenzo[de,h]quinolin‐7‐ol and 5,6‐dihydro‐4H‐dibenzo[de,h]quinolin‐7‐ol were prepared by catalytic hydrogenation of oxoisoaporphines or their 2,3‐dihydro derivatives over PtO₂ in acetic acid under mild conditions. Their structures were confirmed and ¹H and ¹³C NMR spectra were completely assigned using a combination of one‐ and two‐dimensional NMR techniques. Copyright © 2003 John Wiley & Sons, Ltd.     

Über Pterinchemie. 69 Mitteilung [1]. Konformationsanalyse von 5,6,7,8-Tetrahydropteroinsäure und 5,6,7,8-Tetrahydro-L-folsäure

Conformational analysis of 5,6,7,8-tetrahydropteroic acid and 5,6,7,8-tetrahydro-L -folic acid In the 360-MHz-¹H-NMR.-spectrum of (6R, S)-9,9-dideuterio-5, 6, 7, 8-tetrahydropteroic acid (racemic) (XIII) (AMX-System, Fig. 4) and (6R, S)-9,9-dideuterio-5, 6, 7, 8-tetrahydro-L -folic acid (diastereomeric) (XVI) the H_a–C(6) and H_a–C(7) show a vicinal coupling constant of 6,7 Hz and the H_a–C(6) and H_e–C(7) one of 3,2 Hz. The first coupling constant provides evidence for an approximate trans-diaxal arrangement of H_a–C(6) and H_a–C(7), and the second for a gauche conformation of H_a–C(6) and H_e–C(7). The tetrahydropyrazine ring in the racemic 5, 6, 7, 8-tetrahydropteroic acid (III) and in the diastereomeric 5, 6, 7, 8-tetrahydro-L -folic acid (XVII) exists therefore in a half-chair conformation with a pseudoequatorial position of the side chain at C(6) (Fig.5).     

Directed regiospecificity of 1,3-dipolar cycloaddition of 2-diazopropane to 4- and 5-substituted pyridazin-3(2H)-ones

6-Methoxy-2-methylpyridazin-3(2H)-one ( 1 ) gave with 2-diazopropane ( 8 ) a mixture of 3H-pyrazolo[3,4-d]-pyridazin-4(5H)-one derivative 12 , as the main product, and -7(6H)-one derivative 10 , as the minor product. On the other hand, 4-substituted pyridazin-3(2H)-ones 2, 3 , and 4 gave 3H-pyrazolo[3,4-d]pyridazin-7(6H)-one 10 , exclusively, while 5-substituted pyridazin-3(2H)-ones 5, 6 , and 7 produced only the isomeric 3H-pyrazolo[3,4-H]pyridazin-4(5H)-one 12 . The 5-phenylsulfonyl derivative 13 gave with 8 by elimination of a molecule of nitrogen, followed by rearrangement, 1,2-diazepine derivative 15 and with an excess of 8 3H-pyrazolo[3,4-d][1,2]diazepine derivative 16. 1 ,2-Dimethylpyridazine-3,6-(1H,2H)-dione and its derivatives 18 and 19 produced 3H-pyrazolo[3,4-d]pyridazine-4,7(5H,6H)-dione derivative 23 , while from 17 and 1-diazoindane ( 24 ) the spiro compound 27 was obtained. The 1,2-dihydro and 3a,7a-dihydro intermediates 21 and 25 were isolated.     

Synthesis of 2-S-dioxo isosteres of purine and pyrimidine nucleosides. II

The preparation of glucosyl and ribosyl derivatives of 3,4,5-triamino-2H-1,2,6-thiadiazine 1,1-dioxide ( 2 ), 7-amino-4H-furazano[3,4-c]- and 7-amino-1H, 4H-imidazo[2,3-c][1,2,6]thiadiazine 5,5-dioxide ( 3 and 4 respectively) is described. Different synthetic approaches have been used.     

Synthesis of 7H-tetrazolo[1,5-c]pyrrolo[3,2-e]pyrimidines and their reductive ring cleavage to 4-aminopyrrolo[2,3-d]pyrimidines

Some new 7,9-disubstituted 7H-1,2,3,4-tetrazolo[1,5-c]pyrrolo[3,2-e]pyrimidines 5 have been synthesized either by diazotization of 4-hydrazino-5,7-disubstituted-7H-pyrrolo[2,3-d]pyrimidines 4 obtained by hydrazinolysis of 4-chloro-5,7-disubstituted-7H-pyrrolo[2,3-d]pyrimidines 3 or via a substitution reaction between 3 and sodium azide. 5,7-Disubstituted-7H-pyrrolo[2,3-d]pyrimidin-4(3H)-ones 2 were obtained by cyclocondensation of 2-amino-3-cyano-1,4-disubstituted pyrroles 1 with formic acid which on chlorination using phosphorus oxychloride afforded 3 . A novel route for the synthesis of 4-amino-5,7-disubstituted-7H-pyrrolo[2,3-d]pyrimidines 6 by the reductive ring cleavage of 5 has been reported.     

Regioselective synthesis of polyfluoroalkyl-substituted 7-(1<Emphasis Type="Italic">H</Emphasis>-1,2,3-triazol-1-yl)-6,7-dihydro-1<Emphasis Type="Italic">H</Emphasis>-indazol-4(5<Emphasis Type="Italic">H</Emphasis>)-ones

3-Polyfluoroalkyl-6,6-dimethyl-7-(1H-1,2,3-triazol-1-yl)-6,7-dihydro-1H-indazol-4(5H)-ones were synthesized with high regioselectivity by 1,3-dipolar cycloaddition of terminal alkynes (phenylacetylene, hex- 1-yne, hept-1-yne, and but-3-yn-1-ol) to 7-azido-6,6-dimethyl-3-polyfluoroalkyl-6,7-dihydro-1H-indazol-4(5H)-ones which were prepared by bromination of 6,6-dimethyl-3-polyfluoroalkyl-6,7-dihydro-1H-indazol- 4(5H)-ones with N-bromosuccinimide in anhydrous carbon tetrachloride, followed by treatment of the corresponding 7-bromo derivatives with sodium azide.     

Chlorination of 1H-pyrrolo[3,2-c]pyridin-4,6(5H,7H)dione (3,7-dideazaxanthine) and its 5-methyl derivative

The chlorination of 1H-pyrrolo[3,2-c]pyridin-4,6(5H, 7H)dione (3,7-dideazaxanthine) ( 2 ) and 5-methyl-1H-pyrrolo[3,2-c] pyridin4,6(5H,7H)dione (1 -methyl-3,7-dideazaxanthine) ( 3 ) with phenylphosphonic dichloride has yielded 4,6-dichloro-1H-pyrrolo[3,2-c]pyridine (2,6-dichloro-3,7-dideazapurine) ( 1 ). A mechanism for the demethylation of the 5-methyl derivative under these conditions is proposed. Ammonolysis of 4,6-dichloro-1H-pyrrolo[3,2-c] pyridine was unsuccessful while catalytic reduction of this dichloro derivative produced 1H-pyrrolo[3,2-c]-pyridine ( 4 ).     

A density functional theory study of the Au7Hn (n = 1–10) clusters

The geometries, electronic, and magnetic properties of the Au₇H_n (n = 1–10) clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. It is found that the Au₇ on the whole retains its triangle structure after hydrogen atoms adsorption and adsorbing hydrogen atoms can stabilize the Au₇ structure. The Au₇H₇ cluster is much higher stability than the neighboring clusters. The pronounced even–odd alternation of the magnetic moments is observed in the Au₇H_n systems indicating Au₇H_n clusters possess tunable magnetic properties by adding even or odd number of H atoms.     

Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Binuclear cycloheptatrienylchromium carbonyls of the type (C₇H₇)₂Cr₂(CO)_n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η⁷-C₇H₇ rings are not found for (C₇H₇)₂Cr₂(CO)_n structures having two or more carbonyl groups. This result stands in contrast to the related (C_nH_n)₂M₂(CO)_n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C₇H₇)₂Cr₂(CO)_n structures have bent trihapto or pentahapto C₇H₇ rings and CrCr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C₇H₇ ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C₇H₇–Cr bond are predicted for (C₇H₇)₂Cr₂(CO)_n (n = 6, 4, 2). The global minima found for the (C₇H₇)₂Cr₂(CO)_n derivatives for n = 6, 5, and 4 are (η⁵-C₇H₇)(OC)₂CrCr(CO)₄(η¹-C₇H₇), (η³-C₇H₇)(OC)₂CrCr(CO)₃(η^2,1- C₇H₇), and (η⁵-C₇H₇)₂Cr₂(CO)₄, respectively. The global minima for (C₇H₇)₂Cr₂(CO)_n (n = 3, 2) have rearranged C₇H₇ groups. Singlet and triplet structures with heptahapto η⁷-C₇H₇ rings are found for the dimetallocenes (η⁷-C₇H₇)₂Cr₂(CO) and (η⁷-C₇H₇)₂Cr₂, with the singlet structures being of much lower energies in both cases.     

Synthesis of C-nor-4,6-secocamptothecin and related compounds

C-nor-4,6-Secocamptothecin 2 , 4-ethyl-4-hydroxy-6-(2-quinolinyl)-1H-pyrano[3,4-c]pyridine-3,8(4H,7H)-dione, lacking the C-ring of camptothecin 1 , and its related compounds 3 and 4 were prepared from ethyl quinoline-2-carboxylate 7 . By an analogous reaction sequences, synthesis of 6-(2-pyridinyl)-1H-pyrano[3,4-c]-pyridine-3,8(4H7H)-dione derivatives 5 and 6 , which contain the B, D, and E ring of 1 , were achieved.