A specific oxygenation of phenols to ortho-quinones can be effected by a combination of the transition metal complexes Ti(OiPr)4, Vqacack, Zr(acac),, Zr(0nPrh and tert-butylhydroperoxide (TBHP) or [Mo(0&0] . Py . HMPT. Naphthols, anthracenols, phenanthrols and donor substituted mononuclear phenols are readily converted into the corresponding 1,2qui-nones. Unhindered ortho-naphthoquinones can yield binaphthyls with unreacted starting material by Michael addition.In an attempt to prepare an optically active building block for rhodomycinone synthesis, the Sharpless reaction" was tried with the phenolic olefin 1. Instead of the expected epoxide, the orthoquinone 2 was isolated in 70% yield. Scheme 1 Me MeMechanistic considerations indicated that this transition-metalmediated oxygenation with tert-butylhydroperoxide (TBHP) that was followed by dehydrogenation had considerable potential as a highly selective ortho-quinone synthesis. No reagents for this transformation were known until now; in fact, the 1977 Houben-Weyl volume on quinones states: ,,Man kennt noch kein Oxydationsmittel, das spezifisch zu o-Chinonen fiihrt"21. A few recent examples illustrate the increasing interest for ortho-quinones in ultrasoundpromoted Diels-Alder reactions3), in the preparation of tumorigenic benzo[c]phenanthrenes4', ben~ofurandiones~), or biologically active phenazines6). Furthermore, phenols as starting materials have become generally available by a new selenium-catalyzed variation of the Bayer-Villiger reaction7). We now report on the results of our investigations of the transition-metal-mediated specific transformation of phenols to ortho-quinones.Two general methods of quinone synthesis are available (reviews28-'0'). A variety of reagents are available for dehydrogenation of hydroquinones (for example silver oxide, lead tetroxide, hypervalent iodine'", hydrogen peroxide/iodine12), bis( 1,3-propanediamine)copper(I1) chloride/oxygen13), tetrachloro-ortho-q~inonel~); reviewg5)). Some reagents, such as silver (I1) Phenanthrole und Donor-substituierte einkernige Phenole werden glatt in die entsprechenden 1,2-Chiwne iibergefiiii. Ungehinderte ortho-Napthochinone konnen p t e r Michael-Addition mit nicht umgesetztem Ausgangsmaterial zu Biarylen abreagieren. monium nitrate17-"), are capable of converting hydroquinone dior monomethyl ethers to the corresponding quinones. In the second, more dificult quinone synthesis at least one additional oxygen atom has to be introduced into the aromatic ring. As a rule, this oxygenation is not selective, and mixtures of ortho-and para-quinones result from the reaction of phenols with potassium nitrosodisulfonate20,21), peracetic acid22', potassium pero~ydisulfate~", or ceric ammonium nitrate24) (compare also ref.2)). In the oxidation of alkyl aryl ethers with more conventional oxidants such as lead tetraacetate, dibenzoyl peroxide, chromium(1V) oxide or nitric acid "no derivatives of benzo 1,2-quinones have been obtained from such reactions" 'I. However, high ortho selectivities are obtained in many...

Treatment of I-hydroxy[2.2]paracyclophane (1) with the Mim-(4). This dienone dimerizes at room temperature to afford the oun molybdenum oxodiperoxo complex [Mo(O&O] . Py . Diels-Alder adduct 5, whose structure was confirmed by X-ray HMPT gives 3,4-dihydro-3-hydroxy-4-oxo[2.2]paracyclophane analysis. Recently, we have described the specific oxidation of phenols to ortho-quinones using tert-butyl hydroperoxide and transition metal complexes or the Mimoun molybdenum oxodiperoxo complex [Mo(O2),0]. Py . HMPT']. This reaction is initiated by replacement of one ligand of the complex by the phenol, as depicted in I, followed by oxygenation and dehydrogenation processes ' I. The exact mechanism of the oxygen transfer is as yet unknown. In this paper the reaction of 4-hydroxy[2.2]paracyclophane (1) with [MO(O')~-01. Pya HMPT is described; it provides insight into the nature of the oxygen-transfer step and yields a structurally new type of paracyclophane dimer.Miyahara et aL4) have described the isolation of a 1 : 1 mixture of the ortho-, 2, and the para-quinone 3 obtained by the reaction of 1 with diphenylselenic anhydride (Barton's reagentIn an attempt to improve the yield of 2, which is needed for further transformations, 1 was treated with the Mimoun molybdenum complex. However, only small amounts ( < 1 YO) of the deep red ortho-quinone 2 are formed with two equivalents of [Mo(O2),0]. Py . HMPT at 0°C in CH2C12, the major part of the product mixture consisting of two polar colorless compounds. The ratio of the new products depends strongly on the reaction conditions and the workup procedure. It soon became clear from the mass spectra that the less polar product is monomeric and an intermediate in the formation of the stable, dimeric oxidation product. Traces of acid or Lewis acid catalyze the dimerization.For instance, the NMR measurements in CDC13 have to be carried out immediately after termination of the oxidation since the monomer decomposes under these conditions (ca. 40% after 14 h). The monomer is more stable if the workup is carried out under neutral rather than acidic conditions, the molybdenum catalyst being removed by rapid filtration through a column of silica gel. In this case the monomer can be isolated in 75% yield. On the basis of spectral data (see experimental section) this compound is assigned the structure 4. Hydroxyketones of this type have previously been discussed as reaction intermediates in the epoxidation of various phanes6s7), but they have never been isolated. Interestingly, related hydroxylation products have been identified recently by Effenberger et a1." in the autoxidation of phloroglucinophanes. The structural assignment of 4 is supported by the structure of the dimer as determined by Xray analysis. As shown in Figure 1 this second compound has structure 5, i.e. it is formally a Diels-Alder dimer of 4. For the formation of 5, whose very complex NMR spectral data (experimental section) clearly show its highly asymmetric nature, it must be assumed that the terminal double r

Die intensiven Untersuchungen zur Synthese der klinisch bedeutsamen Anthracycline aus der Daunorubicin-Familie2) haben dazu gefuhrt, daD nahezu alle retrosynthetisch moglichen Ringverknupfungen auch tatsachlich verwirklicht wurden3'. Zur Synthese der enantiomerenreinen Verbindungen hat sich besonders die Verknupfung rnit optisch aktiven, mehr oder weniger voll funktionalisierten AB-Bausteinen bewahrt. Unter den drastischen Bedingungen (AlC13/NaClSchmelze, 180 "C) der Friedel-Crafts-Reaktion rnit Phthalsaureanhydriden als zukunftigen CD-Ringen konnen wegen der Aromatisierung durch Wassereliminierung keine Bausteine rnit benzylischer Sauerstoff-Funktion wie 29 eingesetzt werden4). AuDerdem sprechen neuere Befunde fur eine zumindest partielle Racemisierung der Reaktionsprodukte 5,6). Racemisierungsfrei konnen dagegen die bei tiefen Temperaturen ( -70 "C) durchgefiihrten Michael-Additionen von Cyanophthaliden rnit funktionalisierten Benzochinonmonoacetalen durchgefiihrt werden7-"). Diese Methode erlaubt auch eine Kontrolle der Regiochemie bei nicht symmetrisch substituierten Tetracyclen, da die Chinon-monoacetale sich meist rnit guter Regioselektivitat aus den entsprechenden Bi~acetalen~,'" oder den Hydrochinonmonomethylethern12) erhalten lassen. Fur die symmetrisch substituierten Vertreter hat sich dagegen die oft noch einfacher durchzufiihrende Cycloaddition von chinoiden ABBausteinen rnit ortho-Chinodimethanen bewahrt ' Verfahren war jedoch die nachtragliche oxidative Einfiihrung beider Sauerstoffatome in den 1,4-Anthrachinon-Zwischenstufen zu den 9,10-Anthrachinonen notwendig. Wir hatten uns zum Ziel gesetzt, systematisch zu untersuchen, ob sich der kurzlich durch Einbau von Apfelsaure hergestellte enantiomerenreine AB-Baustein 1815) in einem Schritt zu den voll funktionalisierten optisch aktiven Anthracyclinonen 30 -32 umsetzen IieD.Von den als CD-Bausteinen benotigten Benzocyclobutenonen wie 3 und 4 oder -dionen 9 -11 war bekannt, daD sie sich photochemisch oder thermisch reversibel zu exocyclischen ortho-chinoiden Mono-oder Bisketenen offnen 16,17). Schema 1Die ortho-chinoiden Vinylketene konnen unter CO-Abspaltung in sauerstofffreie Produkte ubergehen 18) oder auch rnit sich selbst zu Dimeren reagierent9). In Gegenwart von Dienophilen werden sie jedoch rnit recht guten Ausbeuten abgefangen, wie insbesondere die Umsetzung rnit Naphthochinon belegt 20). Es schien daher lohnenswert, diese Reaktion ebenfalls mit hochfunktionellen AB-Bausteinen fur die Daunomycinon-Synthese zu untersuchen, zumal im Basler Arbeitskreis mit der Gasphasen-Kurzzeit-Thermolyse von Liebigs Ann. Chem. 1988, 943 -948 0 VCH Verlagsgesellschaft mbH, D

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Zirconium‐catalysed Oxidation of Primary Aromatic Amines to Nitro Compounds Using tert‐Butylhydroperoxide A broad range of primary aromatic amines (1a–x) with electron donating and accepting substituents are oxidized in good to excellent yields to the nitro compounds 3a–x using tert‐butylhydroperoxide as the oxidant and Zr(OtBu)4 as the catalyst. The corresponding nitroso compounds 2m, 2n, 2s and 2u can be isolated in the conversion of electron‐rich anilines 1m, 1n, 1s and 1u. The aminopyridines 5a–d are also converted to the corresponding nitropyridines 6a–d, but in lower yields (41–47%).