Tropolone, reactions rearrangement

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

The [2 + 2] cycloadduct of tetrafluoroethene and 1,3-cyclopentadiene, when pyrolyzed at 700° to 750° and 5-mm pressure, produces (as the result of a sigmatropic rearrangement Section 21-10) a mixture of two new substances, each having two double bonds. The pyrolysis mixture, when heated in aqueous ethanoic acid containing potassium ethanoate, forms tropolone in 70% yield. Write equations for the reactions involved, with particular attention to possible structures for the pyrolysis products. 

Irradiation of a-tropolone methyl ether (Formula 8a) first gives the valence tautomer 9a, then the rearranged bicyclic product 18a, and finally the ester 19a (12). The reactions occur in sequence, and each stage requires light (the final transformation of Formula 18 to Formula 19 can also be achieved by acid). The initial reaction has ample precedent (11). The final transformation simply involves addition of water to the enol ether followed by a reverse aldol reaction. 

Dynamics of Intramolecular Metal-Centered Rearrangement Reactions of Tris-Chelate Complexes B. Tris(tropolonates)... 

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

Other conversions of the C ring of colchicine to a benzenoid system have been discussed in Chapter 10, Volume IT, in connection with the preparation of A -acetylcolchinol (344b) and V-acetyliodocolchinol (344a, 344c). Many similar rearrangements have been reported for simpler tropolones. A table of these reactions and a detailed discussion of the possible mechanisms of such transformations have been presented (321). 

Cyclopropanation.1 The key step in a biomimctic synthesis of (i)-colchicine (4) is a regio- and stereoselective reaction of the tricyclic I, prepared in several steps from 5-bromo-2-methoxyphenol, with dimethylsulfoxonium methylide to give a single product (2) in 75% yield. When treated with trifluoroacclic acid at 25°, this product rearranges in 89% yield to the a-tropolone O-methyl ether 3. This product is converted into ( )-co chicine (4) in four known steps. 

Rearrangement to the proper orientation of the tropolone system was accomplished by reaction with p-toluenesulfonic acid in pyridine and then treatment with ammonia to form LXXIV, whose UV-spectrum was identical with that of colchicinamide. Hydrolysis converted LXXIV to desacetamidocolchiceine (LXII), identical with the authentic degradation product (mixed melting point and spectra). 

A related reaction type is the high pressure (6.2 kbar) Diels-Alder reaction of 2-pyrones with cyclopropenone ketals 16 to give isolable bicycloadducts 17, which then undergo cycloreversion, electrocyclic rearrangement, and ketal hydrolysis leading to tropones. Tropolones also have been formed in this way. This tropolone annulation chemistry was the partial subject of a review in 1990, and it was exploited in the syntheses of some naturally occurring tropoloisoquinolines 18-20. ... 

Chlorine also reacts readily with tropolone by substitution, entering the 5-position more readily than does bromine [291,293]. Trichlorotropolones undergo addition reactions with more chlorine to give adducts which rearrange with formation of a six-membered ring [294]... 

Benzotropolones take part in electrophilic substitution reactions, for example halogenation [293,328,331] and coupling with aryldiazonium salts [332]. They may undergo rearrangement to benzenoid compounds in some instances, for example treatment of 3,4-benzotropolone with excess bromine gives 2,3-dibromonaphtho-quinone and with excess concentrated nitric acid leads to 2,4-di-nitro-l-naphthol [333]. Like other tropolones, 3,4-benzotropolone is photochemically converted into a bicycloheptadienone [334]... 

Oxidative para-para coupling of autumnaline (45) delivers the dienone isoandrocymbine (47), which is O-methylated in the subsequent enzymatic step. This very same para-para coupling reaction has also been biosyn-thetically employed in the route to reticuline (19), key intermediate for the preparation of morphine and other important tyrosine-derived alkaloids (see Section 12.2.3). Next, the tropolone ring is established via the aforementioned skeletal rearrangement. Most likely, this process is achieved by a cytochrome P-450-mediated oxidation with subsequent formation of an intermediary cyclopropane ring... 

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