A-Tropolones

Wahrend Tropon mit Lithiumalanat zu 6-Oxo-cycloheptadien-( 1,3) (s. oben) und Tro-polon zu 4,5-Dioxo-cyclohepten reduziert wird erhalt man aus a-Tropolon-methylather unter Ringkontraktion Benzaldehyd10 ... 

The transformation of 7,7-diMeO-CHT to a-, and y-tropolones is also achievable by using anodic oxidation in the key step (equation 18), namely the electrochemical oxidation of an isomeric mixture of diMeO-CHTs prepared by the thermal rearrangement of 7,7-diMeO-CHT yields a mixture of methyl ethers of ji- and y-tropolones. On the other hand, the thermal rearrangement of the ethylene acetal of tropone gives 3,4-dioxyethylene-CHT as a single product due to the difficulty of formation of other isomers, and it yields the ether of a-tropolone upon anodic oxidation. 

Upon exposure to UV light, a-tropolone methyl ether (142), included within chirally modified Y zeolite, has been found to undergo 4 7r-electron disrotatory electrocyclic ring closure to afford " the bicyclic photo-isomer (143). 

An example of the use of this approach in the disrotatory photocyclization of tropolone alkyl ethers is shown in Scheme 2. Thus, irradiation of the 1 1 complex of a-tropolone alkyl ether 10a with host compound 8 in the solid state gave photoproducts 11a of 100% ee and 12a of 91% ee in 11 and 26% yields, respectively [17]. 

Most pyrogallol derivatives did not precipitate either (Table III). A small precipitation occurred with pyrogallol (which may be considered as a hydroxyresorcinol) and it reacts with vanillin/HCl also (7). However, any additional substitution eliminates this reactivity, e.g., gallic acid (Table IV). Purpurogallin did precipitate with formaldehyde. It is not very soluble in the reaction mixture, and it is a tropolone. Structures of this type might interfere, but in tea and probably in wine the known tropolone derivatives are flavonoids (I, 25). 

Simple tropolones also photoisomerize. 7-Tropolone methyl ether (Formula 6) gives the bicyclic photoisomer (Formula 7) on irradiation in aqueous solution (11). In similar fashion the simple a-tropolone methyl... 

The photochemical transformations of simple tropolone methyl ethers can be more complex than simple valence tautomerization. Irradiation of a-tropolone or its methyl ether (Formulas 16a,b) in aqueous solution in quartz vessels gives 4-oxo-2-cyclopentenylacetic acid (Formula 17a) or the methyl ester (Formula 17b) (15,16). 

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. 

A novel (3,3) sigmatropic rearrangement of a hexacoordinate allyl-silicon complex (neutral tetraoxyspirosilicate) to a pentacoordinate complex was recently described242. The allyl group migrates from silicon to the a-carbon of a tropolone ligand242. 

The reactions show that y-tropolones are much more reactive with radical species than a-tropolones thus they suffer attack at the 1-, 5-, and 7-positions that result in dimeric products, among others. 

Dauben, W.G., Koch, K., Smith, S.L., and Chapman, O.L. (1963) Photo-isomerizahons in the a-Tropolone Series The mechanistic path of the a-tropolone methyl ether to methyl 4-Oxo-2-cyclo-pentenylactetate conversion. Journal of the American Chemical Society, 85, 2616-2621. 

The next example deals with the enantioselective photocyclization of a-tropolone alkyl ethers [182]. Cyclization occurs through an allowed 4e -disrotation that can result in opposite optical isomers depending on the direction of rotation (Sch. 37). Racemic products are obtained in solution, but irradiation of crystalline inclusion complexes of a-tropolone alkyl... 

Hinokitiol is a tropolone type natural compound isolated from the wood of Chymacyparis taiwanesis. The compound has been utilized as a natural antimicrobial agent in hair tonics, toothpastes, cosmetics and food supplements. Hinokitiol was evaluated on five different arachidonic acid metabolic pathways for the mechanism of action of anti-inflammatory effects. It has been found to be a potent inhibitor with IC50 values of 0.1 pM against platelet-type 12-LOX and 50 pM against leukocyte-type 12-LOX. It also inhibited soybean 5-LOX enzyme (IC50 = 17 pM). However, hinokitiol had almost no effects on COX-1 and COX-2 enzymes. Similar inhibition profiles were also observed on synthetic tropolone derivatives [168]. 

Also, interesting 7,10-dithiasesquifulvalene-l, 2-quinone derivatives (204) can be obtained by condensation of 2-methylthio-l,3-dithiolylium iodide with a-tropolone derivatives (203) (81CL805). With j8-tropolone (205) and 1,3-dithiolylium iodide (195) in the presence of pyridine the reaction proceeds with formation of the 7,10-dithiasesquifulvalene-1,6-quinone (206) (77CL287). 

The inclusion complex 26, shown in Scheme 11, has been used as a host by Toda and coworkers to carry out a number of enantioselective reactions [231]. For example, irradiation of a 1 1 host-guest assembly of a-tropolone methyl ether 27 and (S,S)-( — )-26, in the solid state gave (lS,5R)-( — )-28 of 100% ee. The authors state that the high stereoselectivity is a result of the steric hindrance to disrotatory ring closure from one direction due to the structure of the host. This leads to the formation of only one enantiomer of the product. More details on this topic are available in Chap. 13. 

Hicks and Brookhart reported the amination of a tropolone triflate using the BINAP/Pd-catalyst as a strategy to prepare new ligand precursors [106]. For example, the coupling the sterically hindered 2,5-difso-propylaniHne proceeded in 86% isolated yield, Eq. (123). 

Hydroxy-1-oxo- Vl/la, 1, 136f. 8-Hydroxy-1-oxo- IV/lc, 183 a-Tropolon 7-Isopropenyl- V/2c, 730 Zimtaldehyd 4-Methoxy- XIII/2a,... 

Caprard HG,Brossi A. Tropolonic colchicum alkaloids. In The Alkaloids. Brossi A, ed. 1984. Academic Press, New York. pp. 48-54. 

An isoquinoline system condensed to a tropolone is the skeleton for a tropoloisoquinoline alkaloid. The general structure of the five alkaloids of this type known so far are derivatives of the tautomers of 10-hydroxy-9H-azuleno[l,2,3-i,j]isoquinolin-9-one (42) and 9-hydroxy-10H-azuleno[l,2,3 i,j]isoquinoline-10-one (43) [88]. Recently, the first tropone-isquinoline alkaloid, pareitropone (44) has been isolated from the roots of the South American Menispermaceae, Cissampebs pareira [48]. 

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