Y-Tropolones

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. 

Furthermore, the homologous complex 286 condenses with ethyl orthoformate to give, in the course of a new y-tropolone synthesis, tropolone ether 287 by O-ethylation it is transformed to tropylium salt 288 (Scheme 72 70JA6382) the corresponding tropone complex was synthesized from dibenzotroporic and Cr (CO)6 [83AG(S)734]. 

The photochemical reactions of three a/3-unsaturated cyclic ketones have been reported. In these cases, it appears that the reaction is that of the olefinic group although the wavelength of the incident radiation is at the ultraviolet absorption of the carbonyl group. Photolysis of 2-cyclo-pentenone (16), carvone (XXXV) (7,12) and y-tropolone methyl ether... 

Another Claisen rearrangement leads from y-tropolone 2-chloroallyl ether 629 to furotropone 631 [93H(36)1725 94MI2], TTie same product can be obtained from the corresponding diastereomeric 3-chloroallyl ethers 630 when oxygen is excluded (94BCJ2803). 

In an open vessel, y-tropolone allyl ether (632) predominantly gives allyltropolone 633 (and, by dehydrogenation, traces of 634 and 635) 633, however, affords 634 and 635 as the main products (Scheme 171). Pyran 634 suffers autoxidation (Section IV,A,4,a). On oxidation by CAN, y-tropolone and allyl derivative 633 dimerize to ditroponofurans 637 and 635, respectively. 

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. 

Oxabicyclo[3.2.1]oct-6-en-3-ones (75) are also readily transformed to tropones (97) or y-tropolones (98) in a few steps. The tricyclic adduct (75 R = (CH2)9) can be readily converted to troponophane (101). Successful use of tetrabromoacetone and tribromo derivatives of methyl alkyl ketones in these reactions opens a new route to various naturally occurring troponoid compounds, viz. nezukone (102), a-thujaplicin (103) and hinokitiol 0-thujaplicin) (104), = respectively (Scheme 24). 

Diols, e.g. 78, may be converted in quantitative yield to y-tropolone derivatives in a similar two-step sequence. 

The method has been used to synthesize 4-hydroxytropone (y-tropolone). Thus catalytic decomposition of 1,7-bis(diazo)-4-chloroheptane-2,6-dione (3) leads to 6-chlorocyclohept-2-ene-l, 4-dione (4) loss of HC1 on treatment with weak bases affords 4-hydroxytropone (5). 

The copper-catalysed decomposition of bisdiazoketones has been developed into a synthesis of seven-membered rings for example, treatment of 1,7-bisdiazoheptane-2,6-dione and l,7-bisdiazo-4-chloroheptane-2,6-dione with copper bis(acetylace-tonate) in benzene gave cyclohept-2-ene-l,4-dione and 6-chlorocyclohept-2-ene-l,4-dione respectively the latter gave y-tropolone when treated with triethylamine. Treatment of the vinylic chloride (21) with n-butyl-lithium gave bicyclic methylene cyclopropane (22) which could be reduced selectively at the C-5—double bond by di-imide. ... 

---