Thujaplicins synthesis

The iron carbonyl-promoted [3 + 4] cycloaddition of bromo-ketones to. 1,3-dienes provides a convenient way, via 2-oxyallyIs, of synthesizing tropones and hence thujaplicins. The synthesis of nezukone (154 X = H)253 has been reported from a,a,a, a -tetrabromoacetone and 3-isopropylfuran. Similarly, from 2-isopropylfuran, /3-thujaplicin (154 X = OH) has been synthesized from the corresponding tropone by known methods, and a-thujaplicin synthesis requires isopropyl substitution in the ketone moiety.254 Karahanaenone (155) is a minor product from pyrolysis of the benzoate (156).149... 

Two new thujaplicin syntheses have appeared. The first (Scheme 20) depends on the presence of trimethylchlorosilane in the acyloin ring-closure in order to avoid Dieckmann condensation to the five-membered ring, and gives pure products. The scheme shows the )5-thujaplicin synthesis, but choice of the appropriate isopropylcyclohexanone leads to a- or y-thujaplicins. A mixture of j - (245)... 

This electroreductive alkylation is successfully applied to the synthesis of /1-thujaplicin. 

The mechanism of this rearrangement has been studied in details by Bartlett85). And this strategy has been applied to the synthesis of P-dolabrin (255)86) as well as P-thujaplicin (257)87). 

Franck-Neumann has reported the synthesis of naturally occurring tropolones -thujaplicin and 3-do-labrin via the readily available [Fe(tropone)(CO)3] as shown in Scheme 23.43 A key step in the sequence is regiospecific acetylation of the [Fe(tropone)(CO)3] complex, a reaction which cannot be carried out on... 

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

CLXXII), which serves as an intermediate in the synthesis of a-thujaplicin (CLXXIII) (Noyori et al., 1975b). 

Cycloheptatriene.—Synthesis. Details have been published on the preparation of specifically substituted cycloheptatrienes by the cycloadditon of cyclopropenes to thiophene-1,1-dioxides followed by expulsion of S02. Substituted tropones have been obtained from the furan adducts of bromo-oxyallyl cations e.g. treatment of l,l,3,3-tetrabromo-4-methylpentan-2-one with di-iron nonacarbonyl in the presence of furan gave adduct (141) which was converted into a-thujaplicin (142). Acid hydrolysis of the norbornyl ketal (143) followed by warming to 70 "C gave 7,7-dimethyl-cycloheptatriene. ... 

The synthetic method for tropolone involving 1,2-cycloaddition of halogenated ketenes to cyclopentadiene followed by solvolytic fragmentation of the resulting adduct provides a very convenient synthesis of tropolone derivatives from readily available starting materials. The synthesis of -thujaplicin (hinokitiol) illustrates this approach. ... 

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