A-Thujaplicin

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]. 

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... 

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). 

Cycloheptanes.—N.m.r. spectral data for beryllium bis-(a-iso-propyltropolonate) have been recorded. The full paper on Noyori s syntheses of nezukone, a-thujaplicin, and jS-thujaplicin or hinokitiol (175 R = Pr ) has been published. Compound (175 R = Pr ) has also been synthesized according to Scheme 4, Other syntheses of /3-thujaplicin (175 R = Pr ) have been... 

This modification has been used to synthesize the tropolones hinokitiol (2, from the adduct of tetrabromoacetone and 2-isopropylfurane) and a-thujaplicin (3, from the adduct of l,l,3,3-tetrabromo-4-methylpentane-2-one and furane). 

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. ... 

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