Karahanaenone

Karahanaenone, a terpenoid isolated from oil of hops, has been synthesized by the thermal reaction shown. Identify the kind of pericyclic reaction, and explain how karahanaenone is formed. 

Cycloheptanes.— The C-1—C-2 bond in -y-thujaplicin is essentially single, Co"-/3-thujaplicin-amine complexes have been described, and thermodynamic data on the U -/3-thujaplicin complex have been calculated. The biomimetic cyclization of the silyl enol ether (191) to karahanaenone (192), using methyl-aluminium bis(trifluoroacetate) is almost quantitative (192) is also synthesized by thermolysis followed by desilylation of the silyl enol ether (193) which is readily available from l-bromo-2-methyl-2-vinylcyclopropane and isobutyraldehyde. Dehalogenation of 3-bromo-l-iodo-3-methylbutan-2-one with Zn-Cu couple on alumina in the presence of isoprene yields (192) and minor amounts of the isomers (194) and (195) however, dehalogenation with Fc2(CO)9 favours (195). Acetolysis of karahanaenol tosylate yields anticipated p-menthane derivatives and no filifolene. ... 

The chiral C5 synthetic block 45, the synthetic unit of dolicol, is prepared in the reaction sequence (42 - 45) in which the electrooxidative oxyselenation and the EGA catalysis are effectively employed Karahanaenone 51, an aroma constituent... 

The epoxidation of dehydrolinalyl acetate 35 provides the corresponding epoxide 36 (75 %) which can lead to karahanaenone 38, a key odorous component of hop oil621. The epoxide 36 can be converted to the keto acetate 37 (82%) by an electrogenerated acid (EG Acid)-catalyzed rearrangement. Hydrogenation followed by alkaline hydrolysis of 37 gives 6-hydroxy-2,6-dimethyl-7-octen-3-one (86%) and subsequent thermal dehydration at 200 °C affords 38 (Scheme 3-12). 

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... 

Karahanaenone is formed by a [3,3] sigmatropic rearrangement (Claisen rearrangement). 

Synthesis ofkarahanaenaue. The ionic reaction of NBS in CCI4 at room temperature with a y-ethylcnic tertiary alcohol leads to an -bromotetrahydrofurane. The reaction has been used in a convenient synthesis of karahanaenone (4), a constituent of hop oil, from linalool (1). Thus reaction of (1) with NBS affords 2-mcthyl-2-vinyl-5-(l-bromo-l-methylethyl)tetrahydrofurane (2) in 85% yield. Dehydrohalogena-tion of (2) with collidine at 110 leads to the allyl vinyl ether (3), which immediately... 

This isomerization reaction has been used in the synthesis of some norcarane derivatives. Thus treatment of the epoxide (4) of karahanaenone with sodium ethoxide in ethanol (reflux IS min.) yielded stereospecifically one bicyclic hydroxy ketone (S S-enrf( -hydroxy-3,3.6-trimethylnorcarane-2-onc) in over 80% yield. 

Cycloheptanes.—Two full papers have appeared on subjects mentioned in Vol. 1 Demole and Enggist s karahanaenone synthesis,and the irradiation of eucarvone. " Another paper concerning the irradiation of eucarvone has appeared in this case the substrate was protonated by carrying out the reaction in fluorosulphonic acid at — 78 °C. The products obtained after quenching the reaction with potassium carbonate in methanol are shown in Scheme 5, together... 

An unusual observation was made when diol (31) was treated with BF3-OEt2. The ring-enlarged ter-pene karahanaenone (32) was formed (along with a double-bond positional isomer) the authors reported that other catalysts gave little or no ring-expanded product (equation 19). 

Pulegone (46) undergoes a Baeyer-Villiger reaction with m-chloroperbenzoic acid, and the product, 748, can be rearranged to the cycloheptanedione 749 with diethylaluminum phenyl sulfide, this dione being a potential source of substances related to karahanaenone (737). ... 

Cycloheptanes.—Karahanaenone (2(X)) has been synthesised from linalool (16). The bromohydrin obtained by the reaction of linalool with iV-bromosuccinimide cyclises during the reaction and, on refluxing the resulting furan (199) in collidine, loss of hydrogen bromide occurs and rearrangement to the cycloheptenone takes place on heating the crude product. 

Addition of thiophenol to cyclopropylallene 12, a less common method for the preparation of cist tram mixtures of divinylcyclopropanes, is used in the synthesis of karahanaenone850, an odoriferous constituent of Japanese hop and cypress oil. Karahanaenone is also prepared via a silyloxy-Cope rearrangement of 15 in 54% overall yield (starting with isobutyraldehyde and a mixture of cis- and /rpyridinium chlorochromate oxidation to give 14)851-852,... 

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