Vinylcyclopropanes, thermolysis

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. ... 

Corey and Wollenberg have found it possible to introduce a THP-blocked trans allylic alcohol unit via reaction of 2-cyclopentenone and other cyclic enones with the highly reactive mixed cuprate 28.58 Through hydrolysis, mesylation, and base-promoted cyclization of adduct 29, the vinylcyclopropane 30 was obtained (Scheme 8). Thermolysis at 600 °C caused ring expansion to 31. 

Cycloheptane annelation (7, 212). The mixed cuprate 1 reacts with acid chlorides to afford vinylcyclopropyl ketones. Previously these ketones were prepared from aldehydes by condensation with l-lithio-2-vinylcyclopropane followed by oxidation (7, 192-193). These compounds are rearranged to 4-cycloheptenones on conversion to trimethylsilyl enol ethers, thermolysis, and hydrolysis. ... 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

Even in vinylcyclopropane derivatives in which the vinyl group is anti to the CH group, thermolysis often leads to 1,4-dienes as a result of isomerization to the syn isomer, followed by l,S-shift (e.g. equation 17). ... 

Usually the reaction occurs either on heating the vinylcyclopropane in the liquid phase in a sealed tube at 300°C for 30 min by passing it through a conditioned hot tube at 330°C with a contact time of 4s " or by flash thermolysis at 600°C for 10 However, careful choice of the pyrolytic conditions may be necessary. Thus, as shown in equation 130, thermolysis in lead conditioned Vycor glass at 580°C or in Pyrex glass at 400°C, leads either to the expected bicyclic ketone or to a retroene product. ... 

Vinylcyclopropanes are also suitable starting materials for the construction of five-membered cycles. The vinylcyclopropane rearrangement first discovered in the late 1950s191 192) immediately became the subject of intense mechanistic investigations 173 193,194) ancj was use(j for tjje CyCiopentene anelation 165.171 195 197> Gr cycloheptane anelation 198 200). This reaction was successfully applied to vinylcyclopropanes bearing a silyl moiety by Paquette 137,138> and was found particularly useful for the synthesis of vinylsilanes as part of a five-membered cycle. These have been in some cases subjected to electrophilic substitution 137,138 201 202) (Schemes 58 b, 65, 82-84). The ring expansion takes place when the thermolysis is conducted between 570° and 670 °C and only occurs with those compounds which do not possess 137)sterie inhibition around the carbon-carbon bond (Scheme 76, compare a and e). Incorporation of the double bond (Scheme 76, compare a and e). Incorporation of the double... 

Decomposition of a diazoalkene in the presence of an alkene either by thermolysis," " photolysis or in the presence of transition metals " generates a vinylcyclopropane. The diazo compound can be isolated or formed in situ from the tosylhydrazone of an a, -unsaturated aldehyde or ketone. 

Thus, ( + )-isocomene was prepared in five steps from 2-bromotoluene, methyl vinyl ketene and (Z)-2-bromobut-2-ene. According to the synthetic strategy the key step is the intramolecular meta photocycloaddition of (Z)-3-methyl-6-(2-tolyl)hept-2-ene. By irradiation in cyclohexane at room temperature for four hours, a 1 1 mixture of isomeric photoadducts was obtained in 72 % yield, the constituents of which were easily interconverted photochemically. Thermolysis of the vinylcyclopropanes at 235-240 C gave dehydroisocomene in 57% yield, which was finally hydrogenated over 5 % palladium on carbon to give the expected sesquiterpene in 98% yield. ... 

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