Cyclopropane enolate

The simplest example of such a system is depicted in Eq. (5.28). The lithium enolate of 2-(trimethylsilyl)cyclopropanone is formed by the reaction of [l-(tri-methylsilyl)vinyl]lithium with carbon monoxide (Eq. (5.29)). Treatment of the vi-nyllithium with CO, at atmospheric pressure, in THF at 15 C for 2 h followed by quenching with trimethylchlorosilane at -78 "C afforded a somewhat labile product, which decomposed during the usual hydrolytic work-up. Quenching with tert-butyldimethylchlorosilane/HMPA instead allowed isolation of the products. The major product was a silylated cyclopropane enolate. It is noteworthy that the overall sequence follows a formal [2-1-1 jcycloaddition (Eq. (5.28)). The silylated alle-nolate was also formed as a by-product as the result of a 1,2-anionic silicon rearrangement. 

The competing side reaction is a cyclopropyl anionic ring opening (for a review see ref 13). In the case of isomeric l-benzoyl-2,3-diphenylcyclopropanes, no base-induced isomerization (MeOH/ NaOMe, 65 °C, 4 h) was detected. In many cases these rearrangements require rather drastic conditions but the interconversion can be facilitated by the participation of an adjacent substituent. For example, with an acyl-substituted cyclopropane, enolization permits cisitmns isomerization to proceed without ring opening and therefore without competing anionic structural isomerization. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Tab. 3.4 Cyclopropanation of the chiral enol ether 89 under Simmons-Smith conditions...

Tab. 3.5 Cyclopropanation of the chiral enol ethers 92-95 under Furukawa conditions...

The Simmons-Smith cyclopropanation method has also found application for the a-methylation of ketones via an intermediate cyclopropane. The starting ketone—e.g. cyclohexanone 9—is first converted into an enol ether 10. Cyclopropanation of 10 leads to an alkoxynorcarane 11, which on regioselective hydrolytic cleavage of the three-membered ring leads to the semiketal 12 as intermediate, and finally yields the a-methylated ketone 13 ... 

For cyclopropanation of enol ethers with in situ-generated acyloxycarbene complexes of chromium and in the absence of CO, see reference [10a]... 

The Simmons-Smith reaction has been used as the basis of a method for the indirect a methylation of a ketone. The ketone (illustrated for cyclohexanone) is first converted to an enol ether, an enamine (16-12) or silyl enol ether (12-22) and cyclopropanation via the Simmons-Smith reaction is followed by hydrolysis to give a methylated ketone. A related procedure using diethylzinc and diiodomethane allows ketones to be chain extended by one carbon. In another variation, phenols can be ortho methylated in one laboratory step, by treatment with Et2Zn and... 

This procedure illustrates a new three-step reaction sequence for the one-carbon ring expansion of cyclic ketones to the homologous tt,/3-unsaturated ketones. The key step in the sequence is the iron(III) chloride-induced cleavage of the central bond of trimethyl-silyloxycyclopropanes which me obtained by cyclopropanation of trimethylsilyl enol ethers. The procedure for the preparation of 1-trimethylsilyloxycyclohexene from cyclohexanone described in Part A is that of House, Czuba, Gall, and Olmstead. ... 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

Key features of the cyclopropanation include the ylide acting as a mild base to isomerize the 1,2-dioxines into cis-y-hydroxy enones, followed by Michael addition of the ylide and last by cyclization of the intermediate enolate [35]. It must be noted that the trans-y-hydroxyenones do not give the cyclopropanation. 

Several steps are involved in these reactions. First, the enolate of the (1-kelocstcr opens the cyclopropane ring. The polarity of this process corresponds to that in the formal synthon B because the cyclopropyl carbons are electrophilic. The product of the ringopening step is a stabilized Wittig ylide, which can react with the ketone carbonyl to form the carbocyclic ring. 

Longifolene has also been synthesized from ( ) Wieland-Miescher ketone by a series of reactions that feature an intramolecular enolate alkylation and ring expansion, as shown in Scheme 13.26. The starting material was converted to a dibromo ketone via the Mr-silyl enol ether in the first sequence of reactions. This intermediate underwent an intramolecular enolate alkylation to form the C(7)—C(10) bond. The ring expansion was then done by conversion of the ketone to a silyl enol ether, cyclopropanation, and treatment of the siloxycyclopropane with FeCl3. 

Nitrocyclopropane-l-carboxylate is prepared in 71% yield by nitration of the enolate derived from the cyclopropane carboxylate with isoamyl nitrate (Eq. 2.24). It is a precursor of... 

The tandem Michael and cyclopropanation reaction of lithium enolates with nitroalkenes gives tricyclic ketones in one pot, as shown in Eq. 7.42.43... 

For cyclopropanations with ethyl diazoacetate, a rather weak influence of the olefin structure has been noted 59 60, (Table 7). The preference for the sterically less crowded cyclopropane is more marked for 1,2-disubstituted than for 1,1-disubstituted olefins. The influence of steric factors becomes obvious from the fact that the ratio Z-36/E-36, obtained upon cyclopropanation of silyl enol ethers 35, parallels Knorr s 90> empirical substituent parameter A.d of the group R 60). These ZjE ratios, however, do not represent the thermodynamic equilibrium of both diastereomers. 

Diverging results have been reported for the carbenoid reaction between alkyl diazoacetates and silyl enol ethers 49a-c. Whereas Reissig and coworkers 60) observed successful cyclopropanation with methyl diazoacetate/Cu(acac)2, Le Goaller and Pierre, in a note without experimental details u8), reported the isolation of 4-oxo-carboxylic esters for the copper-catalyzed decomposition of ethyl diazoacetate. According to this communication, both cyclopropane and ring-opened y-keto ester are obtained from 49 c but the cyclopropane suffers ring-opening under the reaction conditions. 

Participation of 69 in the reaction scheme would also explain why cyclopropanes are obtained from diazoacetic esters, but dihydrofurans from diazoketones 121 In the latter case, the enolate oxygen in 69 is more nucleophilic, thus favoring 1,5-over 1,3-ring closure. 

---