The copper-catalyzed carbomagnesiation (or hydrometalation) reaction of chiral cyclopropenylcarbinol derivatives, obtained by means of a kinetic resolution of secondary allylic alcohols, leads to an easy and straightforward preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn-carbometalation followed by a syn-elimination reaction. To gain further insight into the reaction mechanism of the carbometalation, the diastereoselective formation of cyclopropylcarbinol was also achieved and was found to be very sensitive to the nature of the organometallic species used for the addition reaction. Cyclopropylcarbinol could also be prepared through a diastereoselective reduction of cyclopropenylcarbinol derivatives. Finally, functionalization of enantiomerically enriched cyclopropenylcarbinols into the corresponding acetate or phosphinite derivatives leads, under mild conditions, to various enantiomerically pure heterosubstituted alkylidenecyclopropanes.

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[reaction: see text] Cyclopropenylcarbinol derivatives are regio- and diastereoselectively reduced with LiAlH(4) in Et(2)O into trans-cyclopropylcarbinols as a single isomer. The regioselectivity of the hydroalumination reaction on the cyclopropenyl ring has been mapped out by deuterium labeling experiments.

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Diastereoisomerically pure methylenecyclopropane and methylenecyclopropane carbinol derivatives were easily prepared, in a single-pot operation, from 1,1,2-tribromocyclopropane via a three-component reaction.In the past decades, methylenecyclopropanes (MCP), which are highly strained but stable molecules, have been extensively studied. 1 They are of synthetic interest due to the multiple possibilities for reaction either of one of the three strained bonds (two proximal and one distal bonds) in the cyclopropane ring and by the nature of the reactive exomethylene double bond. 1 Particular attention has been paid to the metal-catalyzed reaction (Cu, 2 Pd, 3 Ni, 4 Pt, 5 Rh, 6 Sn, 7 Mg 8 ) of methylenecyclopropane for the construction of more elaborated carbon skeleton. However, most of these transformations were based on racemic methylenecyclopropanes as very few methods were reported for the preparation of chiral methylenecyclopropanes. 1c,9 On the other hand, Padwa has reported the reaction of 1-phenylsulfonyl-2,3,3-trimethylcyclopropene 1 with n-BuLi followed by alkylation with various electrophiles to produce the corresponding arylsulfonyl-substituted methylenecyclopropanes 2 as the exclusive product (Scheme 1). 10 Scheme 1The product 2 has been rationalized by a removal of the acidic proton with a strong base followed by alkylation via the formation of the allyllithium species. As cyclopropene derivatives are easily prepared and that their usefulness has been increased significantly by recent developments from our group 11 and others, 12 we though to combine the isomerization reaction described above with an easy source of chirality. In this regard, we recently found that the chiral sulfinyl group was the best chiral inductor for controlling the metallotropic equilibrium of allyl species, even for the creation of chiral quaternary centers. 13 In this letter, we would like to disclose our preliminary results for the preparation of chiral methylenecyclopropane derivatives, in a three-component condensation reaction from 1,1,2-trihalogenocyclopropanes, chiral sulfinyl ester and electrophiles. Cyclopropenyllithium 4 is obtained from 1,1,2-tribromocyclopropane 3 (prepared by reaction of 2-bromo-3-methyl-2-butene derivative with bromoform in the presence of cetrimide as phase-transfer catalyst) 14 by a successive 1,2-dehalogenation reaction followed by a halogen-lithium exchange as described in Scheme 2. Then, after addition of (-)-menthyl-(S)-p-toluenesulfinate, 15 the corresponding cyclopropenyl sulfoxide is formed as intermediate. Then, we found after several optimizations, that the optimal condition for the in situ formation of the allyl species 7 was obtained by a deprotonation reaction of the cyclopropenyl sulfoxide 6 with cyclopropenyllithium 4 itself with concomitant formation of the volatile 1,2,2-trimethylcyclopropene derivative. Therefore, two equivalents of 4 are necessary for the completion of the reaction (Scheme 2). After addition of water, the corresponding chiral methylenecyclopropane is obtained in ...