Zinc cyclopropane opening

The Clemmensen reduction of j8-diketones (1,3-diketones) is rather complicated. The first step in the reaction of 2,4-pentanedione with zinc amalgam is an intramolecular pinacol reduction leading to a cyclopropanediol. Next the cyclopropane ring is opened in the acidic medium, and a rearrangement followed by a reduction gives the final product, a ketone, with a changed carbon skeleton [924, 925]. The ketone is usually accompanied by small amounts of the corresponding hydrocarbon [924] or an a-hydroxy ketone [925]. 

The esters, 7-ethoxycyclopropyl acetate (7 a) and benzoate (7b) have been synthesized by the addition of the Simmons-Smith reagent 22> to 1-ethoxy vinyl acetate and benzoate, respectively.4) A potential difficulty in this reaction lies in the fact that zinc iodide, a Lewis acid, is generated in the process and may induce cyclopropane ring opening (Section 4.3.3). However, when glyme is used as a solvent, the acid-labile bonds remain intact since the zinc salt is insoluble in this medium.23)... 

One well defined compound in this series8 is the cyclopropane 64 made9 from chloro-propionate esters 63 and sodium in the presence of Me3SiCl. On treatment with ZnCl2, the ring is opened and a zinc homoenolate 65 with internal coordination is formed. This reacts with... 

Reductive opening with zinc only occurs with vicinal diactivated cyclopropanes whereas lithium in liquid ammonia also works with compounds having one activating substituent. In this case the bond overlapping most effectively with a C=0 group is broken The degree of stereoselectivity, however, is largely dependent on the proton source present and on the reaction temperature (equation 35) as well as on the nature of more remote substituents . ... 

Use of the Simmons-Smith reagent for the formation of cyclopropanone precursors has been reported in the case of ethoxy vinyl carboxylates (1) and ketene ketals (2) (Scheme 2). Using 1-ethoxyvinyl acetate and benzoate, it was found that the zinc iodide generated in the reaction tends to favor opening of the cyclopropane ring. However use of glyme as a solvent prevents attack on the ring by this Lewis acid since the zinc salt is insoluble in that medium . 

Zinc in a protic solvent has also been applied to the reductive ring opening of activated cyclopropane derivatives. Usually the most activated cyclopropyl bond was cleaved. Treatment of arylcyclopropyl aryl ketones 7 with zinc in ethanol alforded aryl propyl ketones in excellent yields. The reduction of 1,2-dibenzoylcyclopropane (7f) with zinc/zinc(II) chloride produced 1,3-dibenzoylpropane (81) in quantitative yield. Dimethyl 2-benzoyl-3-phenylcyclop-ropane-l,l-dicarboxylate (7e) was converted to the corresponding (2-benzoyl-1-phenylethyl)malonate (8e) when heated with zinc in methanol. ... 

Formation of al icy die hydrocarbons by the action of sodium (Freund reaction) or zinc (Gustavson reaction) on open chain dihalo compounds 1,3-dichloropropane derived from the chlorination of propane obtained from natural gas is cyclized in the Hass cyclopropane process by treating with zinc dust in aqueous alcohol in the presence of catalytic sodium iodide ... 

Cheap and readily available cyclopentadecanone is alpha-brominated to (30) and ketalised with (29), and the resulting a-bromoketal (31) subjected to base induced elimination of HBr, giving (32). The key asymmetric step is now at hand. The Simmons-Smith cyclopropanation proceeds with very high facial selectivity, giving almost entirely the diastereomer of (33) shown. The result is rationalised on the basis of zinc chelation by the oxygens of the ketal. The synthesis is completed by acid hydrolysis of the ketal (the chiral diol may be recovered for reuse), reductive opening of the cyclopropane ring under Birch conditions, and re-oxidation of the alcohol back to the ketone. 

A special case of stereoelectivity has been encountered in polymerizing a racemic monomer with an optically active one. Minoura et al. (158) carried out the ring-opening copolymerization of propylene oxide with d-camphoric acid anhydride using diethyl zinc and triethyl amine as catalyst. It was found that the products were alternating copolymers which were optically active. The propylene oxide recovered from the copolymerization system was also optically active. An attempt to incorporate preferentially in a polyamide chain one of the optical antipodes of trans-cyclopropane-1,2-dicarbonyl chloride, by polycondensing this racemic monomer with optically active 1,2-propylene diamine failed (159). 

Bis()8-carboalkoxyalkyl)zinc reagents can also be prepared by ring opening of the cyclopropane ring of mixed cyclopropanone acetals ... 

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