Cyclopropane ring formation

The next step is not immediately obvious. The generation of an ethyl ester from a lactone can be accommodated by transesterification (we might alternatively consider esterification of the free hydroxyacid). The incorporation of chlorine where we effectively had the alcohol part of the lactone leads us to nucleophilic substitution. That it can be SnI is a consequence of the tertiary site. Cyclopropane ring formation from an Sn2 reaction in which an enolate anion displaces a halide should be deducible from the structural relationships and basic conditions. 

Cyclopropane ring formation under electron transfer conditions shows no stereoselectivity. Reduction in dimethylformamide of pure meso- or ( )-2,4-dibromopentene gives the same mixture of cis- and rra s-l,2-dimethylcyclo-pentane [92], Cis- and /ranj-l,3-dibromocyclohexane are both satisfactory substrates for formation of bicyclo[3.1.0]hexane and either isomer of 1,3-dibromocyclopentane affords bicyclo[2.1.0]pentane [93]. Endo-2,endo-6-dibromobomane 16 gives a mixttire of tricyclene and bomane on electrochemical... 

Cyclopropane ring formation has been achieved by electrochemical reduction of the nucleoside derivative 17 [95]. Very highly strained cyclopropanes 18 [90] and... 

Cyclopropane ring formation has been brought about by Michael addition initiated ring closure (sometimes called MIRC). The reaction of methyl 4-bromocrotonate (3) with different nucleophiles either gives the SN2-displacement product 4 or the Michael addition intermediate 5 which finally forms the cyclopropane carboxylate 697 (the configuration of 6 has not been determined). This reaction has been studied with different nucleophiles in the solvent systems tetrahydrofuran-HMPA (20 1) and tetrahydrofuran the bromocrotonate 3 was reacted with 1 equivalent of a 1 M solution of the lithium enolates at —78 for 12 h at room temperature97. 

Other examples indicate how additions of RFI can be used to instigate more extensive chemical transformations such as an oxiranyl carbinyl radical ringopening and cyclopropane ring formation [254,262-264],... 

In the second, aprotic decomposition of tosylhydrazone 101 was shown to proceed with conventional cyclopropane ring formation.148 On catalytic hydrogenation, one of the two products (102) was converted to tricyclo[3.3.0.03, 7]-octane (103) (Scheme 20). This hydrocarbon is not only a dehydrobicyclo[3.3.01-octane but is also of interest because of its bisnor relationship to adamantane. 

Scheme 24). A Li/SeMe exchange must be involved in the intermediate cycloal-kylmethyllithium to explain the cyclopropane ring formation. This lithium me-thaneselenolate displacement has been extended to the stereoselective synthesis of functionalized arylcyclopropanes by reaction with olefinic substrates [30] (Scheme 25). 

Grogan DW, Cronan JE Jr. Cyclopropane ring formation in membrane lipids of bacteria. Microbiol. Mol. Biol. Rev. 1997 61 429— 441. 

Phenylpropanal reacts with allylsilane with 1 2 stoichiometry and cyclopropane ring formation when NbCls is used as the Lewis acid catalyst (Sch. 22). 

C]-mevalonic acid (56a) in Pinus palustris or P. sylvestris showed that the Relabeling is predominantly at (4) . Namely, the C(4) of the monoterpene is derived from C(2) of mevalonic acid (56) rather than C(4) as had been assumed and therefore, the cyclopropane ring formation must be accompanied by a double bond migration if a-car-3-ene (61) is indeed dervied from a-terpinyl ion (62) or its biosynthetic equivalents. 

Figure 26 Cyclopropane ring formation by CmaC in coronamic acid biosynthesis, (a) Characterization of 7-CI loss by ESI-FTMS. (b) Characterization of a-H exchange by deuterium solvent exchange and PEA. CmaC promotes deuterium incorporation at a-C position, (c) Proposed cyclopropane ring formation mechanism.55...

Various types of starting materials leading to homoallylmetal complexes are available as precursors for cyclopropane formation of this type. Instead of an insertion step within the organometallic species other mechanisms of cyclopropane ring formation from homoallylmetal complexes are known, e.g. radical pathways. ... 

Cyclopropanation. The derived ylide (1) is able to participate in cyclopropane ring formation with enones. It is not necessary to use the salt stoichiometically as diisobutyl telluride can be made catalytic. 

Natural Cyclopropanes.—A useful review of the synthesis of pyrethroid acids has been published which includes a discussion of general methods for cyclopropane ring formation. ... 

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