Rearrangements of Cyclopropanes and Epoxides

Gansauer A, Pierobon M (2001) Rearrangements of cyclopropanes and epoxides. In Renaud P, Sibi MP (eds) Radicals in organic synthesis, vol 2. Wiley, Weinheim, p 207 Gansauer A, Rinker B (2002) Tetrahedron 58 7017 Gansauer A, Narayan S (2002) Adv Synth Catal 344 465... 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Carbanions. These add to the C-3 position of (5)-(l), affording epoxides (3) after intramolecular displacement of the tosylate group and in situ ring closure of the ring-opened intermediate (eq 4). Deprotonation of oxirane (3) leads to rearrangement to cyclopropane derivatives. ... 

This transformation has found extensive use in converting cyclopentenone and cyclohexenone ring systems to the rearranged allylic alcohols during the course of the total syntheses of natural products. In the preparation of ( )-quadrone (Scheme 12), the tricyclic enone was epoxidized and the resulting a,P-epoxy ketone treated with hydrazine to afford the allylic alcohol. The cyclopropane-directed epoxidation shown in Scheme 13 gives an allylic alcohol that is taken on to (-)- and (+)-carenones. In the total syn-... 

The reaction involving 0-hydroxyalkyl selenones is by far the less attractive route to qroxides among those cited above due to the difficulties usually encountered in the oxidation step and the easy rearrangement of the p-hydroxyalkyl selenones to carixmyl compounds (Scheme 184, b and c). This method is, however, particulariy useful for the synthesis of oxaspnopentanes (especially for those bearing a fully alkyl-substituted epoxide ring) ftom l-seleno-l-(r-hydroxyalkyl)cyclopropanes (Scheme 163, e Scheme 18S), which caruiot be obtained under other conditions. ... 

It is noteworthy that irradiation of tetracyclo[6.2.0.0 . 0 ]decan-9-one resulted in cycloreversion and formation of ketene and vinylcyclopropane tricyclo[5.1.0.0 ]oct-5-ene instead of ring cleavage and decarbonylation. A C-C double bond a to a cyclopropane ring also resulted when spiro bicyclo[2.2.1]hept-5-ene-7,l -cyclopropane -2-one eni/o-epoxide underwent a deep-seated rearrangement upon irradiation through Corex to give 5-oxo-spiro[2.4]hept-6-ene-4-ylacetaldehyde in 50% yield. ... 

Cycloheptanes.—One synthesis of karahanaenone (231) depends upon thermal rearrangement of a 2-methylene-5-vinyltetrahydrofuran, and the conditions for this type of reaction have been examined on a simpler model (232), which arises from the dihydrofuran (233) at 140—200 °C. The reaction to the cycloheptenone (234) occurs rapidly at active sites on a glass surface, but is arrested in tubes coated with sodium hydroxide. Higher temperatures and lower pressures give two other compounds (235) and (236). All these reactions involve the biradical (237), as does the conversion of the cyclopropane (238) into the cycloheptenone (234). Karahanaenone (231) has also been made by isomerization of terpinolene epoxide (239) with boron trifluoride etherate. Eucarvone (240 R = H) should not be... 

Ph3Si ethylene oxide catalytically adds PhSH to give a- and -adducts in 6 1 ratio, and serves as a vinyl dication equivalent with RCu to give alkenes stereoselectively, while epoxides rearrange in the presence of silylated nucleophiles Diphenylmethylsilylation of cyclopropane carboxylates gives the C-silylated ester which then form C-silylated cyclopropyl ketones, P-silyl cyclopropyl carbinols, and alkylidene cyclopropancs, which ring expand to the... 

For example, 1-donor-substituted cyclopropancmethanols may be efficiently produced by cyclopropanation of suitably substituted enol ethers, by reaction of 1-donor-substituted 1-lithio-cyclopropanes with carbonyl compounds, or by addition of carbon nucleophiles to 1-donor-substituted cyclopropanecarbaldehydes. Oxaspiropentanes, important precursors of cyclobutanones, may as easily be obtained by epoxidation of methylenecyclopropanes, or by reaction of carbonyl compounds with diphenylsulfonium cyclopropanide and l-bromo-1-lithiocyclopropanes, respectively. Moreover, as the stereochemistry of most rearrangements may be efficiently controlled, asymmetric syntheses begin to appear. 

The required oxaspiropentanes are obtained by epoxidation of alkylidenecyelopropanes (A Section 3.2.2.1.), by reacting aldehydes and/or ketones with diphenylsulfonium cyclopropanide (B Section 3.2.2.2.), and by reacting ketones with 1-bromo-l-lithiocyclopropanes (C Section 3.2.2.3.) or diazocyclopropane (D Section 3.2.2.4.). Most widely used is diphenylsulfoni-um cyclopropanide, but for higher than 2,2-disubstituted cyclobutanones the use of alkylidene-cyclopropanes or 1-bromo-l-lithiocyclopropanes may be advantageous. Once formed, oxaspiropentanes rearrange with extreme ease. 

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