Dihydropyrans cyclopropanation

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

All reactions listed in Tables 5-7 were carried out under a nitrogen atmosphere, but with the rhodium or palladium catalysts no noticeable or only minor reduction in cyclopropane yields was observed when air was present. In contrast, air clearly had a yield-diminishing effect in the CuCl P(0-/-Pr)3-catalyzed reactions, especially with cyclohexene and 3,4-dihydropyran. Cyclohexene was oxidized to 2-cyclohexen-l-one, and 3,4-dihydropyran gave 5,6-dihydro-4-pyrone and 5,6-dihydro-2-pyrone, albeit in yields below 8 % 59). 

Reipig (39,40), Pfaltz (41), and Andersson and their co-workers (42) independently showed that these catalysts are capable of effecting the selective cyclopropanation of enol ethers and enolsilanes. Methyl vinyl ketone and acetophenone enolsilanes provide high selectivities in the cyclopropane products, but both isomers are formed equally. The trisubstituted dihydropyran 65 leads to cyclopropane adducts in high diastereoselectivities and enantioselectivities using 55c CuOTf as catalyst. 

For cyclopropanation of very electron-rich alkenes such as vinyl ethers copper(II) trifluoroacetate, copper(II) hexafluoroacetylacetonate or rhodium(II) acetate are the catalysts of choice. Copper trifluoroacetate catalysed cyclopropanation of vinyldia-zomethane with dihydropyran gives the corresponding vinyl cyclopropane adduct in low yield (equation 17). In contrast, catalytic decomposition of phenyldiazomethane in the presence of various vinyl ethers results in high-yield phenylcyclopropane formation (equations 18 and 19)27. 

Where there is an electron-rich olefinic double bond in the diazoketone as in the 2-substituted 3,4-dihydropyran derivative 47, the intramolecular cyclopropanation produces oxatricyclic ketones, e.g. 48 (equation 57)86 87. Rhodium(II) acetate is the catalyst of choice for this transformation. An interesting application of this method is found in a stereoselective synthesis of ( )-/ -chamigrene 49 (equation 5S)88. 

This procedure has been used for the cyclopropanation of cyclohexene, cyclooctene, a-pinene, /8-pinene, hex-l-ene, oct-l-ene, ( )-but-2-en-l-ol, and 2,3-dihydropyran. 

Zeise s dimer can catalyze a rearrangement of the fused cyclopropane 217 to afford the 3,4-dihydropyran 218 in moderate yield (Equation 101) <2003T2765>. 

Treatment of the diazofuran 905 with dirhodium tetraoctanoate leads to decomposition affording 5,6-dihydropyran-2-one 907. The reaction proceeds via ring opening of a zwitterionic cyclopropane intermediate 906 (Scheme 247) <1997TL5623, 2000JOC4261>. 

Diazo-3,6-dihydropyran-2-one 21 is a stable vinylcarbene precursor. Its reaction with 1,4-cyclohexadiene is highly dependent on the chiral Rh catalyst used but results in both C-H insertion and cyclopropanation. Asymmetric cyclopropanation also occurs with various alkenes and reduction of the products provides a route to cycloheptadienes <06JA16038>. 

Condensation of the enol ethers of P-dicarbonyl compounds with dimethylsul-phonium methylide generally takes place by attack on the carbonyl group, leading to furans. However, enol ethers derived from P-keto-aldehydes are attacked first at the double bond to give cyclopropanes. These further react at the carbonyl group, the resulting cyclopropyl epoxides rearranging to dihydropyrans (Scheme 1). 

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