Silyl ethers cyclopropyl

This reaction is extended to the intramolecular ring closure of the intermediate radical 224 with olefinic or trimethylsilylacetylenic side chains [121]. Cu(BF4)2 is also effective as an oxidant (Scheme 89) [122]. Conjugate addition of Grignard reagents to 2-eyclopenten-l-one followed by cyclopropanation of the resulting silyl enol ethers gives the substituted cyclopropyl silyl ethers, which are oxidized to 4-substituted-2-cyclohexen-l-ones according to the above-mentioned method [123]. (Scheme 88 and 89)... 

F. Trimethylsilyl enol ethers are treated with Simmons-Smith reagent to form cyclopropyl silyl ethers. These rings undergo oxidative cleavage and ring expansion when treated with... 

Cyclopropyl silyl ethers undergo ring opening to give allylic fluorides when treated with DAST at room temperature (see Table 5).35... 

Table 5. Treatment of Cyclopropyl Silyl Ethers with...

Conversion of Cyclopropyl Silyl Ethers to Allylic Fluorides General Procedure 55... 

To a solution of a cyclopropyl silyl ether (1 mmol) in CH2C12 (3 mL) was added DAST (1 mmol) at rt under an inert atmosphere. After stirring for 30 min, HaO was added and the resulting mixture was extracted with CH2C12. The extract was dried (Na2S04), filtered and evaporated to afford the crude product. Chromatography (silica gel) gave the pure product yield 45-96%. 

Exposure of cyclopropyl silyl ethers to 4 in acetic acid results in an oxidative Grob fragmentation to give unsaturated acids [Eq. (49)] [86]. 

To a dry DMF (40 ml) solution of cyclopropyl silyl ether (2 mmol) was added drop wise a mixture of anhydrous FeCl3 (4.4 mmol) and dry DMF (40 ml) over 40 min at 0 °C. The yellow solution obtained was further stirred for 1 h under an argon atmosphere at 0 °C. After the reaction, the mixture was poured into water (500 ml) and extracted with ethyl acetate three times (3 X 200 ml). The combined organic layer was washed with water (2 X 200 ml) and dried. After removal of the solvent, the residue was chromatographed on silica gel (ether/petroleum ether = 1/10) to give a bicyclic ketone in 64% yield [325]. 

Hasegawa et al. [85] have explored the electron transfer-promoted regioselective ring opening of cyclopropyl silyl ethers 183 for the generation of cyclopropoxy radical 185 which, upon intramolecular radical cyclization, leads to the formation of spirocydic compound 184 in moderate yields (Scheme 8.51). 

Bridgehead bicyclic cyclopropanols or cyclopropyl silyl ethers 103 (R=H) cleaved to ring-expanded mixtures of cyclic (1-hydroxy ketones 105 and (1-diketones 106 in good overall yields catalyzed by 10 mol% of VO(acac)2 in the presence of oxygen (Fig. 32) [191, 192]. With 3-substituted substrates 103 (R=Me), mixtures of bicyclic endoperoxide hemiketals 104B and (1-hydroxy ketones 105 arose. In separate experiments it was shown that 105 is not oxidized to 106 under the reaction conditions, and thus the products most likely form from... 

Scheme 77 Further example of cascades starting with oxidative opening of cyclopropyl-silyl ethers...

A number of cyclopropyl silyl ethers have been prepared by treating cyclopropanols with chloro-silanes. Most frequently the alcohol is reacted with chlorotrimethylsilane in the presence of a weak base such as triethylamine, or pyridine, but reactions without base present have also been performed. The yields are often good thus, l-(l-ethylidenehexyl)cyclopropanol reacted with chlorotrimethylsilane in diethyl ether and dimethyl sulfoxide containing triethylamine to give l-(l-ethylidenehexyl)-l-trimethylsiloxycyclopropane (1) in excellent yield. ... 

Vinyl (or cyclopropyl) silyl ethers have also been used to generate carbon-centered radicals by treatment with CAN. 3,6-Dihydroxyphthalate esters are produced by dimerization when bisenolsilylated 1,3-diketones are treated with CAN. An elegant example is the three-component condensation of cyclopropyl silyl ether, cyclopentenone, and methyl vinyl ether (eq IS). Oxidation of cyclopropyl silyl ether gives the 8-ester radical, which undergoes tandem radical addition processes apparently controlled by electronic effects. Subsequent oxidation and trapping affords the 2,3-disubstituted cyclopen-tanone in an excellent stereoselectivity. Other substrates include tertiary aminocyclopropanes, iV-(silylmethyl)amides, and VUV-dialkylanilines. For example, CAN-mediated oxidation of IVUV-dialkylanilines in water affords the coupling products -tetraalkylbenzidines. ... 

Wittig rearrangement of allyl silyl ethers, followed by Simmons-Smith cyclopropana-tion and Collins oxidation, produces a-cyclopropyl acyl silanes, e.g. 22, in 10-85% yields (Scheme 55)85. 

The transition metal-catalyzed allylation of carbon nucleophiles was a widely used method until Grieco and Pearson discovered LPDE-mediated allylic substitutions in 1992. Grieco investigated substitution reactions of cyclic allyl alcohols with silyl ketene acetals such as Si-1 by use of LPDE solution [95]. The concentration of LPDE seems to be important. For example, the use of 2.0 M LPDE resulted in formation of silyl ether 88 with 86 and 87 in the ratio 2 6.4 1. In contrast, 3.0 m LPDE afforded an excellent yield (90 %) of 86 and 87 (5.8 1), and the less hindered side of the allylic unit is alkylated regioselectively. It is of interest to note that this chemistry is also applicable to cyclopropyl carbinol 89 (Sch. 44). 

Substituted cycloheptenones 25 can be prepared by thermolysis of neat cyclopropyl silyl enol ethers 23 under an inert atmosphere, followed by hydrolysis of the resulting crude silyl enol ethers 24. This procedure is quite general for low molecular weight compounds. The enol ethers 23 are generated from either the cis- or trani-isomers of 22 and rearrange in excellent yields, albeit at different temperatures. 

Ester homoenolates can be made from 3-chloroesters 20 with sodium in the presence of Me3SiCl which traps them as the cyclopropyl silyl ethers4 21, analogues of silyl enol ethers, in a step reminiscent of the acyloin condensation.5 Reaction with aldehydes and ketones again gives y-lactones 19 and Kuwajima has shown that the titanium homoenolate 22 is a true intermediate in this reaction.6... 

Scheme 11. Iron(IlI)-mediated tandem reaction of cyclopropyl silyl enol ether 32...

Vinylcyclopropanols have been prepared by the addition of alkenyl Grignard reagents to a variety of cyclopropanone equivalents. Upon treatment with acid, the vinylcyclopropanols rearrange to o-substituted cyclobutanones. Alternatively, a variety of o-heteroatom-substituted cyclopropyl lithium reagents have been developed. These react with aldehydes and ketones to afford cyclopropylcarbinols which also rearrange to cyclobutanones under acid catalysis.Lastly, vinylcyclopropanols and cyclopropylcarbinols have been prepared by the cyclopropanation of enol silyl ethers and allylic alcohols. 

Even [3-1-2] cycloadditions occur when cyclopropyl ketones are treated with (1) under photolytic or Lewis acid conditions to produce cyclopentane systems. Cyclopropanations of (1) are well known, in all cases adding to the more electron-rich sUy-loxy alkene. The vinylcyclopropanol silyl ethers can be converted into a number of different products, e.g. cyclopentanones, 2-methylcyclobutanones, vinyl ketones, and -alkoxy ketones (eq 10). One clever use of (1) in synthesis is the tandem [2 + 21-Cope process which converts (34) into (35) (eq 11). ... 

Cleavage of l-(trimethylsilyloxy)bicyclo[n.l.O]alkanes (8, 269-270). Lead tetraacetate fragmentation of the exo- and endo-methyl substituted silyl cyclopropyl ethers (1) is essentially stereospecific.2 Thus exo-1 is fragmented to the (E)-alke-noate 2 exclusively and endo-1 is converted into (Z)-2 exclusively. 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

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