Carbenes, furyl

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

Azidofurans lose nitrogen giving products reminiscent of those formed from furyl carbenes and biradicals.278 External nitrenes apparently add to furan double bonds just as carbenes do, and again the initial products collapse very readily to give nonheterocyclic products. The nitrene produced oxidation of N-aminophthalimide reacts as in Scheme 54. The products are relatively stable derivatives of but-2-endial and are therefore of potential synthetic value, initially the butene link is Z but easily isomerizes to E on silica columns.279... 

Experimental Procedure 2.2.9. [4 + 3] Cycloaddition of a Chromium Carbene Complex to a 2-Aminodiene 6-(2-Furyl)bicyclo[5.4.0]undecan-2,4-dione [264]... 

The new class of carbene complexes, acyloxy-carbenes [(CO)sCrC(X)OAc] (X = 2-furyl or CH2SiMe3) have been isolated. These compounds extend the synthetic range of these carbene systems and afford the nitrile [(CO)5CrNCC4-H3O] and isonitrile [(COijCrCNMe], respectively, with HN3. ... 

This greater resistance of the thienyl system to ring opening is reflected in the fact that this carbene is captured more efficiently (27%) by cyclooctane than is the furyl-carbene (7%). Only dimeric products were obtained from the carbene (375) attached to position 3 of thiophene no ring fragmentation was observed (Scheme 119). The reactions of some other carbenes derived from diazomethylthiophenes have been referred to in Section 3.14.2.2. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Surprisingly, the reactipn of (C0)5Cr[C(0)furyl] "Li with (Me3Si)3SiBr in ether to give (CO)5Cr C[OSi(SiMe3)3]furyl is reversible and results in an equilibrium. However, using the tetramethylammonium instead of the lithium salt the conversion of the acylate complex to the carbene complex is quantitative. ... 

Chiral Fischer-type (menthyloxy)(3-furyl)carbene complexes underwent nucleophilic 1,4-addition with lithium reagents in a regioselective and diastereoselective manner. In particular, a reaction with (Z)-2-phenylethenyllithium provided the (Z)-alkene product shown below in excellent %ee <03CEJ5725>. 

Dioxol 2-(2-Furyl)-4-methoxy-carbonyl-5-methyl- E19b, 1316 (Carben-l,3-dipolare Cycloaddition)... 

Acetyl-6-(2-methyl-propenyl)-E19h, 1339 (Carben + En) 1,3-Pentadien l-(2-Furyl)-2-methoxy-4-methyl- E15/1, 171 [ —CH(OR) —R +... 

Photolysis of matrix isolated diazo(2-furyl)methane led to the aldehyde (79) by stereospecific rearrangement of the carbene (80). " The corresponding 3-furyl compound gave the (5-Z)-methylenecyclopropene (81) by ring closure of the initially formed vinylcarbene to give cyclopropene (82) followed by ring opening of the furan. 

With this restriction in mind, other solutions of the same central idea and without the participation of free radical species are conceivable. For instance. 1,4 elimination of acetic acid shown in route H would yield ketene V directly, a contention that finds support in the favorable 1,5 elimination portrayed in XII (see Scheme 54.5). Analogously, a benzylic carbene precursor would also be in a position to give an acetylene if route F of Scheme 54.3 is handled in such a way as to prevent charge development. In fact, carbene XIV that would result from the 1,1 elimination of acetic acid from I, has been shown to give aldehyde III when furyl-phenyl diazomethane (XV) (an efficient carbene generator) was used as precursor. ... 

Generation of chloro(2-furyl)carbene and chloro(2-thienyl)carbene from 2-dichloromethyl-furans 1 or 2-diehloromethylthiophenes 2 and a base, followed by addition to an alkene, gave l-chloro-l-(2-furyl)- or l-(2-thienyl)cyclopropanes, respectively "(for earlier work, see Houben-Weyl, Vol. E19b, p 1001). The reactions were carried out using potassium /crt-butoxide as the base with, or without, 18-crown-6, or solid potassium hydroxide and a catalytic amount of benzyltriethylammonium chloride (TEBAC) (solid-liquid variant of a phase-transfer catalytic system ). Under both conditions, yields of cyclopropanes were usually good, yet the phase-transfer catalytic system seems to be more convenient from a practical point of view. Addition of the above described carbenes to alkenes was stereospecific. 

During the preparation of l-chloro-l-(2-furyl)cyclopropanes a small amount (up to 12%) of 5-tert-butoxy-2-chloromethylene-2,5-dihydrofuran, the addition product of potassium tert-butoxide to the bipolar structure of chloro(2-furyl)carbene, was formed.Cyclopropanes substituted at C 3 in the thiophene ring are sometimes unstable to column chromatography. The sterically more crowded chloro(3-phenyl-2-thienyl)carbene failed to undergo addition to 2,3-dimethylbut-2-ene. ... 

The following is a complete list of citations describing the benzannulations of furyl carbene complexes refs. 82,99,107,118,119,121,130, 132 and 133. 

Perhaps flash vacuum thermolysis of 342 should be tried, for the 2-furyl-carbenes derived from 343 do give ring-opened acetylenes (344-345) in 20-60% yields under such conditions (Eq. 98).381... 

The common alkyne trapping reagent 1,3-diphenylisobenzofuran was used as a precursor towards the synthesis of new analogs of famesyltransferase inhibitor RPR 130401 <04JOC7220>. A rhenium isobenzofuryl carbene complex was also synthesized recently <04OM4121>. As depicted below, thermal rearrangement of the ri -(o-ethynylbenzoyl)-rhenium complex produced the benzo[c]furyl rhenium carbene complex, presumably via a nucleophilic attack of the carbonyl oxygen on the rhenium-bound alkyne. The alkyne rhenium complex and the rhenium carbene complex were both observed at equilibrium. Like all other benzo[c]furans, the rhenium isobenzofuryl carbene species reacted smoothly with... 

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