Cyclic enol carbonates

Mercuric triflate has been shown to be a powerful catalyst for the cyclization of alkynyl /-butylcarbonates (36) and (37) giving rise to cyclic enol carbonates under mild conditions. Internal alkynyl carbonate (36) afforded the endo-dig cyclization (g) product (38) selectively, whereas terminal alkynyl carbonate (37) provided only the exo-dig product (39).48... 

Benzylidene-l,3-dioxolan-2,4-dione treated at room temp, with aniline in benzene -> product. Y 83%. - The startg. m., which is a cyclic enol carbonic anhydride and contains two very reactive non-equivalent carbonyl groups can react with nucleophiles by several paths to give a variety of hitherto unavailable structures. F. e. s. B. W. Dominy and R. G. Lawton, Chem. Commun. 1968, 1448. 

Cyclic enol carbonates have been converted to enol triflates in excellent yields using Tf2NPh (eq 28). ... 

Methylene ethylene carbonate can be prepared in a good yield by mercury-catalyzed cyclization (54). Mercuric triflate (Hg(OTf)2), also called mercury(ll) trifluoromethanesulfonate, is a powerful catalyst for the cyclization of alkynyl terf-butylcarbonates giving rise to cyclic enol carbonates imder mild conditions. The S5mthesis is shovm in Figure 2.10. 

The oxidation of the cyclic enol ether 93 in MeOH affords the methyl ester 95 by hydrolysis of the ketene acetal 94 formed initially by regioselective attack of the methoxy group at the anomeric carbon, rather than the a-alkoxy ketone[35]. Similarly, the double bond of the furan part in khellin (96) is converted ino the ester 98 via the ketene acetal 97[l23],... 

Alkylation of enamines with epoxides or acetoxybromoalkanes provided intermediates for cyclic enol ethers (668) and branched chain sugars were obtained by enamine alkylation (669). Sodium enolates of vinylogous amides underwent carbon and nitrogen methylation (570), while vicinal endiamines formed bis-quaternary amonium salts (647). Reactions of enamines with a cyclopropenyl cation gave alkylated imonium products (57/), and 2-benzylidene-3-methylbenzothiazoline was shown to undergo enamine alkylation and acylation (572). A cyclic enamine was alkylated with methylbromoacetate and the product reduced with sodium borohydride to the key intermediate in a synthesis of the quebrachamine skeleton (57i). 

Note 1. The term glycal is a non-preferred, trivial name for cyclic enol ether derivatives of sugars having a double bond between carbon atoms 1 and 2 of the ring. It should not be used or modified as a class name for monosaccharide derivatives having a double bond in any other position. 

Rhodium( I)-catalyzed hydroformylation of cyclic enol acetals 1 leads to acetal-protected syn-3,5-dihydroxyalkanals 2 with extraordinarily high levels (>50 1) of diastereoselectivity (Scheme 5.2) [2]. The diastereoselectivity cannot be ascribed to any obvious steric bias, and serves as a powerful demonstration that the hydroformylation reaction may be subject to exquisite stereoelectronic control. Indeed, while the addition of a pseudo-axial methyl group to the acetal carbon (as in acetonide 3) has a deleterious effect on the rate of the reaction, the sy -diastereomer 4 is still produced selectively, in what is surely a contra-steric hydroformylation reaction. 

Scheme 10.14 rationalizes the divergent behavior of the two catalytic systems in these selective transformations of pent-l-yn-ols. The presence of phosphine ligands promotes the formation of ruthenium vinylidene species which are key intermediates in both reactions. The more electron-rich (p-MeOC6Fl4)3P phosphine favors the formation of a cyclic oxacarbene complex which leads to the lactone after attack of the N-hydroxysuccinimide anion on the carbenic carbon. In contrast, the more labile electron-poor (p-FC6H4)3P) phosphine is exchanged with the N-hydroxysuccinimide anion and makes possible the formation of an anionic ruthenium intermediate which liberates the cyclic enol ether after protonation. 

Carbon-13 shift of common non-aromatic heterocycles with endo- and exocyclic double bonds are reviewed in Table 4.66 [416-432], - Deshieldings of / -carbons induced by carbonyl groups in heterocyclic a, /1-enones due to (—)-M electron withdrawal (e.g. 2-pyrones, coumarins) and shieldings of [ carbons in cyclic enol ethers arising from (+ )-M electron release (e.g. 2,3-dihydrofuran and oxepine derivatives in Table 4.66) fully correspond to the effects described for the open-chain analogs. Outstandingly large shift values are observed for the lithiated carbon in cyclic a-lithium enol ethers (Table 4.66). In terms of its a and / carbon-13 shifts, 2,7-dimethyloxepine is also a typical enol ether [420], Further, 2,6-dimethyl-4-pyrone [421] and flavone [422] display similiar shift values for the a, /1-enone substructure. 

Cyclic nitrone (52) has a cyclic bridge and cyclic enol ether dipolarophile thus all three elements are present in rings (Scheme 13).24 Models show that direct cyclization of (52) is not possible because of the relative positions of the dipole and dipolarophile. However, a pentacyclic cage isoxazolidine was formed by heating (52) presumably epimerization at the phenyl-bearing bridgehead carbon preceded cycloaddition. 

Several interesting synthetic applications of hydroformylation are known. As one example, hydroformylation of enol ethers to afford -hydroxy aldehydes can be extended to catalytic aldol synthesis. The Rh-catalysed hydroformylation of the cyclic enol ether 42 affords the protected. vy -3,5-dihydroxyaldchydc 43 without forming the anti product 44. The regioselectivity of attack on the terminal carbon is also very high [28],... 

The mechanism of this three-component coupling reaction is probably analogous to the aforementioned insertion of acyl chlorides (above). One can imagine assembling an intermediate acylpalladium species either by oxidative addition to an acyl chloride or, in this case, by oxidative addition to the aromatic iodide followed by migratory insertion into carbon monoxide. Once formed, the acylpalladium intermediate can insert into the SCB to furnish a 7-(chlorosilyl)propyl ketone, which cyclizes in the presence of the amine to afford cyclic enol ethers. 

The catalytic system [A] based on RuCl(Cp)(tris(p-fluorophenyl)phosphine)2 (5 mol%), tris(p-fluorophenyl)phosphine (20 mol%), (BU4NPF6, 15 mol%) and N-hydroxysuccinimide sodium salt (50 mol%) led to the selective transformation of pent-4-yn-l-ols into cyclic enol ethers via intramolecular anh-Markovnikov addition of the hydroxy group to the terminal carbon of the triple bond [21],... 

Along these lines, cyclic enol esters, after reaction with dichlorocarbene, were found to undergo smooth expansion to the 2-chlorocycloheptenones on room temperature treatment with ethanolic potassium carbonate (Scheme 41). ... 

Keto stannylenolates can be prepared by the reaction of Sn-O or Sn-N bonded compounds with diketene, which can be regarded as a cyclic enol ester. The adducts formed from bis(tributyltin) oxide can undergo further reaction, with subsequent decarboxylation, to give the same products as those from the simple enolates. Alkylation with alkyl iodides or benzyl or allyl bromides is strongly catalysed by lithium bromide (e.g. Scheme 14-5). Double alkylation can be achieved with HMPA as solvent.120 The product of alkylation before the final hydrolysis is itself a tin enolate, which can be used in reactions with further carbon electrophiles. 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

The cyclic enolate-type anions 35, 39 and 42, prepared from 4-methyl-2-phenyloxazol-5(4/7)-one,< t.62 2-chlorotropone, " and ) -tropolone, respectively, added to the cyclo-propenylium ions at the enolate carbon. Product 43 from the -tropolone reaction further underwent an intramolecular [4 + 2] reaction to give the cage compound 45. ... 

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