Addition to ketene

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

The following reactions are believed to result from addition to ketene intermediates produced from type I cleavage reactions ... 

Michael addition in the absence of any catalytic agent has been reported for dialkyl and diaryl phosphites and thiophosphites with a-cyanoacrylate esters and a-cyanoacrylic acid.444 Yields of the conjugate addition products were moderate to good. The regiochemistry of this process is the opposite of that previously reported for similar additions to ketene acetals, the latter presumably proceeding by initial protonation of the distal olefinic carbon site.445... 

Ovchinnikov, V.V., Cherezov, S.V., Cherkasov, R.A., and Pudovik, A.N., Reactivity of cyclic and acyclic hydrophosphoryl compounds in reactions of electrophilic addition to ketene acetals and enamines, Zh. Obshch. Khim., 55,1244, 1985. 

Fig. 6.6. HOMO-LUMO interactions in the [2 + 2] cycloaddition of an alkene and a ketene. (a) Frontier orbitals of alkene and ketene. (b) [2ks + 2na Transition state required for suprafacial addition to alkene and antarafacial addition to ketene, leading to R and R in cis orientation in cyclobutanone products, (c) [2ns + (2ns + 271,)] alternative transition state.

New examples of [n2 +, 2] photocycloaddition of maleic anhydride (318) to alkenes have been reported.262 The major product of addition to ketene (319), for example, is the spiro cyclobutane (320).263 The stereoselective addition of dichlorovinylene carbonate to phenanthrene has been described,264 and the photoaddition of this carbonate (321) to the alkene (322)... 

Type II Asymmetric Alcohol, Phenol, Enol, and Amine Addition to Ketenes... 

Addition to ketenes. n-Butyllithium adds to diethylketene to form an enolate la), which can be alkylated (equation 1). The product is not available by conventional base-catalyzed alkylation.°... 

Use as a Chiral Auxiliary. (5)-Ethyl lactate has been used as a chiral auxiliary in a variety of simple Diels-Alder reactions. As the fumaric acid diester, the de employing cyclopentadiene can almost be completely reversed by addition of Titanium(IV) Chloride (eq 8). In general, superior de values are achieved using (R)-Pantolactone in this context, and also for base-mediated addition to ketenes. ... 

The stereoselectivity of chloro(niethyl)carbenoid addition to ketene alkyl silyl acetals has been investigated. ... 

Several moihanistic studies on the reactivity of sulphur-containing quinquevalent phosphorus acids have been recorded. The reactions investigated include addition to keten acetals, protodemetallation with tetraethylstannane, addition to methyl propiolate (for which there is c/j-addition in propanol but a lack of stereospecificity in aprotic media), and addition to acrylonitrile, with which five-membered-ring acids react faster than six-membered, which in turn react faster than acyclic acids. 

Addition to ketenes,35 When a cold (—78°) methylene chloride solution of diazomethane is added to an excess of ketene, cyclopropanone can be identified as the product by various spectral properties and chemical reactions. In the same way,... 

Luning. U. Miiller. M. Gelbert. M. Peters. K. von Schnering, H.G. Keller. M. Concave reagents. 15 New concave 1.10-phenanthrolines Catalysts for the alcohol addition to ketenes. and ligands in transition metal complexes. Chem. Ber. 1994. 127. 2297-2306. 

The ketene 25g prepared by dehydrochlorination has been produced on an industrial scale and gives a highly stereoselective reaction with R-panto-lactone forming the product ester in 64% yield (Eqn (4.11)). Related chiral 2-arylacetic acid derivatives have important pharmaceutical appHca-tions, of which ibuprofen, now sold as Advil, and naproxen, now sold as Aleve, are synthesized by this general route.Stereoselective additions to ketenes of appropriate chiral alcohols provide usefiil routes to the particular enantiomers needed for pharmaceutical application. 

Quantitative information for the kinetics of nucleophUic additions to ketenes is available from hydration reactions as well as from aminations. Detailed summaries of these studies are available in previous reviews, - and only selected data are given here. 

Mechanistic investigations of additions to ketenes continue to focus on which double bond reacts first, and on the role of the solvent many theoretical studies probe the latter by systematic incremental inclusion of a series of solvent molecules in the calculation. Experimentally, similar effects for the catalyst are often seen in its kinetic order. 

Synthon 14 An analogue of the previous pattern, a nucleophilic addition to ketene carbon with a participation of the nitroso-group the formation of the C -C bond might be carried out by a nucleophilic addition to... 

The maneonenes 50 are tricyclic diethers found in the marine alga Laurencia nidifica [97-99] (Fig. 6). Synthetic analogues like 54 have been prepared by Renaud and Vionnet through radical addition to ketene acetal 51 (Scheme 7) [100]. This generates radical 52 which isomerizes into 7-oxa-2-norbomyl radical 53. Allylic quenching of radical 53 with allyltributyltin produces tricyclic compound 54. 

The stereochemical outcome of numerous fundamental chemical transformations such as decarboxylation of malonates, tandem hetero-Michael addition and nucleophilic addition to ketenes, photodeconjugation of unsaturated esters, pro-... 

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