Phenyl(trimethylsilyl)ketene. Some ketene reactions with diazomethane

Phenyl(trimethylsilyl)ketene was prepared by the zinc dehalogenation of phenyl(trimethylsilyl)bromoacetyl chloride. This ketene parallels tirmethylsilylketene in stabibility and lack of reactivity in cycloaddition reactions. The reaction of phenyl(ethyl)-, phenyl(trimethylsilyl)- and trimethylsilylketenes with diazomethane at ?78°C is described. Only the $\text{2}\text{1}$ cycloadducts, the cyclobutanones, could be isolated.     

Photoaddition reactions of 1,2-diketones with silyl ketene acetals. formation of beta-hydroxy-gamma-ketoesters

Photochemical reactions taking place between 1,2-diketones and silyl ketene acetals and their excited state reaction mechanisms have been explored. Irradiation of benzene, acetone, or acetonitrile solutions containing 1,2-diketones and silyl ketene acetals is observed to promote formation of 1,4-dioxenes, resulting from [4 + 2]-cycloaddition, oxetanes, arising by Paterno-Buchi processes, and beta-hydroxy-gamma-ketoesters, generated by SET-promoted Claisen-type condensation. These competitive pathways leading from the excited states of the 1,2-diketones to these products are influenced by solvent polarity and the nature of the silyl ketene acetal and 1,2-diketone. The Claisen-type condensation process, following an SET desilylation pathway and predominating when the photoreactions are carried out in the polar solvent acetonitrile, represents an efficient method to prepare a variety of diversely substituted beta-hydoxy-gamma-ketoesters.     

Synthesis of azoxabicyclo[3.3.1]nonanones based on diastereoselective reactions of 1,1-bis(trimethylsilyloxy)ketene acetals with isoquinolines and quinolines

Densely functionalized azoxabicyclo[3.3.1]nonanones were prepared by regio- and diastereoselective condensation of 1,1-bis(silyloxy)ketene acetals with isoquinolinium and quinolinium salts and subsequent regioselective and stereospecific iodolactonization.     

Synthesis of degradable poly(methyl methacrylate) star polymers via RAFT copolymerization with cyclic ketene acetals

Copolymerization of the cyclic ketene acetal 5,6‐benzo‐2‐methylene‐1,3‐dioxepane (BMDO) with methyl methacrylate (MMA) is studied with respect to its copolymerization parameters and the suitability to control BMDO/MMA copolymerizations via the reversible addition‐fragmentation chain transfer (RAFT) technique to obtain linear and 4‐arm star polymers. BMDO shows disparate copolymerization behavior with MMA and r₁ = 0.33 ± 0.06 and r₂ = 6.0 ± 0.8 have been determined for polymerization at 110 °C in anisole from fitting copolymer composition vs. comonomer feed data to the Lewis–Mayo equation. Copolymerization of the two monomers is successful in RAFT polymerization employing a trithiocarbonate control agent. As desired, polymers contain only little amount of polyester units stemming from BMDO units and preliminary degradation experiment show that the polymer degrades slowly, but steadily in aqueous 1 M NaOH dispersion. Within ten days, the polymers are broken down to low molecular weight segments from an initial molecular weight of M_n = 6000 g mol^?1. Star (co)polymerization with an erythritol‐based tetra‐functional RAFT agent following the Z‐group approach proceeds efficiently and polymers with a number‐average molecular weight of 10,000 g mol^?1 are readily obtained that degrade in similar manner as the linear copolymer counterparts. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1633–1641     

Reactivity of pyridines bearing EWG with bis-(TMS)ketene acetals. Substituent-induced lactonization reaction

The nucleophilic addition of bis-(TMS)ketene acetals to pyridines substituted by electron-withdrawing groups, activated with triflic anhydride, leads to their corresponding dihydropyridine carboxylic acids in a first instance. These acids can be efficiently turned into new bicyclic lactones by either of three different methods: first, bicyclic γ-lactones were successfully obtained by an unprecedented Michael-type ring closing procedure assisted by silica gel; second, bicyclic δ-bromolactones were achieved by a NBS-promoted process; and third, the unexpected spontaneous formation of new hydroxy γ-spirolactones was observed. Along the work presented herein, the scope, limitations, and regioselectivity of these lactonization reactions were studied, confirming the utility of bis-(TMS)ketene acetals as 1,3-dinucleophiles in this type of reactions, extending thus the range of synthesis of a wide variety of new aza-compounds.     

(Trifluoromethyl)ketene silyl acetal as an equivalent to the trifluoropropionic ester enolate: preparation and aldol-type reactions with acetals

The title reagent, readily prepared from methyl β,β,β-trifluoropropionate with trimethylsilyl triflate, is shown to react with a broad variety of acetals to provide the corresponding α-CF₃ β-alkoxy esters in good yields.     

Synthesis of dimethyl acetals. diethyl acetals. and cyclic acetals catalyzed by aminopropylated silica gel hydrochloride (APSG-HCL)

The aminopropylated Silica-Gel hydrochloride (APSG.HCl) proved to be an efficient catalyst for the rapid conversion of carbonyl compounds in the corresponding acetals with high yields and in mild and selective conditions. In addition to the obvious advantages offered by heterogeneous catalysis, the present method results very useful when the presence of a weakly-acidic function chemically bonded on the catalyst surface (alkyl ammonium salt). is necessary (compounds which contains functions unstable in acidic media).     

Acid catalyzed rearrangement of vinyl and ketene acetals

Substituted vinyl and ketene acetals undergo smooth oxygen-to-carbon rearrangement with a catalytic amount of TMSOTf to afford chain-extended ketones or esters, respectively. The developed procedure has been applied to the stereoselective synthesis of C-glycosides from the corresponding anomeric vinyl ethers.     

Isothiourea-catalysed asymmetric C-acylation of silyl ketene acetals

Screening of a range of chiral isothioureas and acyl donors to promote the asymmetric C-acylation of silyl ketene acetals indicates that C(2)-aryl-dihydropyrimidobenzothiazole-derived isothioureas and propionic anhydride give optimal reactivity and enantioselectivity in this process. Under optimised conditions 3-acyl-3-aryl or 3-acyl-3-alkylfuranones are prepared in good yields and moderate to excellent enantioselectivities (up to 98% ee; ee=enantiomeric excess).     

Theoretical studies of five-membered ring ketene acetals

Cyclic ketene acetals are a class of organic molecules characterized by a nucleophilic exo-methylene carbon attached to a carbon with two adjacent O, N, or S atoms. We have carried out a systematic computational study of a series of five-membered cyclic acetals like 2-methylene-1,3-dioxolane and its OS, SS, NO, NS, and NN analogs as well as all the protonated species. The calculations were performed at the MP2 level using a triple zeta plus polarization basis set. The nucleophilicity was discussed in terms of geometrical factors, calculated atomic charges, calculated chemical shifts, and proton affinities. All the six neutral species were strong nucleophiles. The NN analog was predicted to be the strongest and the SS analog the weakest nucleophile.     

Synthesis of N-allylideneamines and their use for the double nucleophilic addition of ketene silyl (thio)acetals and trimethylsilyl cyanide

N-Allylideneamines 1a, b were prepared from acrolein and diphenylethyl or trityl amine in the presence of Ti(OEt)4. Double nucleophilic addition of various ketene silyl (thio)acetals and trimethylsilyl cyanide to these imines proceeded efficiently to give, after workup with TFA, homoglutamic acid derivatives 3 and valerolactam 5.     

Aldol reactions,conjugate additions,and alkylation reactionsof O-silylated ketene acetals under neutral conditions

Aldol reactions, conjugate additions, and alkylation reactions of O-silylated ketene acetals take place under neutral conditions by using a high pressure technique.     

Triplex-catalyzed diels-alder and [2+2] cycloaddition reactions of enol ethers and ketene acetals

Irradiation of benzene solutions containing a cyanoarene sensitizer, 1,3-cyclohexadiene, and a trimethylsilyl enol ether or ketene acetal leads to intermolecular [4+2] and [2+2] cycloaddition products.     

Straightforward access to tetrahydropyridine and piperidine-fused fluorolactones from pyridines and bis(trimethylsilyl)ketene acetals

The interaction of bis(trimethylsilyl)ketene acetals with various pyridines provides a direct, general, diastereoselective access to fluorolactones via the formation of dihydropyridine-substituted carboxylic acids. These in turn reacted with selectfluor as the source of electrophilic fluorine. Whereas a discrimination of the double bonds devoid of fluorine and those bearing fluorine was observed in the case of electrophiles such as iodine, bromine and peracids, no such differentiation took place in the case of selectfluor since, besides 3,5-difluorolactones, the formation of gem-difluorolactones also took place. Moreover, the formation of two stereoisomeric fluorolactones during the lactonization of tetrahydropyridine-substituted carboxylic acids, obtained upon the interaction of (trimethylsilyl)ketene acetals with the previous lactones, could be ascribed to conformational modifications. In all the cases examined a trans, diaxial addition of the electrophile and of the carboxylate is observed. The stereochemical outcome of these reactions was assessed both by NMR spectroscopy and by X-ray crystallography.     

Bis(trimethylsilyl)ketene acetals as 1,3-dinucleophiles. Formation of an anhydride from (pentacarbonyl)(phenylethoxy)tungstacarbene

The carbene complex (CO)₅WC(Ph)OEt (1) reacts with the ketene acetal Me₂CC(OSiMe₃)₂ (2) in the presence of t-BuOK to give the anhydride C₁₂H₁₂O₃ (3) characterized by an X-ray analysis, upon cleavage of both oxygen-silicon bonds, elimination of the ethoxy group, insertion of CO, and elimination of W(CO)₄.     

Lewis base activation of Lewis acids. Vinylogous aldol addition reactions of conjugated N,O-silyl ketene acetals to aldehydes

N,O-Silyl dienyl ketene acetals derived from unsaturated morpholine amides have been developed as highly useful reagents for vinylogous aldol addition reactions. In the presence of SiCl4 and the catalytic action of chiral phosphoramide (R,R)-3, N,O-silyl dienyl ketene acetal 8 undergoes high-yielding and highly site-selective addition to a wide variety of aldehydes with excellent enantioselectivity. Of particular note is the high yields and selectivities obtained from aliphatic aldehydes. Low catalyst loadings (2-5 mol %) can be employed. The morpholine amide serves as a useful precursor for further synthetic manipulation.     

Crystallization-induced reactions of copolymers. I. Ring-size isomerization of poly(ester acetals)

The isomerization reactions of five- and six-membered acetal rings in copolyesters containing these structures along the backbone were studied both in the melt and in the semicrystalline solid state. For the latter, isomerization of the solid copolymer at a temperature well below the melting point was in a direction opposite to that expected from the data for the acetal ring equilibration in a homogeneous melt. This apparent anti-equilibrium behavior was rationalized on the basis of the continuous removal of the five-membered ring units by crystallization during the reaction, thereby forcing the reaction in the direction controlled by the ability of that unit to crystallize, irrespective of the normal thermodynamic properties of the reaction itself. The probable requirements to be met for the operation of this type of crystallization-induced reaction are discussed.     

Reactions of ethoxyethynylphosphines with phenyl azide and nucleophiles. 1,5-Alkylotropic migration in phosphorus(IV)-substituted ketene acetals

Ethoxyethynyl-di-tert-butylphosphine reacts with phenyl azide in the presence of amines, alcohols, phenol, or water by addition of the nucleophile to the triple bond of the unstable ethoxyethynyldi-tert-butyl-N-phenyliminophosphorane. It has been found that iminophosphorylated ketene acetals are usually unstable, undergoing 1,5-alkylotropic migration to give alkoxycarbonylmethyl-ene(alkylphenylamino)phosphoranes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2850–2855, December, 1991.     

Ab initio studies on the mechanism of dimerization reactions of ketene imine and bis(trifluoromethyl)ketene imine

The dimerization reactions of ketene imine and bis(trifluoromethyl)ketene imine were studied theoretically. All the dimerization processes take place in a concerted but asynchronous manner, each proceeding through a four-membered ring transition state. For the ketene imine dimerization reactions, three different processes have almost equal activation barriers, while for the three bis(trifluoromethyl)ketene imine dimerization processes the reaction giving symmetrical a four-membered heterocyclic product has the lowest activation barrier. Received: 15 July 1998 / Accepted 3 September 1998 / Published online: 17 December 1998