Ketenes reaction with aldehydes

Carbonyl Compounds. Cychc ketals and acetals (dioxolanes) are produced from reaction of propylene oxide with ketones and aldehydes, respectively. Suitable catalysts iaclude stannic chloride, quaternary ammonium salts, glycol sulphites, and molybdenum acetyl acetonate or naphthenate (89—91). Lactones come from Ph4Sbl-cataly2ed reaction with ketenes (92). 

The aromatic primary and secondary stibines are readily oxidized by air, but they are considerably more stable than their aHphatic counterparts. Diphenylstibine is a powerful reducing agent, reacting with many acids to Hberate hydrogen (79). It has also been used for the selective reduction of aldehydes and ketones to the corresponding alcohols (80). At low temperatures, diphenylstibine undergoes an addition reaction with ketene (81) ... 

We have studied this reaction in considerable detail (88) and have found that when one uses quinine (eq. [25]) or any one of the chiral bases, a variety of aldehydes react with ketene to form the corresponding p-lactones in excellent chemical and nearly quantitative enantiomeric yields. Equation [25] exemplifies the reaction. Note that mild basic hydrolysis of the lactone furnishes a trichlo-rohydroxy acid that was prepared earlier by McKenzie (89). If one uses quinidine as catalyst, the process furnishes the natural (S)-malic acid. Note that ketene first acylates the free hydroxyl group of quinine, so that the actual catalyst is the alkaloid ester. 

Ketones and aldehydes insert into B-X bonds, and the products vary with the substituents on the substrateU Thus, with BX3 either mono-, bis-, or tn s-inserted products may be obtained in which B-0 bonds are formed. Electron-withdrawing substituents favor insertion. Reactions with ketenes lead to insertion into both the C=C and C=0 bonds . 1,3-Addition of PhBCU occurs with a-diazoethylacetates to form PhClBOC(OEt)=CRCl under mild conditions. Cyclic perfluoroketones react with... 

In the presence of 10 mol % of the chiral catalyst, aldehydes react with ketene at -78 °C in toluene for 1 h to afford oxetan-2-one derivatives 21 in 14 to 74% ee as listed in Table 7. This suggests that the reactions of bulkier aldehydes proceed in better optical purity. Although the obtained enantioselectivity is not high, there is a room for the future improvement. 

The reaction of the aldehyde 219 with ketene, generated from acetyl chloride and i-Pr2NEt in the presence of a chiral quinidine catalyst, affords the /3-lactone 220 in 85 % yield (94 % e,e). ... 

A range of aliphatic aldehydes have been converted to the corresponding chiral P-lactones by the reaction with ketene in the presence of the chiral oxazaborolidine and tributyltin triflate as catalysts with up to 84% ee. 

The reaction of ketene itself with tettaalkyl titanates followed by a ketone R R C=0 gives P-hydroxy-esters, R R C0HCH2C02R. Polyinsertion of ketene and aldehyde into the Ti—O bond leads to di-, tri-, and tetraesters, eg, H0CR R CH2C02CR R CH2C02R (200). 

The initial reaction between a ketene and an enamine is apparently a 1,2 cycloaddition to form an aminocyclobutanone adduct (58) (68-76a). This reaction probably occurs by way of an ionic zwitterion intermediate (75). The thermal stability of this adduct depends upon the nature of substituents Rj, R2, R3, and R. The enolic forms of 58 can exist only if Rj and/or R4 are hydrogens. If the enamine involved in the reaction is an aldehydic enamine with no 3 hydrogens and the ketene involved is di-substituted (i.e., R, R2, R3, and R4 are not hydrogens), then the cyclo-butanone adduct is thermally stable. For example, the reaction of dimethyl-ketene (61) with N,N-dimethylaminoisobutene (10) in isopropyl acetate... 

The reaction between an aldehydic enamine with no (3 hydrogens and ketene yields a cyclobutanone adduct which is not thermally stable (Rj and R4 H, Ri = R2 = H) (67,70,72). Thermal decomposition gives just one... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Dihydro-2H-pyran-2-ones (e. g., 4-195) are valuable intermediates in the synthesis of several natural products [67]. Hattori, Miyano and coworkers [68] have recently shown that these compounds can be easily obtained in high yield by a Pd2+-catalyzed [2+2] cycloaddition of ct, 3-unsaturated aldehydes 4-192 with ketene 4-193, followed by an allylic rearrangement of the intermediate 4-194 (Scheme 4.42). In this reaction the Pd2+-compound acts as a mild Lewis acid. a,(3-unsaturated ketones can also be used, but the yields are below 20%. 

Chiral bis-phosphine acylplatinum complex 210 with a strong acid such as TfOH serves as an effective enantio-selective catalyst for aldol-type reactions of aldehydes with ketene silyl acetals (Equation (127)).486 The presence of water and oxygen in the catalyst preparation step is required to obtain the highly enantioselective catalyst. The intermediacy of a C-bound platinum enolate was suggested by IR and 31P NMR spectroscopies. 

Silyloxy)alkenes were first reported by Mukaiyama as the requisite latent enolate equivalent to react with aldehydes in the presence of Lewis acid activators. This process is now referred to as the Mukaiyama aldol reaction (Scheme 3-12). In the presence of Lewis acid, anti-aldol condensation products can be obtained in most cases via the reaction of aldehydes and silyl ketene acetals generated from propionates under kinetic control. 

Besides their application in asymmetric alkylation, sultams can also be used as good chiral auxiliaries for asymmetric aldol reactions, and a / -product can be obtained with good selectivity. As can be seen in Scheme 3-14, reaction of the propionates derived from chiral auxiliary R -OH with LICA in THF affords the lithium enolates. Subsequent reaction with TBSC1 furnishes the 0-silyl ketene acetals 31, 33, and 35 with good yields.31 Upon reaction with TiCU complexes of an aldehyde, product /i-hydroxy carboxylates 32, 34, and 36 are obtained with high diastereoselectivity and good yield. Products from direct aldol reaction of the lithium enolate without conversion to the corresponding silyl ethers show no stereoselectivity.32... 

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