Ketene selectivity

Laboratory studies of these reactions indicate that the ketene selectivity S (kmol ketene formed per kmol acetone converted)... 

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) ... 

Reaction in a Centrifugal Pump In the reaction between acetic acid and gaseous ketene to make acetic anhydride, the pressure must be kept low (0.2 atm) to prevent polymerization of ketene. A packed tower with low pressure drop could be used but the required volume is very large because of the low pressure. Spes (Chem. Ing. Tech., 38, 963-966 [1966]) selected a centrifugal pump reactor where... 

The Lewis acid-catalyzed addition of silyl kelene acetals occurred m high yield, and when the ketene acetal bore a substituent, the reactions occurred with modest diastereofacial selectivity [d] (equation 7) (Table 3)... 

A few a/j/r -selective amide and imide enolates which arc able to provide high induced diastereo-selectivity have been uncovered very recently. The /V-propionylsultam 1 w hich opens a way to sryn-aldols as described in Section D.1.4.3.2.3.1. also allows the synthesis of r/nh-adducls. For this purpose. 1 is converted into the silyl-iV.O-ketene acetal 2 and subsequently added to aldehydes in a Mukaiyama-type aldol reaction106 to give awi-adducts 310<>f. 

Considerable efforts have been devoted to the stereoselective introduction of a /(-methyl function in intermediates for the synthesis of 1 jS-methylcarbapenems. While the trimethylsilyl trifluoromethanesulfonate catalyzed reaction of a 4-acetoxyazetidinone derivative with ketene acetals shows no selectivity, ketene thioacetals lead to stereoselective formation of the a-methyl isomer108. The zirconium enolate, however, shows high /(-methyl selectivity. 

Excellent /(-methyl selectivity is observed in the zinc chloride mediated condensation with 0-silyl enol ethers of 2-pyridinylmethyl thiopropionates109. Supposedly, chelate formation of zinc(II) with the sulfur and the nitrogen atom of the pyridinylmethyl thioester is essential for the high /(-selectivity. The geometry of the ketene acetal also seems to have some influence. 

Silyl enol ethers and ketene acetals derived from ketones, aldehydes, esters and lactones are converted into the corresponding o/i-unsaturated derivatives on treatment with allyl carbonates in high yields in the catalytic presence of the palladium-bis(diphenylphosphino)ethane complex (32). A phosphinc-free catalyst gives higher selectivity in certain cases, such as those involving ketene acetals. Nitrile solvents, such as acetonitrile, are essential for success. 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

While the steric explanation is consistent with the observed selectivity, it nonetheless presents an incomplete explanation, as alkylation of 2-methyl-4-cyano-l,3-dioxane 17 also proceeded with very high syn-selectivity [11] (Eq. 5). The selective equatorial alkylation can be rationalized as an anfz-anomeric effect that disfavors axial alkylation of the ketene iminate through filled-shell repulsion. Simple lithiated nitriles are known to exist as ketene iminates, but it would be easy to rationalize the preference for equatorial alkylation by considering the relative stability of hypothetical equatorial and axial alkyllithium reagents, vide infra. Preferential equatorial alkylation was also observed by Beau... 

Much rarer than the inclusion of phenols is complex formation between alcohols and pyridinomacrocycles. In some combinations (selected macro-cycles and alcohols) complexes could be isolated (Weber and Vogtle, 1980). TTie hydrogen bond formation between pyridines and alcohols is the basis for an application, the additions of alcohols to ketenes catalysed by concave pyridines [13] (Liining et al., 1991b Schyja, 1995). 

Claisen rearrangement. As for the mechanism, the reaction begins with intramolecular cyclopropanation the resulting bicyclo[2.1.0]pentan-2-one then undergoes fragmentation to a p,y-unsaturated ketene which finally is trapped by the added alcohol to afford a p,y-unsaturated ester (Scheme 41). The intermediates could be observed in selected cases. 

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. 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

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... 

In a related publication, Kobayashi and his team reported on Zr-catalyzed asymmetric Mannich reactions that utilize the more electron-rich oxygenated ketene acetals shown in Scheme 6.28 [93], A noteworthy aspect of this study was that the levels of syn/anti diaste-reocontrol proved to be dependent on the nature of the alkoxide substituent whereas the (3-TBS acetals predominantly afforded the syn isomer, the OBn derivatives afforded a larger amount of the anti isomer. As before, the presence of an additive, this time 1,2-dimeth-ylimidazole (DMI), proved to be important with regard to the level of Ti-facial selectivity. The phenol activating group can be removed by the same procedure as reported previously, with essentially identical degrees of efficiency (see Scheme 6.27). 

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

The silyl ketene acetal 2, prepared from ethyl 3-hydroxybutanoate, undergoes a similar condensation with benzylideneaniline but with syn-selectivity. The product cyclizes to a cis-fl-lactam (4). 

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