Ketene dialkyl acetals

It is important to note that a possible photochemical ring expansion by [1,3]- or [l,5]-acyl shifts in the 5-membered vinyl lactams 191 has not been observed,4,116 nor have [l,5]-shifts of acyl groups involving an aromatic ring been reported for either enol esters or enamides. However, an example of a similar [l,5]-alkyl shift, 193 -> 194, in related ketene dialkyl acetals is known.24... 

Stork and Borowitz36 were able to rule out an alternative mechanism, in which an initially formed mesylated enamine underwent cyclization, by preparing the mesylated species (87) and showing that it did not yield the cycloadduct (equation 67). Borowitz36 also demonstrated that only electron-rich olefins (such as enamines, ketene dialkyl acetals, etc.) give [2 + 2] cycloadducts when the sulfene is generated from alkenesulfonyl chlorides and triethylamine, and no cycloadduct was formed in the presence of cyclohexene, ethoxyacetylene or anthracene. 

Silyi enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in DMSO or certain other polar aprotic solvents." The reaction has been performed with R , R = hydrogen or alkyl, though best yields are obtained when r = r = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyi enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-Bp3-Et20," ceric ammonium nitrate," and lead tetraacetate." If R =0R (in which case the substrate is a ketene silyi acetal), dimerization with TiCU leads to a dialkyl succinate (34, r =0R)." ... 

Michael addition.1 This ketene silyl acetal undergoes Michael addition to a,fl-enones in acetonitrile in the absence of a Lewis acid to afford the corresponding O-silylated Michael adduct in high yield. These O-silyl enolates undergo site-specific electrophilic substitution. This sequence was used for vicinal dialkylation of cyclohexanone (equation I) and of cyclopentanone. It is particularly useful for synthesis of methyl jasmonate and related compounds from cyclopentenone. 

Wenzel and Jacobsen reported the thiourea 15b-catalyzed Mannich-type reaction of a ketene silyl acetal with an N-Boc-aldimine, furnishing (3-amino esters with excellent enantioselectivities (Equation 10.30) [59]. Subsequently, Jacobsen and co-workers reported the hydrophosphonylation of dialkyl phosphites with aldimines to yield a-amino phosphonates [60]. 

The fragmentation of acetylated dialkyl dithioacetals follows similar routes, but the spectra are more complex, owing to the secondary fragments formed by elimination (s) of the 0-acetyl groups as acetic acid, ketene, or acetic anhydride. The mass spectrometry of these compounds has been reviewed. ... 

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... 

Okamoto, Y., Azuhata, T., and Sakurai, H., Dialkyl 3-alkoxy-3-(trimethylsi-loxy)-2-propenephosphonate a one step preparation of (dialkoxyphosphi-nyljmethyl-substituted ketene alkyl trimethylsilyl acetal, Chem. Lett., 1265, 1981. 

Dialkyl(trimethylsilyl)phosphines undergo 1,4-addition to a,/3-unsaturated ketones and esters to give phosphine-substituted silyl enol ethers and silyl ketene acetals, respectively. A three-component coupling reaction of a silylphosphine, activated alkenes, and aldehydes in the presence of a catalytic amount of GsF affords an aldol product (Scheme 76).290 291... 

The procedure illustrates a general method for the preparation of ketene S,N-acetals via thioamides and their crystalline quaternary iodides some other examples are shown in Table I. 2,2-Dialkyl-substituted ketene S,N-acetals cannot be prepared by this method because the nature of the products makes rigorous purification impractical. Ketene S,N-acctals are useful starting materials for many syntheses.11,13 15... 

Abe and coworkers have also observed this stereochemical effect in the Patemo-Btichi reaction of aromatic aldehydes with cyclic ketene acetals [158]. The addition of benzaldehyde to the 5-silyloxy-substituted 2,3-dihydrofuran resulted in the exophenyl product, albeit in low yields. Higher yields and selectivity were obtained with naphthaldehydes and acceptor-substituted benzaldehydes as carbonyl addends. Interesting substrates in this context were also the 2,3-dialkylated ascorbic acid acetonides investigated by Kulkarni and coworkers [159]. Photocycloaddition of benzaldehyde with these substrates resulted in the formation of two regioisomeric products (Scheme 41). 

The preparation of acetals of the unknown formylphosphonic acid was reported by treating a dialkyl hydrogenphosphonate or a dialkyl trimethylsilyl phosphite wth trialkyl orthoformate in the presence of boron trifluoride. Another method which is reported to yield acylphosphonate ketals is the addition of dialkyl hydrogenphosphonates to ketene acetals (equation 55). For dithioketals, see Section II. C. 4. b. 

Syndiesis of Diesters.—Dialkyl esters of 3-methoxy-2,4-dialkylpentanedioic acids (81) are readily prepared, in 78—98% yield, from the ketene acetal (80) (Scheme 43). ... 

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