Ketene acetals structure

The formation of methanolysis product 25 (R = Me) was easily understood in terms of ketene acetal structure 24. However, the isolated thermal product 26 is not directly derived from the ketene acetal 24. It is not unreasonable to suggest that thermal transformation of the [2 + 4] adduct 24 into the [2 + 2] adduct 23 takes place, followed by intramolecular migration of the trimethylsilyl group to afford vinylsiloxane... 

The synthesis of 2-alkylidene-substituted l,3-benzoxathiepan-5-one 267 possessing a 0,V-ketene acetal structure was achieved by heterocyclization of 266 in the presence of BF3 NEt3 (Scheme 82) <2001RCB1255>. The reaction corresponds to a formal type a (0—C—C—C—C—0-C) synthesis. 

Scheme 38 Formation of a ketene acetal structure (135) or a racemized ligand system (136) in the complex formation of SiCl4 with chiral Schiff-base ligands...

Intermediate 7, a viable precursor of intermediate 6, possesses a y,<5-unsaturated ester, the structural prerequisite, or retron, for the ortho ester Claisen transform.5 In the synthetic direction, the convergent union of intermediates 9 and 10 could give mixed-ketene acetal 8 the intermediacy of 8 should be brief, for it should readily... 

A prominent structural feature of 21 and its precursor 22 is the trans C16-C17 trisubstituted double bond. The particular relationship between the ethoxycarbonyl function and the A16 17 double bond in 22 is significant because it satisfies the structural prerequisite for the Johnson ortho ester Claisen rearrangement transform.2130 Mixed ketene acetal 23 thus emerges as the immediate... 

Entries 4 and 9 are closely related structures that illustrate the ability to control stereochemistry by choice of the Lewis acid. In Entry 4, the Lewis acid is BF3 and the (3-oxygen is protected as a f-butyldiphenylsilyl derivative. This leads to reaction through an open TS, and the reaction is under steric control, resulting in the 3,4-syn product. In Entry 9, the enolate is formed using di-n-butylboron triflate (1.2 equiv.), which permits the aldehyde to form a chelate. The chelated aldehyde then reacts via an open TS with respect to the silyl ketene acetal, and the 3,4-anti isomer dominates by more than 20 1. 

The ester 7-1 gives alternative stereoisomers when subjected to Claisen rearrangement as the lithium enolate or as the silyl ketene acetal. Analyze the respective transition structures and develop a rationale to explain these results. 

When the lactone silyl ketene acetal 18-1 is heated to 135° C a mixture of four stereoisomers is obtained. Although the maj or one is the expected [3,3] -sigmatropic rearrangement product, lesser amounts of other possible C(4a) and C(5) epimers are also formed. When the reaction mixture is heated to 100° C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic triene ester. Suggest a structure for the triene ester and show how it can be formed. Discuss the significance of the observation of the triene ester for the lack of complete stereospecificity in the rearrangement. 

The structure of the a-methylenecyclopropanone ketal 185 is reminiscent of the addition mode of the corresponding TMM to C=0 [196]. The ester 186 is probably the product of silica-gel-catalyzed hydrolysis of the ketene acetal 187 (Figure 4.8), which is the expected product in the reaction ofTMM with electron-deficient olefins [197]. At higher temperatures 185 isomerizes into 187 [195], NMR spectroscopic investigations of these adducts reveal that the cycloadditions occur at the [6,6] double bonds. Analogous products to 185-187 have been observed for the reaction of the... 

Wenzel and Jacobsen, in 2002, identified Schiff base thiourea derivative 48 as catalyst for the asymmetric Mannich addition [72] of tert-butyldimethylsilyl ketene acetals to N-Boc-protected (hetero)aromatic aldimines (Scheme 6.49) [201]. The optimized structure of 48 was found through the construction of a small, parallel... 

Quinolone undergoes photochemical addition of tetramethylethylene to give (434) (70AHC(ii)50), l,3-oxazin-4-ones photocycloadd ketene acetals to give (435), and irradiation of 2,6-dimethylpyran-4-one yields the cage dimer (436). 2-Pyranones form [2 + 2] photodimers whose structure is similar to that of uracil dimers (432 or 433). 

To obtain polymer with low MWDs in a living polymerization the rate of initiation must be faster or similar to the rate of propagation. This can suitably be accomplished if the structure of the initiator is the same as that of the growing chain end. For GTP this is a silyl ketene acetal (Scheme 9). The... 

Methyl trichlorosilyl ketene acetal reacts with aromatic and aliphatic ketones (the former enantioselectively), using chiral pyridine bis-N-oxide catalysts.134 Computations and an X-ray crystal structure of a catalyst-SiCU complex have helped to elucidate the mechanism. 

Reactions of trimethylsilyl enol ethers with diazo ketones give cyclopropanes contaminated by ring opened compounds 60,61). Use of the more stable tert-BuMe2Si-derivatives or of Rh2 (0Ac)4 as a catalyst might eventually improve the situation. O-Silylated ketene acetals and O.S-ketene acetals, respectively, did not provide products with cyclopropane structure 61 ... 

Silylated ketene acetals are more reactive than silyl enol ethers (Scheme 46), and the higher reactivity of cyclopentenes compared to cyclohexenes, which has already been reported for the hydrocarbon series (Scheme 41), is also observed for this class of compounds. The negative inductive effect of oxygen, which operates at the position of electrophilic attack, makes the bisenol ether (Scheme 46, right column, bottom) 20 times less reactive than the structurally analogous monoenol ether. 

Evans et al. recently reported the use of structurally well-defined Sn(II) Lewis acids for the enantioselective aldol addition reactions of a-heterosubstituted substrates [47]. These complexes are readily assembled from Sn(OTf)2 and C2-symmetric bis(oxazoline) ligands. The facile synthesis of these ligands commences with optically active 1,2-diamino alcohols, which are themselves readily available from the corresponding a-amino acids. The Sn(II)-bis(oxazoline) complexes were shown to function optimally as catalysts for enantioselective aldol addition reactions with aldehydes and ketone substrates that are suited to putatively chelate the Lewis acid. For example, use of 10 mol % Sn(II) catalyst, thioacetate, and thiopropionate derived silyl ketene acetals added at -78 °C in dichloromethane to glyoxaldehyde to give hydroxy diesters in superb yields, enantioselectivity, and diastereoselectivity (Eq. 27). The process represents an unusual example wherein 2,3-ant/-aldol adducts are obtained stereoselec-tively. 

Azaborolyl complex (- -)-218 has been used in a stereoselective Mukaiyama aldol reaction as illustrated in Scheme 32 <2005JA15352>. Complex (- -)-218 reacts with electron rich aromatic aldehydes and silyl ketene acetals to generate adduct 220. X-ray structures indicate the stereochemistry is as illustrated. This stereochemistry is... 

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