Vinyl silyl enol ethers

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

Vinylic lithium reagents (26) react with silyl peroxides to give high yields of silyl enol ethers with retention of configuration. Since the preparation of 26 from vinylic halides (12-37) also proceeds with retention, the overall procedure is a... 

In aldol reactions, especially Mukaiyama aldol reactions, TiIV compounds are widely employed as efficient promoters. The reactions of aldehydes or ketones with reactive enolates, such as silyl enol ethers derived from ketones, proceed smoothly to afford /3-hydroxycarbonyl compounds in the presence of a stoichiometric amount of TiCl4 (Scheme 17).6, 66 Many examples have been reported in addition to silyl enol ethers derived from ketones, ketene silyl acetals derived from ester derivatives and vinyl ethers can also serve as enolate components.67-69... 

The scope and efficiency of [4+2] cycloaddition reactions used for the synthesis of pyridines continue to improve. Recently, the collection of dienes participating in aza-Diels Alder reactions has expanded to include 3-phosphinyl-l-aza-l,3-butadienes, 3-azatrienes, and l,3-bis(trimethylsiloxy)buta-l, 3-dienes (1,3-bis silyl enol ethers), which form phosphorylated, vinyl-substituted, and 2-(arylsulfonyl)-4-hydroxypyridines, respectively <06T1095 06T7661 06S2551>. In addition, efforts to improve the synthetic efficiency have been notable, as illustrated with the use of microwave technology. As shown below, a synthesis of highly functionalized pyridine 14 from 3-siloxy-l-aza-1,3-butadiene 15 (conveniently prepared from p-keto oxime 16) and electron-deficient acetylenes utilizes microwave irradiation to reduce reaction times and improve yields <06T5454>. 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

Similarly, tandem hydroformylation/aldol sequences can be applied to the formation of bicyclic and spirocyclic compounds. Thus silyl enol ethers of 3-vinyl and 3-allyl cycloalkanones give ring anellated products (Scheme 33) [86,87]. 

Methyllithium (4.0 mmol, 1.0 M in diethyl ether, 4.0 mL) was added to a suspension of CuCN (2.0 mmol, 0.18 g) in THF (10 mL) at -75°C. The reaction mixture was then stirred until a clear solution was obtained and allowed to warm to room temperature. The appropriate (Z)-vinylic telluride A (2.0 mmol) or B (1.0 mmol) was added and stirred for 45 min. The solution was cooled back to -75°C and the corresponding enone (2.2 mmol) was added. After 20 min, chlorotrimethylsilane (2.6 mmol, 0.60 g) diluted in THF (5 mL) was added. The reaction mixture was stirred for 1 h, allowed to warm to room temperature and then treated with 1 1 solution of saturated aqueous NH4CI and NH4OH (20 mL), extracted with ethyl acetate (3x20 mL), dried, evaporated and the residue was purified by Kiigelrohr distillation affording the silyl enol ethers. 

The reaction of vinylcarbenoids with vinyl ethers can lead to other types of [3 + 2] cycloadditions. The symmetric synthesis of 2,3-dihydrofurans is readily achieved by reaction of rhodium-stabilized vinylcarbenoids with vinyl ethers (Scheme 14.17) [107]. In this case, (J )-pantolactone is used as a chiral auxihary. The initial cyclopropanation proceeds with high asymmetric induction upon deprotection of the silyl enol ether 146, ring expansion occurs to furnish the dihydrofuran 147, with no significant epi-merization during the ring-expansion process. 

Mukiayama aldol reactions between silyl enol ethers and various carbonyl containing compounds is yet another reaction whose stereochemical outcome can be influenced by the presence of bis(oxazoline)-metal complexes. Evans has carried out a great deal of the work in this area. In 1996, Evans and coworkers reported the copper(II)- and zinc(II)-py-box (la-c) catalyzed aldol condensation between benzyloxyacetaldehyde 146 and the trimethylsilyl enol ether [(l-ferf-butylthio)vinyl]oxy trimethylsilane I47. b82,85 Complete conversion to aldol adduct 148 was achieved with enantiomeric excesses up to 96% [using copper(II) triflate]. The use of zinc as the coordination metal led to consistently lower selectivities and longer reaction times, as shown in Table 9.25 (Eig. 9.46). 

This rule has been used for the assignment of some diastereomeric silyl enol ethers, e g., 3-trimethylsilyloxy-2-pentene269, where the chemical shifts of the vinyl protons were so similar that a confident differentiation was not possible. 

THF if CuBr is present.1393 The reaction takes place either with or without allylic rearrangement.1394 Propargylic ethers give allenes.1395 Vinylic ethers can also be cleaved by Grignard reagents in the presence of a catalyst, in this case, a nickel complex.1396 Silyl enol ethers R2G=CROSiMe3 behave similarly.1397... 

Fluoropyridinium triflate (la) exhibits high selectivity in fluorinations. In a steroid 8 with two reaction sites, a conjugated and a nonconjugated vinyl acetate moiety, 1-fluoropyridinium triflate (la) reacts at the conjugated site only. On the other hand, steroid 9 with a silyl enol ether and a conjugated acetate moiety affords the product resulting from reaction at the former site only.44,52... 

Formation of a silyl enol ether will generate an allyl vinyl ether which after rearrangement can be desilylated to give a carboxylic acid. 

Vinyl substitution with silyl enol ethers 840... 

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