Preparation from enol ethers

The silyl enol ether prepared from (-)-menthone is transformed into a mixture of two diastereo-meric optically active cyclopropane derivatives by the diiodomethane/diethylzinc reagent28. 

Some synthetic modification for Pd(II)-catalyzed oxidation of silyl enol ethers is also developed. Silyl enol ethers prepared from aldehydes and ketones are converted to the corresponding a,/3-unsaturated carbonyl compound in good yields by 10 mol % of palladium(II) acetate in the presence of 1 atm pressure of O2 in DMSO as solvent (Scheme... 

Dehydrosilylation of silyl enol ethers prepared from unsymmetrical polyfunctional-ized ketones gives the corresponding acyclic and five-,f f seven-,t t ... 

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. 

Silyl enol ether 139 has also been transformed into D-allose, as shown in Scheme 5. The same methods can be applied to the enantiomeric enol ether derived from camphanate 38, and this allows one to prepare L-allose and its derivatives. Oxidation of 139 with MCPBA in THF (20 °C) led to the product of epoxide acidolysis 147 (69 %) which yielded 148 on heating to 200 °C for 15 min. Addition of 1.1 equiv. of MCPBA converted 148 into lactone 149 which in the presence of MeOH and K2CO3 (20 °C), gave selectively diester 150. Reactions 147... 

Zinc enolate 4, prepared from acetylene ether pyridine i-oxide, mercuric chloride, and zinc, adds to aldehydes to form a-chloro-3-hydroxy esters 5 in good yields ( ). Subsequent treatment with base gives trans-epoxyesters, one of which 6 is converted to 2-amino-2-deoxy-D-ribose stereoselectively in good yields (O. 

Further examination of the fluoride ion-catalyzed asymmetric aldol reaction of the enol silyl ethers prepared from acetophenones and pinacolone with benzaldehyde using 4b and its pseudoenantiomer 4c revealed the dependence of the stereochemistry of the reactions on the hydroxymethyl-quinudidine fragment of the catalyst (Table 9.3) [10,15]. 

Bromopyridine (3.16 g, 20 mmol) is added to an enolate solution prepared from 8.56 g (75 mmol) of 2,4-dimethyl-3-pentanone and 4.12 g (75 mmol) of KNH2 in 300 mL of liquid ammonia. After the mixture has been irradiated for 1 h, diethyl ether is added and the ethereal suspension remaining after evaporation of the ammonia is decanted through a filter and the residual salts are washed with diethyl ether. This reaction affords high yield of the substitution product (Sch. 37). 

Fig. 11.42. Preparation of an allyl enol ether, D, from allyl alcohol and a large excess of ethyl vinyl ether. Subsequent Claisen rearrangement D —> C proceeding with chirality transfer.

ACETYLENIC ETHERS FROM ALCOHOLS AND THEIR REDUCTION TO Z-AND E-ENOL ETHERS PREPARATION OF 1-MENTHOXY-1-BUTYNE FROM MENTHOL AND CONVERSION TO (Z)-AND (E)-1-MENTHOXY-1 -BUTENE ([Cyclohexane, 2-(1 -butynyloxy)-4-methyl-1 -(1 -methylethyl)- [1S-(1a,2p,4p)]-], end [[[Cyclohexane, 2-(1-butenyloxy)-4-methyl-1-(1-methylethyl)-, [1S-[1a,2P(Z),4p]]- and [lS-[1 ,2p(E),4P]]-)... 

It has, for example, been used in the preparation of 3-trimethylsiloxyfuran, and to prepare stereochemically pure E- or Z-silyl enol ethers starting from the corresponding E- orZ-vinyl bromides9 or vinylstannanes.6... 

ACETYLENIC ETHERS FROM ALCOHOLS AND THEIR REDUCTION TO Z- AND E-ENOL ETHERS PREPARATION OF 1-MENTHOXY-1-BUTYNE FROM MENTHOL AND CONVERSION TO (Z)- AND (E)-1 -MENTHOXY-1 -BUTENE... 

Acetylenic Ethers from Alcohols and Their Reduction to Z- and E-Enol Ethers Preparation of 1-Menthoxy-1-butyne from Menthol and Conversion to (Z)- and (E)-1-Menthoxy-1 -butene. 

As vinyl ethers were known to be poor substrates in Ru-catalyzed olefin metath-eses, it has been difficult to obtain cydic enol ethers by RCM of the vinyl ethers. Recently, a novel method to obtain cyclic enol ethers has been reported, which afforded cydic enol ethers directly from easily prepared dienes containing an allyl ether moiety [46]. Treatment of 70 with diene 99 in CH2CI2 in the presence of small amount of H2 resulted in a formation of dihydropyran 101 (Eq. 12.40). Treatment of 70 with H2 has been thought to produce an active catalyst for the olefin isomerization, and only metathesis products are formed until a small amount of H2 is introduced in the reaction. These results implied that this reaction most likely proceeded by way of a formation of the cyclic olefin 100, which was subsequently converted to dihydropyran 101 by the newly formed isomerization catalyst. In addition to the tandem reaction shown in Eq. 12.40, another method for obtaining cydic enol ethers from allyl ethers has also been demonstrated [46b]. This method induded addition of the hydride donor, such as NaBH4, to the reaction solution after the metathesis reaction had been completed. Although attempts to observe an active species for olefin isomerization in the presence H2 failed, these results suggested participation of hydride species in the olefin isomerization. 

The Lewis acid-catalyzed conjugate addition of silyl enol ethers to a,y3-unsaturated carbonyl derivatives, the Mukaiyaraa Michael reaction, is known to be a mild, versatile method for carbon-cabon bond formation. Although the development of catalytic asymmetric variants of this process provides access to optically active 1,5-dicarbonyl synthons, few such applications have yet been reported [108], Mukiyama demonstrated asymmetric catalysis with BINOL-Ti oxide prepared from (/-Pr0)2Ti=0 and BINOL and obtained a 1,4-adduct in high % ee (Sch. 43) [109]. The enantioselectiv-ity was highly dependent on the ester substituent of the silyl enol ether employed. Thus the reaction of cyclopentenone with the sterically hindered silyl enol ether derived from 5-diphenylmethyl ethanethioate proceeds highly enantioselectively. Sco-lastico also reported that reactions promoted by TADDOL-derived titanium complexes gave the syn product exclusively, although with only moderate enantioselectiv-ity (Sch. 44) [110]. 

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

Trimethylsilyl enol ethers prepared in 60-91% yields from ketone enolates and trimethylsilyl chloride are converted into a-hydroxy ketones by chromyl chloride in 62-82% yields (equation 402) [676] (equation 340). 

In contrast to the addition of water, the addition of alcohols to alkynes leads to stable enol ethers. Those of economic importance are almost exclusively the vinyl ethers prepared from acetylene. This preparation is carried out under base catalysis [41] (KOH, alcoholates, and the like). The noble metal-catalyzed alcohol addition does in fact likewise lead, in an intermediate stage, to vinyl ethers, but these react under the prevailing conditions, generally in a quantitative reaction, to give to corresponding acetaldehyde dialkyl acetals [42]. This is illustrated in (eq. (18)), which takes as its example the addition of n-butanol to acetylene in the presence of Na2PtCl6. 

For the synthesis of (69), the enol ether (71) from the indanone (70) was carboxylated with COa-n-butyl-Iithium in THF at —70 C to yield (72). The methyl ester (73) was converted into (75) via the maleic anhydride adduct (74), essentially as described in earlier work. Lithium aluminium hydride reduction followed by oxidation with dicyclohexylcarbodi-imide afforded the aldehyde (76). This was condensed with excess (77) to yield a mixture of the diastereomers (78). Oxidation with chromium trioxide-pyridine in methylene dichloride gave (79), which could be converted into the diketone (80) by treatment with excess benzenesulphonylazide. The diketo-lactam (81) was prepared from (80) as described for the synthesis of the analogous intermediate used in the synthesis of napelline. Reduction of (81) with lithium tri-t butoxyaluminohydride gave the desired dihydroxy-lactam (82). Methylation of (82) with methyl iodide-sodium hydride gave (83). Reduction of this lactam to the amine (84) with lithium aluminium hydride, followed by oxidation with potassium permanganate in acetic acid, gave (69). 

Isocodeine methyl ether, prepared from the methyl ether methiodide [411] or by methylating isocodeine-N-oxide and reducing the product [265], cannot be isomerized to thebainone methyl enolate by heating with sodium ethoxide [265]. On degradation it yields y-codeimethine methyl ether [411]. 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

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