Ether, trimethylsilyl vinyl synthesis

Poly(vinyl alcohol) is very high in head-to-tail structures, based on NMR data. It shows the presence of only a small amount of adjacent hydroxyl groups. The polymer prepared from amorphous poly(vinyl acetate) is crystalline, because the relatively small size of the hydroxyl groups permits the chains to line-up into crystalline domains. Synthesis of isotactic poly(vinyl alcohol) was reported from isotactic poly(vinyl ethers) like poly(benzyl vinyl ether), TOly(f-butyl vinyl ether), poly(trimethylsilyl vinyl ether), and some divinyl compounds. ... 

The synthesis of dichloronorcarane from cyclohexene by the chloroform-base-PTC method has been improved further as has the preparation of a-halogeno-aP-unsaturated ketones via em-dihalogenocyclopropanes by employing trimethylsilyl vinyl ethers rather than ethyl vinyl ethers. The formation of gem-difluorocyclo-propanes proceeds in high yield (60— 90%) when chlorodifluoromethane is treated with halide ion and an epoxide in the presence of an olefin. The epoxide-halide ion combination is employed to produce a base of sufficient strength, and in sufficient concentration, to maximize the production of difluorocarbene oxiran and chloro-methyloxiran afford the most suitable bases when treated with chloride ion (Scheme 4). 

An important stage in the synthesis has been reached. It was anticipated that cleavage of the trimethylsilyl enol ether in 18 using the procedure of Binkley and Heathcock18 would regiospecifically furnish the thermodynamic (more substituted) cyclopentanone enolate, a nucleophilic species that could then be alkylated with iodo-diyne 17. To secure what is to become the trans CD ring junction of the steroid nucleus, the diastereoisomer in which the vinyl and methyl substituents have a cis relationship must be formed. In the... 

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. 

The copper-catalyzed conjugate addition of methyl magnesium iodide to cyclohexenone and trapping the enolate as its trimethylsilyl enol ether, followed by a trityl hexachloro-antinomate-catalyzed Mukaiyama reaction, is apphed to / -(—jcarvone. C-2, C-3 functionalized chiral cyclohexanones are converted into their a-cyano ketones, which are submitted to Robinson annulation with methyl vinyl ketone. Highly functionalized chiral decalones are obtained that can be used as starting compounds in the total synthesis of enantiomerically pure clerodanes (equation 70). 

A stable, readily soluable platinum ir-vinyl alcohol complex has now been synthesized and well characterized. Initially, this compound, chloro(acetylacetonato)(ir-ethenol)platinum(II) (45), was synthesized from the ir-ethylene compound (43) via a vinyl trimethylsilyl ether complex (44) using the same general procedure employed in the synthesis of (41) and (42) (44). 

In a process related to GTP, aldehydes initiate the polymerization of silyl vinyl ethers and silyl diene ethers. Here the silyl group is present in the monomer and transfers to the aldehyde ended chains regenerating aldehyde ends [17] (Scheme 8). A Lewis acid catalyst is required. terf-Butyldimethylsilyl works best as a transfer group for vinyl ether while trimethylsilyl is suitable for diene ethers [18]. Even though aldol GTP provides a route to polyvinyl alcohol segments in the subsequent block polymer synthesis, the projected cost of the monomers discouraged further research aimed at commercialization. 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

The synthesis, starting from a bifunctional initiator followed by quenching the double-headed living ends, gives homotelechelic polymers (method B). Carboxylate-capped telechelic poly(isobutyl vinyl ether) has been obtained in this way [82], where the adduct of a bifunctional vinyl ether with trifluoroacetic acid is the initiator, and the quencher is the malonate anion. For method C, a bifunctional trimethylsilyl enol ether, CH2=C[OSi(CH3)3]C6H4OCH2CH20C6H4[(CH3)3SiO]C=CH2, is a useful terminator (chain coupler) for vinyl ethers [142,147] and a-methyl-styrene [159] (see also Section VI.B.4). 

In a series of elegant studies, Paquette and coworkers demonstrated the potential of the Claisen rearrangement for the stereocontrolled total synthesis of natural products. Dehydrative coupling of (2)-3-(trimethylsilyl)-2-propen-l-ol with cyclohexanone (51) under Kuwajima s conditions, followed by rearrangement of enol ether (52) in decalin, led in excellent stereoselectivity (>99 1) to aldehyde (53 Scheme 8). Concise construction of the eight-membered core of acetoxycrenulidine was achieved by intramolecular phenylseleno etherification of lactone (54), introduction of the exocyclic vinyl ether double bond by selenoxide elimination and subsequent Claisen rearrangement (Scheme 9, 66% from 54). ... 

Triazoles are generally prepared by the cycloaddition of an alkyne with an azide, but the hazardous nature of some alkyl azides limits the method in these cases. A convenient synthesis which leads to A-hydrogen 1,2,3-triazoles utilises the stable (and relatively safe) trimethylsilyl azide." For C-unsubstituted 1,2,3-triazoles, ethyne itself would be required but it is much more convenient to use, as starting material, vinyl acetate instead of the gaseous ethyne, or in general, an enamine or an enol ether as alkyne equivalents. 

An elegant synthesis of unsymmetrical tetroxanes 238 involves ozonolysis of vinyl ethers 239 in the presence of hydrogen peroxide in diethyl ether. This procedure afforded the 1,1-bishydroperoxides 240, which on subsequent trimethylsilylation with A, 0- A(trimethylsilyl)acetamide (BSA) gave 241. TMSOTf-catalyzed cyclocondensation of 241 with carbonyl compounds afforded tetroxanes 238 (method A, Scheme 45 Table 4) <1999J(P1)1867>. 

A section on cardenolides would not be complete without mentioning the total synthesis of (+)-digitoxinin by Stork and his co-workers. Protected Wieland-Miescher ketone 66 was converted into compound 67 via formation of the trimethylsilyl enol ether ozonolysis of the enol ether gave a mixture of hydroxyketones, which was reduced to the corresponding diol and then cleaved with sodium periodate to yield dialdehyde intermediate 67. Compound 67 was then elaborated into Diels-Alder precursor 68 in two steps. After completion of the Diels-Alder reaction, four additional steps led to the preparation of intermediate 69, which was ideally set up for a vinyl radical cyclization. Key intermediate 70 was then elegantly converted into (+)-digitoxinin (71). 

---