Silyl acylation

A number of more readily hydrolysed acetals have been investigated, including lithio bis(methylthio) methanes65 and methoxy phenylthio trialkylsilyl methane (3), which acts as an a-silyl acyl anion equivalent66. This route (Scheme 4) is successful for a wide range of aliphatic electrophiles and is even successful in conjugate addition to enones67. 

A further efficient preparation of these silyl enol ethers proceeds through an intramolecular 1,2-silicon shift in an a-silyl acyl lithium substrate (8), prepared from an a-lithiosilane (Scheme 23)". This method appears very simple to carry out, and produces the silyl enol ethers in good yields with high isomeric purity (usually, E isomer > 90% of mixture). 

SCHEME 2.31 One-pot tritylation-silylation-acylation of (a) methyl a-glycosides and (b) p-thioglycosides. 

Volatile Derivatives for GC. The reactions to produce volatile derivatives can be classified as silylation, acylation, alkylation, and coordination complexation. Examples of the first three types are included in Table 5... 

Hydroxyferrocene is prepared by hydrolysis of ferrocene acetate, and it undergoes many of the same organic transformations (alkylation, silylation, acylation, acetal formation) that are carried out with phenol. Access to thioferrocene or dithioferrocene is easily accomplished by treatment of anions (322) or (323) with elemental sulfur. 

Carbon monoxide insertion into zirconium(IV)-silicon bond has been reported . The trimethylsilyl derivative Zr(/7 -C5H5)2(SiMe3)Cl was carbonylated under CO pressure (about 7 atm) in diethyl ether, giving the following silyl-acyl compound ... 

The molecular structure, as determined by X-ray diffraction methods, shows the type of arrangement of the silyl-acyl derivative (the 0-inside isomer) with Zr-O and Zr-C distances of 2.248(1) and 2.183(2) A, similar to those of Zr( / -C5H5)2()7 -COMe)Me Insertion of carbon monoxide occurs for a solvent stabilized cationic complex of zirconium (IV), [Zr(>j -C5H5)2 (Me)(L)] ... 

As pointed out in the introduction (Section 3.16.1) neutral 1,3-diphospholes and 1,3-benzodiphos-pholes contain a pyramidal phosphorus atom and are not fully conjugated their anions, however, are. Synthesis is therefore generally aimed at these anions, i.e. at alkali metal 1,3-diphospholides (11) and 1,3-benzodiphospholides (3). In some cases they can be converted to neutral diphospholes by protonation, alkylation, silylation, acylation, or oxidation (see Section 3.16.3.5.1). 1,3-Diphos-pholes and 1,3-benzodiphospholes have also been prepared by direct synthesis. 1,1,1,3,3-Pentaphenyl-lA 3A -benzodiphospholes have also been reported <95AG2186>. 

Aldol reactions with specific enol equivalents Contrast with equilibrium methods Aldols with Lewis acid catalysis silyl enol ethers Application to the synthesis of gingerol Reaction at O or C Silylation, Acylation and Alkylation Naked enolates... 

Reaction at Oxygen or Carbon Silylation, Acylation and Alkylation... 

Various derivatization techniques are implemented. The principal ones can be classified into four general groups according to the reagents used and the reaction achieved silylation, acylation, esterification and alkylation. 

Compounds with keto groups often give more than one derivative on silylation, acylation or esterification because of the possibility of geometrical isomerism, and yields may be unsatisfactory. For oc-keto acids the fonna-... 

The reactions to produce volatile derivatives can be classified as silylation, acylation, alkylation, and coordination complexation. Examples of the first three types are included in Table 10.1 which is organized by functional groups including carboxylic acid, hydroxyl, amine, and carbonyl. Amines require special consideration even if they are volatile. Their strong tendency to hydrogen-bond often makes it difficult to elute them from a GC column. Consequently, amines often have to be derivatized whether they are volatile or not. A review of this subject has appeared recently [29]. 

GC is a suitable technique for the analysis of benzodiazepines. Derivatization techniques are used, and silyl, acyl, and alkyl derivatives are formed. Electron capture detector gives the best detection limits (<1 ng/ml) for plasma, although NPD provides detection limits down to 5 ng/ml. GC-MS is an excellent confirmatory method for... 

The Rh(l)-catalysed asymmetric hydrogenation of various olefins (dehydroamido acids, enamides, itaconates, and acrylates) was studied in the presence of a wide range of enantiopure R-SMS-Phos = l,2-bis[(o-RO-phenyl)(phenyl) phosphino]ethane ligands where R was either branched or heteroatom-substituted alkyl, aralkyl, silyl, acyl, sulfonyl, and so on. The results indicated that the reaction rates increased by several times, with increased enantioselectivities, and this system was therefore better than DiPAMP = l,2-bis[(o-anisyl)(phenyl)phosphino]ethane ligand. ... 

In their total synthesis of (-i-)-ophiobolin in 1989, Kishi et al. found that treatment of a cyclopentenyl ester under the typical Ireland conditions gave principally C-silylated ester [63]. Heating of a C-silyl ester (prepared by acylation using a C-silyl acyl chloride) at 230 °C resulted in a 1,3-Brook rearrangement followed by an Ireland-Claisen rearrangement to give the desired product as a 6 1 ratio of isomers at C2 of the pentenoic acid (Scheme 4.63). The major product could have arisen through either a chair transition state of the Z-sUyl ketene acetal or a boat transition state of the E-silyl ketene acetal. 

Three types of derivatization are mainly used silylation, acylation, and alkylation. 

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