Acetylenic silanes synthesis

The synthesis is necessarily long and we shall concentrate on the tandem aspects. The three enantiomerically pure starting materials were the acetylenic silane 135 for C-6, prepared by resolution (chapter 22), the epoxide 136, for C-l and 2, prepared from proline (chiral pool strategy, chapter 23) and the Wittig reagent 137, prepared from ethyl lactate (chiral pool again), for C-ll. 

The synthesis of dihydroagarofuran is a further example of the construction of tricyclic compounds via radical six-ring formation80. ( —)-Carvone is converted to a bridgehead chloride 6 as a radical precursor treatment with tributyltin hydride gives a mixture of products consisting of a 4 1 mixture of the two vinylsilane double-bond isomers and the noncyclized acetylenic silane. Desilylation and stereoselective reduction leads to dihydroagarofuran. 

Quan, Z. Zuju, M. Lizhong, N. Jianding, C. 2007. Novel phenyl acetylene terminated poly(carborane-silane) Synthesis, characterization, and thermal property. J. Appl. Polym. ScL, 104 2498-2503. 

Perhaps the best general method to date for preparing a-haloacyl silanes involves bromi-nation of silyl enol borinates (9) at 0 °C, a reaction which proceeds in good yield and involves no sensitive intermediates. This route offers a most convenient one-pot synthesis of a-haloacyl silanes from readily available starting materials, as the intermediate enol borinates are very easily prepared from silyl acetylenes (Scheme 35)7,117,118. 

A much more general and very simple synthesis requiring a minimum of laboratory manipulation uses a Swern oxidation of the corresponding diols to give the a-ketoacyl silanes directly in useful yields (Scheme 38)7,124. Purification in this case was accomplished in the dark, by chromatography at —78°C or by distillation. a-Ketoacyl silanes appear to be intermediates in the oxidation of silyl acetylenes to a-ketoesters by osmium tetroxide125, and indeed have been isolated from the oxidation of silyl acetylenes by dimethyl dioxirane126. 

In a further development of this approach, the synthesis of cr,/J-acetylenic acyl silanes has been achieved as shown in Scheme 4514. Oxidation of the 3-selenenyl allenyl ethers (16) with m-chloroperbenzoic acid at —78 °C gave the corresponding unstable selenoxides, which underwent in situ [2,3] sigmatropic shift producing acetals (17). Loss of selenenyl ester on work-up gave the cr,/J-acetylenic acyl silanes in ca 50% yields. 

The /3-lactone was formed by the cyclization of a 3-hydroxycarboxylic acid with sulfonyl chloride. An alternative synthesis attempted to control all stereochemical relationships in the molecule using the properties of silyl moieties attached to substrates and reagents <20040BC1051>. Stereoselective reactions of this type included the use of silyl groups in enolate alkylations, hydroboration of allylsilanes, and an anti Se2 reaction of an allenyl silane with an aldehyde and ry -silylcupration of an acetylene. The /3-lactone was again formed by the standard sulfonyl chloride cyclization method. 

A novel Si-macrocycle was prepared by the demetalation of the zirconocene-containing dimeric cyclophane intermediate, which was formed by the coupling of dimethyl-fc(4-trimethylsilylethynylphenyl)silane with [Zr(Cp)2Cl2] <02CEJ74>. Synthesis, X-ray analysis, and characterization of a dibenzo macrocyclic acetylenic diphenylsilane have been reported <02TL2079>. 

Enantioselective reduction of acetylenic ketones is a route frequently used to prepare the starting materials. The synthesis of a dihydrocompactin precursor 10 involves such a reaction sequence via Ireland rearrangement of 8477. Problems arising with considerable C-silylation of the ester enolate in the rearrangement of 8 can be overcome by adding hexamethylphosphoric triamide to the enolate (1.2 mL/mmol), followed by four equivalents of /ert-butylchlorodimethyl-silane in tetrahydrofuran. The product ester 9 is obtained as a 12 1 mixture of diastereomers in 81 % yield ... 

The hydrosilylation of alkynes is a convenient method for the synthesis of unsaturated silyl derivatives. Addition of hydrosilanes to acetylene leads to the formation of monoadduct and/or 1,2-disilylethane (diadduct). Apart from these adducts, the reaction of monoorgano-substituted acetylenes with trisubstituted silanes yields a mixture of vicinal (I) (/3-isomer) and geminal (II) (a-isomer) products, as well as the products of dehydrogenative silylation (III) and C=C migration (IV) (see Scheme 22) (17). 

A limited number of functionalized acyl silanes have been prepared by the use of 1,3-dithiane methodology as described above, but this route is unfavourable for the synthesis of a, -nnsatnrated acyl silanes . Several /8-hydroxy acyl silanes have, however, been prodnced in high yields using this chemistry acid-catalysed elimination of water from these compounds does give a,/S-unsaturated acyl silanes, and Swem oxidation gives -ketoacyl silanes (Scheme 52). Analogous acetylenic 1,3-dioxanes have been nsed as precnrsors to a, -acetylenic acyl silanes, as have formyl silanes (vide supra. Section III.A.l) ". ... 

Among the applications of allyl silanes reported this year their reaction with phenylselenyl chloride and subsequent oxidation of the resulting allyl selenides to allylic alcoholshas been used in the sequence outlined in Scheme IS. 3-Bromoallyltrimethylsilane therefore behaves as a hydroxypropenyl synthon. Nitroalkenes react with allylsilanes to give, after hydrolysis, y,S-unsaturated ketones, and a synthesis of substituted cyclopentenones based on this procedure has been described.Allyl trimethylsilyl ethers undergo a palladium-promoted coupling with aryl iodides to give /3-aryl-a,/3-unsaturated ketones, and palladium also catalyses the reaction of various aryl bromides with trimethylsilyl acetylene to produce ethynylated aromatics. 

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