Cyclopentylidene derivatives

In addition, the cyclopentylidene ketal has been prepared from dimethoxy-cyclopentane, TsOH, CH3CN, or cyclopentanone (PTSA, CUSO4 >70% yield) and can be cleaved with 2 1 ACOH/H2O, rt, 2 h. Certain epoxides can be converted directly to cyclopentylidene derivatives as illustrated in the following reaction ... 

Methyl 4,6-O-methylene-o-D-mannopyranoside was the only product isolated from the LiBr-catalised transacetalation of the unprotected methyl glycoside with dimethojqmiethane. Cyclopentylidene derivatives of pentoses have been prepared in moderate yields by treatment of the free sugars with cyclopentanone in the presence of copper (II) sulphate and sulphuric acid. D-Xylose formed the diacetal (11) (also used in Scheme 3 below), whereas from D-ribose the 2,3-monoacetal (12) was obtained. A novel, selective synthesis of (5)-configurated 4,6-pyruvate acetals of methyl D-hexopyranosides is illustrated in Scheme 1. It relies on transacetalation from the dimethyl acetal of 3,4-dimethoxybenzophenone to give, after acetylation, preferentially the intermediate (13) with an axial aryl substituent which, on oxidation, suffers rapid degradation to a carboxylic acid group. ... 

Alkynyl zincates (59), derived from 5-hexynyl tosylates, undergo a 7r-type endo cyclization to form cyclohexynes (60).58 This reaction takes place in competition with exo cyclization, leading to the formation of l-(cyclopentylidene)alkylzincs (61). 

The chemistry of cycloalkylidene acetals is very similar to isopropylidene acetals but the cyclic derivatives have two advantages over their acyclic counterparts. First, they diminish water solubility of low molecular weight fragments and therefore facilitate isolation. Secondly, they can show a greater bias for the formation of 1,3-dioxolanes over 13-dioxanes Cyclopentylidene acetals are more easily hydrolysed than isopropylidene or cyclohexylidene acetals. 

Thus, 5-hexynyllithiums bearing an alkyl, aryl, or trimethylsilyl substituent on the terminal alkyne carbon cyclize readily upon warming to room temperature and the exocyclic vinylic organolithium 14 derivative may be trapped by reaction with different electrophiles. The alkyl-substituted system such as dec-5-ynyllithium 13a (R = Bu) undergoes clearly first-order cyclization to l-(cyclopentylidene)pentyllithium 14a (R = Bu) with a half-life of 7 min at 28.8 °C, and the activation parameters that characterize this reaction indicate that the rate of cyclization is strongly dependent on the temperature. 

Acetals, including benzylidene, cycloxehylidene, and cyclopentylidene, reveal virtually unlimited stability to basic conditions, which is an important chemical property that is crucial for any synthetic strategy requiring those conditions. On the other hand all the acetals are quite sensitive towards acid. This specific chemical property derives from participation of a lone pair on the adjacent oxygen atom in the cleavage of a resonance stabilized and protonated intermediate, known as an oxonium ion. 

A five-membered cyclopentylidene can be cleaved in the presence of a six-membered derivative. ... 

O-AIkylidene and -arylidene derivatives of D-gluco-, D-allo-, D-manno-, and D-talo-pyranose are difficult to prepare directly. Lem-ieux and Detert59 prepared these from the 1,2-O-ethoxyethylidene orthoester, and were able to recover quantitative yields of 1,2-O-iso-propylidene, 1,2-O-cyclohexylidene, 1,2-O-cyclopentylidene, and diastereoisomeric 1,2-O-benzylidene acetals of D-glucopyranose. In their procedure, the orthoester is allowed to react, with scrupulous exclusion of moisture, with a ketone or aldehyde in the presence of an acid catalyst. 

O-rm-Butyl trichloroacetimidate. prepared in 70% yield by reacting potassium rm-butoxide with trichloroacetonitrile. reacts with carboxylic acids and alcohols in the presence of a catalytic amount of boron trifluoride etherate at room temperature in cyclohexane-dichloromethane [Scheme 6.3S]. 7 The method also converts alcohols to ferr-butyl ethers (see section 4.3.2). A very similar reaction that allows rerr-butylation under essentially neutral conditions on a large scale involves reaction of a carboxylic acid with 3-4 equivalents of N,V-di-isopropyl-0-/er/-butylisourea [Scheme 6.36]. The reaction proceeds via a tertiary carbocation ion intermediate and since capture of the cation is inefficient, excess isourea is required. The presence of alcohols is tolerated but not thiols or unhindered amines. The reaction conditions are compatible with a range of acid sensitive groups such as N-trityl derivatives and cyclopentylidene acetals. ... 

Methyl cyclopentylidene(phenylsulphinyl)acetate (79) undergoes an additive Pummerer rearrangement in methylene dichloride induced by acetyl chloride or trifluoroacetic anhydride and SO Cl to give a near quantitative yield of the useful a,p-bifunctional phenylthio-derivative (83) after reflux of (82) in aqueous dioxan. This chloride is formed via the intermediate (81) derived by exclusive attack of Cl on the p-olefinic carbon in (80) (Scheme 4). 

The synthesis of l,S-dideoxy-l,S-[JV-hydroxy(alkoxy)imino]>D>lyxitol derivative 27 has been achieved by treating l-0-acetyl-2,3-di-0-cyclopentylidene-a-D-() o-pentodialdose-l,4-furanose with hydroxylamine or 0-( )-phenylprop-2-enylhydroxylamine (PhCH=CHCH20NH2) followed by reductive cyclization/ ... 

The first well-defined tungsten(Vl) alkylidenes which serve as highly active metathesis inititors were reported by Osborn and co-workers in 1982 [149]. W(=CH-t-Bu)(OCH2-t-Bu)2X2 (13) in combination with AlBrs or GaBrs polymerizes a variety of cycloolefins [61,62,149-151]. Later it has been reported that the related cyclopentylidene complex W(=C(CH2)4)(OCH2-t-Bu)2Br2 polymerizes numerous cycloolefins, e.g., various methoxycarbonyl derivatives of norbomene, even without addition of a Lewis acidic cocatalyst [152-154]. 

Sugar cyclopentylidene acetals hydrolyse slightly faster than the corresponding isopropylidene derivatives. l,2 3,5-Di-0-cyclopentylidene-a-D-xylose (11) was fully deprotected by use of strong acid resin and methanol, whereas use of methanolic HCl allowed the selective removal of the dioxane ring. This selectivity has been exploited, for... 

Triplet photoinitiation has been used to generate radicals from simple carbohydrates, and these species have been studied using time-iesolved e.s.r.i E.s.r. has also been used to study cuminoxyl radicals derived ftom a series of 3-(lV-arylmethyl-iV-hydroxyamino)-l,2-0-cyclopentylidene-3-deoxy-5-0-toluoyl-a-D-ribofuranose derivatives. 

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