Aldehydes triethyl orthoformate

Alkyl groups at the a- and y-positions of pyrylium salts are, as might be expected, quite acidic reaction at a 4-methyl is somewhat faster than at an a-methyl. Such methyls will, for example, condense with aromatic aldehydes, triethyl orthoformate and dimethylformamide. ... 

Aldehydes can be obtained by reaction of Grignard reagents with triethyl orthoformate. The addition step is preceded by elimination of one of the alkoxy groups to generate an electrophilic oxonium ion. The elimination is promoted by the magnesium ion acting as a Lewis acid.93 The acetals formed by the addition are stable to the reaction conditions, but are hydrolyzed to aldehydes by aqueous acid. 

A big problem in asymmetric hydroformylation is that the chiral aldehyde products may be unstable and may undergo racemization during the reaction. This problem is even more serious for the Pt catalyst systems, which are usually plagued by slow reaction rates. Stille et al.121 tackled this problem by using triethyl orthoformate to trap the aldehyde products as their diethyl acetals and consequently increased the product ee values significantly. 

Synthesis of l-hydroxy-3-oxyimidazolines from a-hydroxylaminooximes has been reviewed " . Therefore, in the present work it will be described very briefly. In general, the reaction of oximes 136 with aldehydes, ketone or triethyl orthoformate leads to imidazolines 137 or 138, respectively (equation 60). 

The use of aldehyde derivatives serves in lieu of the carboalkoxy group in cyclization reactions. The oxime 473, when allowed to react with triethyl orthoformate, gives the -oxide 474 (Equation 177) <1996H(43)389>. 

The hydroformylation of styrene in triethyl orthoformate is slower than that observed in benzene, but a 98% ee is obtained, since racemization of the product acetal does not occur. Hydrolysis of the acetal to the aldehyde can be accomplished without racemization. A number of other substrates are hydroformylated in the presence of triethyl orthoformate. The reactions are slower, but with all substrates tried except norbomene, enantiomerically pure products can be obtained. 

One of the difficulties in achieving high enantioselectivity in asymmetric hydroformylation is the propensity of chiral 2-arylpropanal to racemize under the reaction conditions. Accordingly, if the chiral aldehyde can be converted to a less-labile derivative in situ, higher enantioselectivity might be anticipated. In fact, when the asymmetric hydroformylation of styrene and its derivatives catalyzed by PtCl2(BPPM)/SnCl2 was carried out in triethyl orthoformate, the... 

The TV-amino functions in the derivatives (286) condense with triethyl orthoformate to yield the intermediate condensation products (287) which are readily cyclized by the action of p-toluene-sulfonic acid to yield the tricyclic systems (288) (Scheme 22) <92EJM73>. The isothiazolopyrimidines (290) are formed by condensation of compound (289) with aldehydes (Equation (37)) <83MI 712-02). 

Carbon atom 2 of the oxazole ring is also supplied by aldehydes in their reaction with a- (hydroxylamino) ketones, which proceeds in the presence of sulfuric acid and acetic anhydride (equation 109). Three further oxazole syntheses involving incorporation of a C(2) fragment are the condensation of triethyl orthoformate with the hydrochlorides of a-aminoacetophenones (equation 110), the reaction of acyl chlorides with a-azido ketones or a-azido esters in the presence of triphenylphosphine (equation 111), and the preparation of 2-aminobenzoxazole and benzoxazoleimines from o-aminophenols and cyanogen bromide (equation 112). 

An extension of the Erlenmeyer synthesis is the condensation of acylamino acids with triethyl orthoformate which leads to the ethoxymethylene derivatives (297). These can be hydrolyzed to the corresponding enols, which in turn can be converted into chloromethylene compounds, e.g. (298). The lability of the chlorine atom in this compound (see arrows) can be put to good account reaction with organometallic compounds or with benzene and its derivatives under Friedel-Crafts conditions yields unsaturated lactones (298 aryl or heteroaryl in place of Cl). The method is especially valuable in cases where the aldehyde is not readily available. 

The two metals that have been found to give encouraging conversions and selectivities for the hydroformylation of styrene are platinum and rhodium. The platinum-based catalytic system uses tin chloride as a promoter. It also uses triethyl orthoformate as a scavenger that reacts with the aldehyde to form the acetal. By removing it as soon as it is formed, any further degradative reactions of the aldehyde are avoided. The chirality in these reactions is induced by the use of optically active phosphorus ligands. With the best platinum catalyst, branched and linear aldehydes are produced in about equal proportion, but the former has an e.e. of >96%. 

The hydroxy-binaphthyl functionalised saturated imidazolium salt is readily available from 1-amino-I -hydroxy-binaphthyl in a reaction with a ( oc-protected mesitylamine aldehyde [86] (see Figure 4.24). The resulting Schiff base is reduced to the diamine by Na(OAc)3BH. Subsequent deprotection and ring closure reaction with triethyl orthoformate yields the corresponding hydroxy-binaphthyl functionalised saturated imidazolium salt. Reaction with silver(I) carbonate and subsequent carbene transfer to the ruthenium(II) precursor yields the asymmetric olefin metathesis precatalyst. 

Q A convenietu synthesis of aldehydes involves the reaction of a Grignard reagent with triethyl orthoformate [triethoxymethane, HC(OEt),]. Suggest a mechanism for this reaction and hence explain why the reaction stops at the aldehyde stage. 

Various carbonyl compounds were efficiently converted into the corresponding 1,3-dithiolanes using triethyl orthoformate as a water scavenger and (bromodimethyl)sulfonium bromide as an efficient catalyst under solvent-free conditions <2004EJ02002>. This protocol can be applied for the chemoselective protection of aldehydes in the presence of ketones on a large scale (Equation 73). 

Carbonylation of aromatic olefins also provides an attractive route to profens. The key issues in the hydroformylation reaction (cf. Section 2.1.1 and Scheme 4) are the branched/normal selectivity and the configurational stability of the branched aldehydes. When the Pt -catalyzed asymmetric reaction is performed with triethyl orthoformate, the aldehyde product is immediately removed as... 

Ketones and aldehydes can be interconverted with acetals. (IUPAC now discourages the use of the word ketal.) Acetalizations are usually thermodynamically uphill, so the reaction is usually driven to completion by the removal of H2O from the reaction mixture by azeotropic distillation or by addition of a dehydrating agent like 4 A molecular sieves or triethyl orthoformate (HC(OEt)3). Hydrolyses of acetals are usually executed simply by dissolving the acetal in water and a cosolvent such as THF and adding a catalytic amount of acid. 

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