Preparation of Unsaturated Esters

This four-component reaction represents a straightforward method for the preparation of unsaturated esters and amides [156-159]. The system Pd(OAc)2/SIPr HC1 has been reported to efficiently catalyze the coupling of diazonium salts with boronic acids in the presence of CO and ammonia [160] to yield the corresponding amides in good yields (Scheme 17). 

The ortho-ester Claisen rearrangement continues to provide a versatile method for the preparation of unsaturated esters. Reaction of trimethyl-methoxyorthoacetate with an allyl alcohol furnishes a-methoxy-Y,d-unsaturated esters in 22—55% yields. A stereospecific synthesis of both enantiomers of the insect pheromone methyl E-2,4,5-tetradecatrienoate involves the conversion of the R-acetylenic alcohol (65) (Scheme 53) into the R-allenic ester (66) as its key step. This process was repeated with the 5-alcohol to provide the enantiomer of (66). The optically active erythro-alcohol (67), readily available... 

The Homer-Emmons olefination shows a preference for the formation of the more stable ( )-olefins. On the other hands. Still and Gennari modified HWE olefination shows high (Z)-stereoselectivity and high yield in the preparation of unsaturated esters, 45(Z) 46( ) = > 50 1, from a variety of aromatic, saturated and unsaturated aliphatic aldehydes. ... 

Wittig and Horner-Wittig Reactions. (Me2N)3P may be used in place of Triphenylphosphine in Wittig reactions with aldehydes and ketones (eq 6). It is advantageous because the water solubility of the byproduct, hexamethylphosphoric triamide, renders it readily removable. This method has been used for the preparation of unsaturated esters as well as alkenes (eq 7). 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

For the study of the physical and chemical properties of triply unsaturated fatty acids and for the comparison with y-linolenoic acid [6Z,9Z.12Z)-6,9,l2-octadeca-trienoic acid] coworkers of the Unilever Research Centre in Vlaardingen (Netherlands) synthesized some methyl esters of ( ,Z,Z)-trisunsaturated fatty acids 73). For the preparation of the esters of (2E,9Z, 12Z)-2,9,12-octadecatrienoic acid and of (2 ,11Z,14Z)-2,11,14-eicosatrienoic acid (68 a and b) they used the Wittig reaction to introduce the ( )-2-double bond into the starting diunsaturated aldehyde. Reduction of the acid chlorides of (7Z,10Z)-7,10-hexadecadienoie acid and of linolenoic acid (65a and b) with lithium tri-tert-butoxyaluminium hydride affords the corresponding aldehydes 66 a and b which can be olefinated with the stable ylide 67 to methyl ( ,Z,Z)-alkatrienoates 68a, b with a (Z)-2-isomer content of 4.6%73) (Scheme 13). 

Lx)ng-chain fatty acid cellulose esters were synthesized by the acid-chloride-pyridine reaction with different degrees of substitution. Hydrolyzed soybean oil was used as an unsaturated fatty acid [82]. The preparation of cellulose esters with substituted benzoic acids in the presence of p-toluenesulfonyl chloride in pyridine was investigated [83]. The substituents included NO2, Cl, Me, and MeO. The substituent and its position (ortho, meta, or para) did not influence the reaction significantly. 

Protodesilylation, A new preparation of -unsaturated carboxylic acids involves the amide acetal Claisen rearrangement of a 3-(trimethylsilyl)allyl alcohol such as 1 to allylsilanes (2). Desilylation of 2 by usual methods results in formation of stereoisomers. However, use of liquid HF at low temperatures results only in (E)-3 in 88% yield. The HF-pyridine complex is not so stereoselective. Amides such as 3 can be converted into carboxylic acid esters by Meerwein s trialkyloxonium salts (—70% yield). 

Reaction of a mixture of unsaturated esters 271a and 271b with KHMDS followed by addition of A -sulfonyloxaziridine 33 gave a-hydroxy ester 272 in 72% yield <1995TL413>. The bromo analogue was prepared similarly in 78% yield. 

Dialdehydes result when cyclic olefins are ozonized. Improved directions for the ozonolysis of unsaturated esters in glacial acetic acid to yield aldehyde esters have been given. The same procedure is applied to the preparation of aliphatic aldehydes containing halo, hydroxyl, and ether groups. ... 

An alternate procedure involves the precipitation of mixed copper and chromium hydroxides from a solution of chromium nitrate and copper nitrate by the addition of an sodium bicarbonate and then heating the precipitate to 300°-500°C.30 In this procedure, the copperchromium ratio can be varied over a wide range. A ratio between four and eight was optimum for use in the hydrogenation of esters to alcohols (Eqn. 13.6).30 A related Zn-CrO catalyst prepared by the decomposition of precipitated zinc-copper hydroxides was effective in the hydrogenation of unsaturated esters to unsaturated alcohols (Eqn. 13.7). 0 The presence of a small amount of alumina increased catalyst activity and selectivity. Some of these catalysts, however, tend to become colloidal on use so they can present separation problems. O... 

RCOOH + BF3 Et20 -t- MeOH. This procedure has been used for the preparation of unsaturated methyl esters. ... 

The Michael reaction of sulfonyl carbanions with a,p-unsaturated esters provides a useful preparation of cyclopropane esters, as shown in Scheme 47. In this sequence, the sulfonyl carbanion undergoes conjugate 1,4-addition to the a,p-unsaturated carbonyl compound followed by intramolecular elimination of the benzenesulfinate anion (Scheme 47). 

Carboxylic Acids and Esters. The literature on German methods for the preparation of unsaturated acids and their esters via acetylene, carbon monoxide, and water or alcohol, as well as the application of this reaction to olefins, carbon monoxide, and water or alcohol to give saturated acids and esters, is to be found in references (27, 44, 4, 3,94)- Other references on these methods are Blair-McGuffie (7) and Natta... 

The imide structure generated from trimellitic anhydride (7) and 4,4 -diami-nodiphenylmethane (8) are used for the preparation of unsaturated poly(ester-imide)s having the imide in the polymer backbone. Thermal properties are su-... 

Anhydrides of unsaturated acids could react in pyridine, as shown in the preparation of methacrylic esters.1908 1909 As with acetic anhydride, higher aliphatic acyl anhydrides produced triesters,1910 but benzoic anhydride produced only a diester. Subsequent acylation with benzoic and acetic anhydrides provided a mixed acetyldibenzoyl starch.1911... 

An improved general method for reducing fatty acid esters by sodium was worked out by Hansley,521 in which secondary alcohols serve best as the reducing alcohol. Only a small excess—about 5%— of sodium and reducing alcohol is used, but in addition an inert solvent in the form of an ether or hydrocarbon such as xylene. The ester and reducing alcohol are treated with sufficient of the solvent to keep the reaction mixture liquid, and finely divided sodium, in the solvent, is added with stirring at the boiling point. The yields amount to 80-95%. This method is particularly useful for preparation of unsaturated alcohols from unsaturated esters. 

The Claisen condensation is little used for the preparation of <%,/ -unsaturated carboxylic acids since the methods given above are usually preferable. Cinnamic esters have, however, been obtained by Claisen s method from aromatic aldehydes, alkanoic esters, and metallic sodium, e.g., methyl 4-methoxycinna-mate924 and ethyl 5-(2-furyl)-2,4-pentadienoate925 in yields between 70% and 80%. -Substituted cinnamic esters can also be obtained by this method, from higher alkanoic esters.926... 

Several protocols for the preparation of unsaturated ketones by the reaction of alkenyl- or alkynyl- metal derivatives with esters or with acid chlorides have been reported. Thus a,0-unsaturated... 

A Claisen-Johnson rearrangement with transfer of chirahty has been used by Pearson in a stereoselective synthesis of A-factor 199, an autoregulator of the production of streptomycin in Streptomyces griseus [42]. Allylic alcohol 200 is prepared in 80% yield and 84% ee by oxazaboroUdine reduction of the corresjxjnding ketone. Orthoester rearrangement afforded ester 201 in 75% yield. Ozonolysis of unsaturated ester 201 is then followed by lactonization (Scheme 6.29). 

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