Esters From other starting materials

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

The baking process has remained much the same until the present day at a stoichiometric ratio of 1 4, phthalic anhydride or phthalic acid reacts with an ammonia releasing compound. The reaction may also start from other suitable materials, such as phthalic acid derivatives, including phthalic acid esters, phthalic acid diamide, or phthalimide. Appropriate ammonia releasing agents include urea and its derivatives, such as biuret, guanidine, and dicyanodiamide. The fact that a certain amount of urea decomposes to form side products makes it necessary to use excess urea. Approximately 0.2 to 0.5, preferably 0.25 equivalents of copper salt should be added for each mole of phthalic anhydride. 0.1 to 0.4 moles of molybdenum salt per mole of phthalic anhydride is sufficient. The reaction temperature is between 200 and 300°C. 

In the chemical industry, moreover, cost is an important issue. Any reaction needed to make a large quantity of a useful drug or other consumer product must use cheap starting materials. Direct enolate alkylation usually requires a very strong base like LDA to be successful, whereas the acetoacetic ester synthesis utilizes NaOEt. NaOEt can be prepared from cheaper starting materials, and this makes the acetoacetic ester synthesis an attractive method, even though it involves more steps. 

The reaction depicted was run in THF at 0 C, other solvents having been found to be inferior. The S-(2-pyridyl) thioates may be prepared through reaction of the corresponding acid chloride and 2-pyridine-thiol in the presence of a tertiary amine. They are also available directly from carboxylic acids by reaction with 2,2 -dipyridyl disulfide (Aldrithiol-2) and triphenylphosphine. In the case illustrated above, protection of the ketone would seem unnecessary if Grignard addition was selective for the thiol ester however, the starting material, 5-(2-pyridyl) y-oxopentanethioate, is not stable to the lactonization shown in equation (18). 

One special application comes when silyl enol ethers of allylic esters provide the starting material this is known as the Claisen-Ireland reaction. Esters normally form E-enolates 181 with LDA at low temperature. Because the E/Z nomenclature depends on the hierarchy of the substituents it is particularly ridiculous when applied to enol derivatives of esters. A lithium enolate (Li < C) would have the opposite stereochemical label from a silyl enol ether (Si > C) so a uniform scheme is adopted whereby the metal - O bond always has priority over the other. The mechanism 180 may remind you of the reasons for this - they are discussed in more detail in chapter 4. 

Infrared spectroscopy is used as a monitoring method when the resin is functionalized with linkers that have chemical groups that absorb in the IR spectral region, such as carbonyls, esters, amides, or oximes. IR spectroscopy has also been used to monitor quantitative loading of a resin by comparing the difference in absorbance between two peaks, one from the starting material and the other one from the final product [45]. 

Carboxylic acid esters are obtained instead of carboxylic acids if the intermediate complex formed from the starting material, carbon monoxide and catalyst is decomposed with alcohols instead of water in the second stage. Since fundamentally no other results are obtained with alcohols compared to water in view of yields or the isomer distribution, both reactions will be discussed together in the individual sections of this chapter. 

Concentrates of polyunsaturated fatty acids (as the methyl esters) can be obtained from suitable starting materials, by preparing the mercuric acetate adducts (see Chapter 4 for practical details of the preparation of adducts and regeneration of the original double bonds) and partitioning them between methanol and pentane the methanol layer retains the adducts of the more unsaturated esters which can be regenerated unchanged. For example, methyl linoleate of 95 % purity and methyl linolenate of 90 % purity have been produced on the 50-100 g scale by this method from the esters of safflower and linseed oils respectively. The method could no doubt also be adapted to the preparation of concentrates of other polyunsaturated fatty acids. 

Other approaches to (36) make use of (37, R = CH ) and reaction with a tributylstannyl allene (60) or 3-siloxypentadiene (61). A chemicoen2ymatic synthesis for both thienamycia (2) and 1 -methyl analogues starts from the chiral monoester (38), derived by enzymatic hydrolysis of the dimethyl ester, and proceeding by way of the P-lactam (39, R = H or CH ) (62,63). (3)-Methyl-3-hydroxy-2-methylpropanoate [80657-57-4] (40), C H qO, has also been used as starting material for (36) (64), whereas 1,3-dipolar cycloaddition of a chiral nitrone with a crotonate ester affords the oxa2ohdine (41) which again can be converted to a suitable P-lactam precursor (65). 

The only other functional group is the conjugated unsaturated ester. This functionality is remote from the stereocenters and the ketone functionality, and does not play a key role in most of the reported syntheses. Most of the syntheses use cyclic starting materials. Those in Schemes 13.4 and 13.5 lead back to a para-substituted aromatic ether. The syntheses in Schemes 13.7 and 13.8 begin with an accessible terpene intermediate. The syntheses in Schemes 13.10 and 13.11 start with cyclohexenone. Scheme 13.3 presents a retrosynthetic analysis leading to the key intermediates used for the syntheses in... 

Other preparations of Nazarov s reagent and its analogs have been reported,7 but many of the procedures are labor-intensive and/or require special apparatus. The reported preparation of ethyl 3-oxo-4-pentenoate is facile (2 steps) and efficient (52% overall yield). Ail starting materials are commercially available, relatively inexpensive, and easily purified. The synthesis is also amenable to scale up and has been carried out successfully on a 1-mol scale. Other esters have also been synthesized by this method with overall yields ranging from 45-58% (see Scheme l).8 Finally, methacrolein and crotonaldehyde are also suitable reactants (see Scheme I). 

At that time, as now, the enantiomers of many chiral amines were obtained as natural products or by synthesis from naturally occurring amines, a-amino acids and alkaloids, while others were only prepared by introduction of an amino group by appropriate reactions into substances from the chiral pool carbohydrates, hydroxy acids, terpenes and alkaloids. In this connection, a recent review10 outlines the preparation of chiral aziridines from enantiomerically pure starting materials from natural or synthetic sources and the use of these aziridines in stereoselective transformations. Another report11 gives the use of the enantiomers of the a-amino acid esters for the asymmetric synthesis of nitrogen heterocyclic compounds. 

The (2 + 2) photocyclodimerization of substituted olefins has provided some of the most striking examples of crystal-lattice control of the stereochemistry of a reaction. This may be exemplified by a selection of derivatives of 5-phenylbutadienoic acid (61), for which it is observed that the solid-state photobehaviors of the amide 62, the methyl ester 63, and the dichlorophenyl ester 64 differ entirely from one another each affords a single stereo- and regioisomer in high yield, but with different starting materials giving different types of products (130). In solution, irradiation of 63 or other photoactive dienes yields... 

In the absence of ultrasound, the results show a substantial amount (49 %) of the dimer bicyclohexyl from the one-electron pathway, together with cyclohexylmethyl-ether, cyclohexanol and other products from the two-electron pathway (approx. 30%). The methyl cyclohexanoate ester (17%) can be thought to arise from the acid catalysed chemical esterification of the starting material with the solvent methanol. (As a result of the high current densities needed, (parasitic) discharge of the solvent methanol produces a large quantity of protons around the anode as a competitive reaction [54].)... 

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