3- acyl-, Schmidt reaction

Two molar equivalents of amine are required m the reaction with acyl chlorides and acid anhydrides one molecule of amine acts as a nucleophile the second as a Brpnsted base  [c.859]

Silver haUde crystals grown from a single haUde type have received considerable attention in basic studies. Some studies have focused on the growth of AgBr (59—62) and others on AgCl (63). Although analyses of the growth processes for crystals of mixed haUde content, eg, Ag(Br,I) and Ag(Cl,Br,I), have not been as extensively reported in the Hterature, mixed haUde grains of various compositions commonly are used in practical photographic materials. In the preparation of mixed haUde crystals, the various haUde types may be added to the reaction vessel either simultaneously or sequentially. In the latter case, the most soluble silver haUde crystal, ie, AgCl, is sometimes grown first, and through subsequent haUde additions the silver haUde is converted pardy to a silver haUde of lower solubiUty, eg, AgBr. Photographic (64—66), electron-microscopic (67), and x-ray diffraction studies (68—70) suggest that this conversion involves a surprisingly efficient intragranular anionic diffusion. If the least soluble silver haUde crystals are grown first, epitaxial growth can be achieved. In epitaxial growth the second phase is crystaHographicaHy oriented on the substrate phase (71). For example, AgCl growth on preformed P-AgI microcrystals has been reported (72). The two phases (AgCl and Agl) of the epitaxial crystals are in electrical contact. In fact, mobile photogenerated electrons produced in the Agl phase have been shown to migrate into the AgCl phase.  [c.444]

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15)  [c.109]

A serine proteinase cleaves peptide bonds within a polypeptide to produce two new smaller peptides (Figure 11.4). The reaction proceeds in two steps. The first step produces a covalent bond between Ci of the substrate and the hydroxyl group of a reactive Ser residue of the enzyme (Figure 11.5a). Production of this acyl-enzyme intermediate proceeds through a negatively charged transition state intermediate where the bonds of Ci have tetrahedral geometry in contrast to the planar triangular geometry in the peptide group. During this step the peptide bond is cleaved, one peptide product is attached to the enzyme in the acyl-enzyme intermediate, and the other peptide product rapidly diffuses away. In the second step of the reaction, deacylation, the acyl-enzyme intermediate is hydrolyzed by a water molecule to release the second peptide product with a complete carboxy terminus and to restore the Ser-hydroxyl of the enzyme (Figure 11.5b). This step also proceeds through a negatively charged tetrahedral transition state intermediate (Figure 11.5b). What are the structural requirements for the enzyme to perform these reactions  [c.208]

Kinetic studies of the reaction of alcohols with acyl chlorides in polar solvents in the absence of basic catalysts generally reveal terms both first-order and second-order in alcohol. Transition states in which the second alcohol molecule acts as a proton acceptor have been proposed  [c.486]

Two molai equivalents of anine are requued in the reaction with acyl chlorides and acid anhydrides one molecule of amine acts as a nucleophile, the second as a Brpnsted base.  [c.859]

Two particularly interesting aspects of the pyruvate carboxylase reaction are (a) allosteric activation of the enzyme by acyl-coenzyme A derivatives and (b) compartmentation of the reaction in the mitochondrial matrix. The carboxy-lation of biotin requires the presence (at an allosteric site) of acetyl-coenzyme A or other acylated coenzyme A derivatives. The second half of the carboxylase reaction—the attack by pyruvate to form oxaloacetate—is not affected by CoA derivatives.  [c.745]

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA.  [c.788]

FIGURE 25.12 Elongation of fatty acids in mitochondria is initiated by the thiolase reaction. The /3-ketoacyl intermediate thus formed undergoes the same three reactions (in reverse order) that are the basis of /3-oxidation of fatty acids. Reduction of the /3-keto group is followed by dehydration to form a double bond. Reduction of the double bond yields a fatty acyl-CoA that is elongated by two carbons. Note that the reducing coenzyme for the second step is NADH, whereas the reductant for the fourth step is NADPH.  [c.814]

The reaction of ammonia and amines with esters follows the same general mech anistic course as other nucleophilic acyl substitution reactions (Figure 20 6) A tetrahe dral intermediate is formed m the first stage of the process and dissociates m the second stage  [c.857]

Site-specificity of the reaction is established in the first step since enolate formation involves the carbonyl carbon and the former halide bearing carbon, while the stereospecificity of the incoming deuterium is determined during the second step. It appears that the ketonization in deuterioacetic acid yields mainly the kinetic product (axial attack) although deuteration is  [c.201]

A study of nonsteroidal examples has led to the suggestion that the elimination of vicinal ditosylates involves nucleophilic displacement of one tosy-late by iodide. Reductive elimination then occurs if the geometry is correct otherwise, a second displacement occurs which then gives the required trans arrangement. The reason for the failure of reaction with 2jS (axial) isomers is not clear.  [c.345]

The reaction of anmonia and amines with esters follows the sane general mechanistic course as other nucleophilic acyl substitution reactions (Figure 20.6). A tetrahedral intennediate is fonned in the first stage of the process and dissociates in the second stage.  [c.857]

The side chains of amino acids in proteins offer a variety of nucleophilic centers for catalysis, including amines, carboxylates, aryl and alkyl hydroxyls, imidazoles, and thiol groups. These groups readily attack electrophilic centers of substrates, forming covalently bonded enzyme-substrate intermediates. Typical electrophilic centers in substrates include phosphoryl groups, acyl groups, and glycosyl groups (Eigure 16.9). The covalent intermediates thus formed can be attacked in a subsequent step by a water molecule or a second substrate, giving the desired product. Covalent electrophilic catalysis is also observed, but usually involves coenzyme adducts that generate electrophilic centers. Well over 100 enzymes are now known to form covalent intermediates during catalysis. Table 16.2 lists some typical examples, including that of glyc-eraldehyde-3-phosphate dehydrogenase, which catalyzes the reaction  [c.509]

The final step in the /3-oxidation cycle is the cleavage of the /3-ketoacyI-CoA. This reaction, catalyzed by thiolase (also known as j8-ketothiolase), involves the attack of a cysteine thiolate from the enzyme on the /3-carbonyI carbon, followed by cleavage to give the etiolate of acetyl-CoA and an enzyme-thioester intermediate (Figure 24.17). Subsequent attack by the thiol group of a second CoA and departure of the cysteine thiolate yields a new (shorter) acyl-CoA. If the reaction in Figure 24.17 is read in reverse, it is easy to see that it is a Claisen condensation—an attack of the etiolate anion of acetyl-CoA on a thioester. Despite the formation of a second thioester, this reaction has a very favorable A).q, and it drives the three previous reactions of /3-oxidation.  [c.788]

Mammals can add additional double bonds to unsaturated fatty acids in their diets. Their ability to make arachidonic acid from linoleic acid is one example (Figure 25.15). This fatty acid is the precursor for prostaglandins and other biologically active derivatives such as leukotrienes. Synthesis involves formation of a linoleoyl ester of CoA from dietary linoleic acid, followed by introduction of a double bond at the 6-position. The triply unsaturated product is then elongated (by malonyl-CoA with a decarboxylation step) to yield a 20-carbon fatty acid with double bonds at the 8-, 11-, and 14-positions. A second desaturation reaction at the 5-position followed by an acyl-CoA synthetase reaction (Chapter 24) liberates the product, a 20-carbon fatty acid with double bonds at the 5-, 8-, IT, and ITpositions.  [c.816]

The (V-methyldihydrodithiazine 125 has also been used as an effective formyl anion equivalent for reaction with alkyl halides, aldehydes, and ketones (77JOC393). In this case there is exclusive alkylation between the two sulfur atoms, and hydrolysis to give the aldehyde products is considerably easier than for dithianes. However, attempts to achieve a second alkylation at C2 were unsuccessful, thus ruling out the use of this system as an acyl anion equivalent for synthesis of ketones. Despite this limitation, the compound has found some use in synthesis (82TL4995).  [c.108]

See pages that mention the term 3- acyl-, Schmidt reaction : [c.180]    [c.19]    [c.67]    [c.380]    [c.302]    [c.215]   
Advances in heterocyclic chemistry Vol.2 (1963) -- [ c.397 ]