Linear substrates

Owing to flexibility in the substrate, the TycATE was also used to synthesize a variety of novel cyclic structures. Inclusion of a propargylated amino acid into the linear substrate allowed the synthesis of over 247 macrocyclic glycopeptides, where azido-sugars were coupled onto the cyclized alkyne via copper-catalyzed 1,3-dipolar cycloaddition [44] (Figure 13.12). 

The data given in Table V show not only that pancreatic amylase hydrolyzes unfractionated starch and a linear substrate at different rates but also that, for equivalent time intervals with the same concentration of pancreatic amylase, the relative concentrations of the products formed from these two substrates differ. In addition, Table VIM,M summarizes comparative data for the products of the hydrolyses of potato starch, of com amylose, and of waxy maize starch when equivalent numbers of glucosidic linkages of these substrates had been broken. 

Tables XI and XII summarize data obtained by Myrback86 for the hydrolysis of amylose by purified maltase-free malted barley alpha amylase. The hydrolysis curve with this linear substrate is much the same as those obtained with unfractionated starches, and also is similar to the curves representing the hydrolysis of amylose by pancreatic amylase.41 The flattening of the hydrolysis curves during the later stages (86) K. H. Meyer and P. Bernfeld, Helv. Chim. Acta, 24, 359E (1941).

The inhibitory effects of PVA can also be found in degradation studies of polycaprolactones (PCLs). These polyesters can be readily split by lipase enzymes binding to hydrophobic domains of that linear substrate. PVA/PCL films in contrast are not biodegradable by PCL-degrading microorganisms. It can be assumed that the surface properties of PCL change upon interaction with PVA in a manner that enzymatic accessibility of the hydrolysable PCL backbone motifs is decreased. 

Chiral ketone catalysts of the Yang-type (5a and 5b, see above) and of the Shi-type (10, Scheme 10.2) have been successfully used for kinetic resolution of several racemic olefins, in particular allylic ethers (Scheme 10.4) [28, 29]. Remarkable and synthetically quite useful S values of up to 100 (ketone 5b) and above 100 (ketone 10) were achieved. Epoxidation of the substrates shown in Scheme 10.4 proceeds with good diastereoselectivity. For the cyclic substrates investigated with ketone 10 the trans-epoxides are formed predominantly and cis/trans-ratios were usually better than 20 1 [29]. For the linear substrates shown in Scheme 10.4 epoxidation catalyzed by ketone 5b resulted in the predominant formation of the erythro-epoxides (erythro/threo-ratio usually better than 49 1) [28]. 

The first Pd-catalysed addition of ArB(OH)2 to linear substrates R1CH=CCHCOR2 (R1 = aryl, alkyl, H R2 = alkyl, OEt) with DuPHOS (206) as chiral ligand required 0 50 °C (<99% ee) 47 Its non-enantioselective version, carried out with bipyridine as ligand, has also been reported.248... 

One specific case of favorable conformational benefit for a RCM is the template-directed RCM where non-covalent interactions around an appropriate acidic and/or metal template serve to favor productive conformations. This has been found to be useful for linear substrates devoid of intrinsic conformational constraints [19]. RCM of ether 17 in the presence of lithium ions gives the best yield of product 18, in accordance with the preferential binding of Li+ by [12]crown-4, which resembles diene 17 (Scheme 2.7). 

The Pd-catalyzed allylic substitution reactions proceed via TT-allyl complex, so that branched and linear substrates yield the same products (Scheme 5.7). 

The Staudinger reaction, i.e. the [2+2] ketene-imine cycloaddition, is the most frequently employed procedure of synthesis of the azetidin-2-one nucleus. Base-promoted cyclization of suitable linear substrates is another frequently employed procedure. We wish to report the electrochemical methodologies of synthesis of P-lactams, according to both procedures, and the possible utilization of RTILs (as solvents and/or as parent of intermediates). 

The possible cyclization of linear substrates to P-lactams has been investigated by many authors [100-108]. The cyclization, promoted by bases (NaH, EtjN, CSjCOj, etc.), has been carried out in classical VOCs (MeCN, DCM) solvents (Scheme 16.10). 

Scheme 16.10 Base-promoted cyclization of linear substrates to )3-lactams...

Fig. 8.— Hydrolysis of Linear Substrate by a-Amylases. (Key O—, (1 — 4)-linked a-D-glucose residue , reducing group —, no hydrolysis , slow hydrolsrsis +, rapid hydrolysis.) Data from Ref. 159.

Scheme 30.14 Arborescent polystyrene synthesis linear substrate functionalization by chloromethylation (1) or acetylation (2), and coupling with living polystyrene to yield a GO molecule (3).

The use of enantiomerically pure iridium catalysts allows the enantioselective allylic amination of linear substrates and this has been achieved with high ee using the iridium complex of phosphoramidite (10.84) and both acyclic and cyclic amines, including pyrrolidine and piperidine. ... 

---