Exo-type synthesis

Influences of glucose, cellobiose, sophorose, and cellulose, when they were each used as a C-source, upon the formation of both cell-bound and extracellular cellulases during the growth of this pseudomonad are shown in Figure 1. Glucose supported the bacterium for an excellent growth, but only slightly stimulated the formation of cellulases, and the enzymes produced were distributed almost equally to the cell and the culture medium. In the cellulose and sophorose cultures, the formation of cellulases, particularly that of extracellular component, was enhanced prominently (exo-type synthesis), whereas cellobiose which was a main end-product of enzymatic cellulolysis stimulated the formation of cell-bound component (endo-type synthesis). Thus, an apparent difference in the distribution of extracellular and cell-bound cellulases was noticed between the cultures with cellobiose and sophorose or cellulose. 

Moreover, it is worthwhile to note that cellulase components A and B always occurred in pairs in all fractions of the cell under the cultural conditions to cause an exo-type synthesis of cellulase. The component C was, on the contrary, produced under the cultural conditions to cause both exo- and endo-type syntheses. These facts suggest not only the possible relationship between the components A and B with regard to their formation, but also the possible formation of the component C being independent of the other two. 

Ps. fluorescens var. cellulosa synthesized a large amount of cellulase in 0.5% cellulose or sophorose medium, and more than 90% of the enzyme formed was released in surrounding medium (exo-type synthesis). In contrast, smaller amounts of cellulase were formed upon cultures on 0.5% cello-oligosaccharides, and more than 90% of the enzyme remained within the cells (endo-type synthesis). Since the major soluble end-products of cellulolysis by Pseudomonas cellulases in vitro were cellobiose and cellotriose (28), the direct C-source utilized by the bacterium should be these sugars even if either cellulose or cello-oligosaccharides were used as C-source. The only difference between these cultures, therefore, may be in the concentration of the end-products in each medium. 

This section describes two examples, in which it was very hard to distinguish a compound as either exo-type form or endo-type form by NMR analysis (Table 7). One structure was revised from endo-type form to exotype form and the other vice versa. The first structure revision in this section was one of the most challenging examples of structural reassignment since the original (proposed) and the revised (alternative) structures show essentially the same 2D NMR correlation pattern. Thus, either total synthesis or NMR chemical shift simulations are required to confirm the structure. The first structure revision of zamamistatin... 

As shown in Table VI, only slight changes in the relative CMCase activity were found in the mixtures of the cellulase components A and C with any of the broken spheroplast preparations, while a considerable increase in the activity was noticed in the mixtures with the component B. The enhancement in the mixtures treated with those SI and S3 was particularly high, which were both the broken spheroplasts from the cultures of an exo-type cellulase synthesis the cellulase activity was enhanced five times that of the original. S2 preparation from the culture of an endo-type cellulase synthesis was much lower in the activating... 

Gold-catalyzed 5-exo-type cyclization is also useful for oxazole synthesis. It was reported that AuCls is efficient in the electrophilic activation of carbon-carbon triple bonds of A-propargylamides 144 to afford 2,5-disubstituted oxazoles 150 under mild conditions (Scheme 19.36) [59]. The reaction proceeds through stereospecific oxyau-ration to generate vinylgold(III) intermediates 149 [60], which would be converted to the oxazoles 150 by protodeauration and subsequent isomerization. Vinylgold... 

A ruthenium-catalyzed intramolecular olefin hydrocarbamoylation for the regiodivergent synthesis of indolin-2-ones and 3,4 dihydroquinolin-2-ones was disclosed by Chang and coworkers (Eq. (7.3)) [8]. The reactions underwent smoothly without requiring external CO atmosphere. In the presence of combined catalysts of Ru3(CO)i2/Bu4NI, a 5-exo-type cyclization proceeds favorably to form indolin-2-ones as a major product in good to excellent yields in DMSO/toluene cosolvent (catalytic system A). When the reaction was conducted in the absence of halide additives in NAf-diniethylacetamide (DMA)/PhCl (catalytic system B), 3,4-dihydroquinolin-2-ones were obtained in major in moderate to high yields via a 6-endo cyclization process. An excellent level of regioselectivity was observed with a variety of substrates to deliver 5-exo- or 6-endo-cyclized lactams. 

Furthermore, compounds of the type exo/endo-1 are precursors for the synthesis of the se-ven-membered rings 8. 

Scheme 11.18 Auxiliary control in an Ueno-Stork-type 5-exo-trig cyclization in the synthesis of an enantiomerically pure lactone 60 [77]. a)90 10 mixture of diastereomers. b)>99% ee.

The synthesis of tacamonine, an indole alkaloid of the Iboga type, was accomplished in both racemic and homochiral forms, by incorporating a classical 6-exo-trig radical cyclization in the key step of the synthesis (Reaction 7.57) [52], The cyclization produced piperidinone in a 72% yield as a diaster-eomeric mixture. 

Some A -enecarboxylic acids undergo a rapid intramolecular cyclization reaction of the 5-exo-trig type prior to the radical coupling step. Substituted tetrahydro-furans [95] and pyrrolidines [96] can be prepared by this Kolbe route. It is preferable to protect the amine function in the pyrrolidine synthesis by N-formy ation in... 

Azomethine yhdes have also been subjected to reactions with 165 (Scheme 12.52). Gamer and Ho (288) developed the reaction of the photogenerated azomethine ylide 173 with 165 for the synthesis of quinocarcin. The reaction gave 174 with complete endo/exo selectivity and with more than 90% de. Other types of azomethine ylides have also been used in reactions with 165 and its derivatives (289,290). 

Examples of 5-exo cyclizations in which the hydroxy group is constrained by a five-membered ring are shown by equation (53). The stereoselectivity in these cyclizations ranges from quite high in iodocy-clization (9 1 in H2O and >20 1 in CH2CI2) to only moderate (2—3 1) with mercury and phenylselenenyl chloride. P This type of cyclization has been widely applied to synthesis of prostacyclins.146 Similar cyclizations with dimethyl(methylthio)sulfonium fluoroborate have been reported recently.35... 

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