Endo-type dimer

Biosynthesis of spiroisoozoline was proposed by K. T. Okamoto, J. Clardy et al. [36]. The dimmer structure of zamamistatin (17) may be biologically synthesized by reductive dimerization of the spiroisooxazolin moiety in a bromotyrosine precursor followed by decarboxylation, hydrolysis and recyclization, Fig. (13). Isomerization from an exo-type dimmer (previously proposed structure for zamamistatin) to an endo-type dimmer (revised structure) was supported by computational calculation, in which the latter is more stable than the former [42]. 

Another example of the retention of volatile DA reagents is that of cyclopentadiene in a tandem retro-DA/DA prime reaction [15, 16, 38], This reaction type is the thermal decomposition of a DA adduct (A) and the generation of a diene (generally the initial diene) which is trapped in situ by a dienophile leading to a new adduct (B) [39]. Cyclopentadiene (22) (b.p. 42 °C) is generated by thermolysis of its dimer at approximately 160 °C [40]. An equimolar mixture of commercial crude dicyclopenta-diene (21) and dimethyl maleate was irradiated in accordance with the GS/MW process, in an open reactor, under 60 W incident power, for 4 min (8 x 30 s). The expected adduct 23 was isolated in 40% yield (Scheme 7.1). The isomeric composition of 23 (endo-endoIexo-exo = 65/35) was identical with that obtained under classical conditions from 22 and methyl maleate [41]. The overall yield of this tandem reaction can be increased from pure dimer 21 (61%) and the same tandem reaction has also been reported using ethyl maleate as dienophile [31]. 

Stereospecificity manifests itself in the dimerization of a diene with a diene. The double bond that remains free may deviate from the ring formed (exo configuration) or may approach it (endo configuration). Endo condensation is the predominant pathway in the case of ion radical reactions (Scheme 6-17). As seen, the charge distribution in the reactants dictates the head-to-tail pathway of the reaction. For the cation radical, the position selectivity at the C(l) atom is 100%, regioselectivity being 0%, whereas at the C(4) atom the position selectivity is 0% and regioselectivity is 100%. In other words, only the addition of the D+ -C(l) + D°-C(4) type is observed (symbol D° refers to a neutral diene and D+ to a diene in cation radical form). 

The proposed mechanism for the cucurbit[n]uril synthesis is presented in Scheme 3 and involves the kinetic formation of a mixture of 1,3,5,6-tetrazocane derivatives, endo and exo glycouril dimers 66 and67 <2001JOC8094> (cf. Section 14.09.1 for S- and C-shaped glycouril dimers 1 and 2). Manipulation of the acid concentration, acid type, temperature, and reactant concentrations allows a degree-of-selectivity in the cucurbit[n]uril synthesis to be achieved. It is expected that the number of repeat glycouril units in cucurbit[n]uril significantly impacts the properties of these macrocyclic molecules. 

Figure 7. Complete proteolytic stability of all types of P-and y-peptides towards a variety of peptidases. The P-peptides ranged in size from dimer to ISmer. The enzymes include all common types of peptidases (endo/exo, metallo, serine, threonine, and aspartyl proteases). After 40 hours there was no observable cleavage of any of the homologated peptides and no inhibition of the enzymes [41].

The distribution of the reaction products confirms that most of cellulases from various sources show an endo-acting nature, they predominantly liberate a dp2-10 mixture, made up of dimers, trimers and oligomers from chitosan, though there are few in exo-type, for instance, the bifunctional enzyme purified from a commercial cellulase [29,56] and from r.ree5e [57]showed an exo-(3-D- glucosaminidase activity on chitosan. 

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