Cycloolefinic precursor

Such aldolase-catalyzed bidirectional chain elongation ( tandem aldolization) of simple, readily available dialdehydes has been developed into an efficient method for the generation of higher carbon sugars (e.g. (87)/(89)) by simple one-pot operations (Figure 10.32) [126,156]. The choice offuranoid (87) or pyranoid (89) nature of the products can be determined by a suitable hydroxyl substitution pattern in a corresponding cycloolefinic precursor (86) versus (88)). The overall specific substitution... 

To illustrate the opportunity to pre-determine different types of ring structures by the deliberate choice of substitution patterns within an cycloolefinic precursor, racemic cis-3,4-dihydroxycyclopentene 21 [96,98] was subjected to the routine reaction sequence (ozonolysis, aldolization, dephosphorylation). A bisadduct fraction was obtained in 25% for which NMR analysis showed the presence of two components in a 4 1 ratio [56]. After chromatographic separation, the main component was established to have the mixed furanoid-pyranoid structure 22 with trans-all equatorial ring junction that indeed was to be expected for the thermodynamically more favorable product derived from the (2R,3R) enantiomer of the dialdehyde. The minor product 23 arising from the (2S,3S) enantiomer, however, did not contain the presumed trans-diaxial substitution at C-3/4 of the pyranose substructure which would be forced upon by... 

Cycloolefins having rings with more than four carbon atoms do not homo-polymerise in the presence of Ziegler-Natta catalysts based on titanium or vanadium compounds as precursors and alkylaluminium activators. However, these cycloolefins may copolymerise with ethylene via the double bonds while preserving the cycloolefin ring ethylene is able to compensate the steric hindrance at the Ca atom of the growing chain after and before the 1,2-insertion of the cycloolefin [2],... 

There is a wide variety of transition metal compounds, ranging from group 4 (Ti) to group 8 metals (Ir), that can be applied as catalysts or catalyst precursors for the ring-opening metathesis polymerisation of cycloolefins. However, the most commonly used are W, Mo, Re and Ru compounds tungsten-based catalysts appeared to be the most effective. Other transition metal compounds such as Nb and Ta compounds have also often been used as catalysts, but especially for mechanistic studies [45]. 

Reaction of the arene ruthenium precursor 200 with polystyrene and hydrogen leads to the loss of both cycloolefins and gives a polymeric ruthenium complex, used for catalytic hydrogenation, for which EXAFS indicated a Ru—C distance of 2.05 A. A similar reaction of derivative 200 with 1,3-diphenylpropane gives complex 203 and a compound of composition (diphenylpropane)Ru2 (204) (129,130) [Eq. (18)]. 

We have used cometathesis of cycloolefins with a-olefins (Scheme 1) as a general approach to biologically active natural compounds [26-30], primarily for preparation of l,A-dienes precursors of monoenic insect sex pheromone components having a double bond in 6-11 positions. The size of starting cycloolefin defines the internal double bond position in the target diene. The length of the latter is defined by total number of C -atoms in the cycloolefin and a-olefin. 

Besides naturally occurring cyclic monoolefins, also synthetic analogs were screened. A typical example is cyclooctene (Scheme 6.16). Reaction of syngas with this cycloolefin in the presence of an unmodified Co catalyst under unusually smooth conditions in a >200 g scale gave cyclooctane carbaldehyde [87]. The product was converted into the corresponding acetals, a modification that enhances the intensive green note of the precursor aldehyde. Alternatively,... 

Heterobimetallic complexes involving group 4 metallocene and Ni(0)-cycloolefin moieties have been prepared from Ni-alkyne complexes. For instance, reacting the butadiyne-Ni(O) complex Ni(r7 -Ph-G=G-G=G-Ph)(PPh3)2, 10, with MGp2 precursors (M = Ti and Zr) leads to the species 11, in which metallacyclopentadienes are coordinated in ry -fashion to the Ni center (Equation (2)). It is important to note that these complexes are stable with Ph substituents only. 

Bidirectional chain extension of dihydroxy dialdehydes gives rise to carbon-linked disaccharide mimetics (e.g., 43, 45, 46) by simple one-pot operations [115,117]. The latter may be obtained as single diastereomers in good overall yield even from racemic precursors (especially cycloolefins), if the tandem aldolization reactions are conducted under conditions of thermodynamic control (Fig. 19). Typically, the thermodynamic advantage favors the trans (43) and equatorial attachments (45, 47) of the sugar ring by far, so that the... 

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