Stereoselectivity examples

Another example of the very subtle effect of a ligand is provided by the dimerization of norbornadiene (31, 52). With (PPh3)2Ni(CO)2, under irradiation, the exo,trans,exo dimer is formed stereoselectively (example... 

The great advantage of Diels Alder reaction in organic synthesis lies in its high stereoselectivity. Examples of Diels-Alder reactions are known for almost all types of n bonds acting as dienophiles. The diene system may be all carbon or there may be many variations as shown in the following tables. 

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-175]. In these examples, the surface serves as a template when guest molecules are adsorbed on it. 

However, in most cases, the detailed mechanism is not yet known, i.e., whether CIP, SSIP or even free radical ions are scavenged by the nucleophile. Arnold and Snow [62] suggest an attack of methanol to solvated olefin radical cations, whereas Mariano observed a highly stereoselective example of a direct scavenging of a radical cation — radical anion pair by methanol [63]. Although this process has received relatively little attention, it is obvious that scavenging of different types of radical ion intermediates is not only possible but may be used to differentiate between the various types of radical ion pairs (CIP and SSIP). 

The insertion of the N(2)-G(3) unit in reduced isoquinolines remains a topic of interest, especially stereoselective examples. The iminoglycinate 43 undergoes reaction with the dibromo 44 in the presence of the -symmetric chiral quaternary ammonium bromide phase-transfer catalyst (Equation 130) <2001S1716>. A high-yielding tetrahydro-isoquinoline resulted in excellent enantioselectivity. Reaction of the chiral anion generated from 45 with benzylidene also produces chiral tetrahydroisoquinolines (Equation 131) <1999EJO503>. 

The mechanistic variations in these substitutions are sometimes unexpected. Cremer and Chorvat (1967) gave the stereoselective example, (123). In view of the ring angle of ca. 90°, the kinetic order of hydroxide in analogous attacks, and the single product, we speculate that 32 is the preferred intermediate. Compound 32 is presumably formed in one... 

Aryl esters of other acids show similar stereoselectivity examples are shown in eq 3. In addition, the BHT esters of 0-benzyl1actic acid condense... 

Although nitrile hydratases tend not to be stereoselective, examples of enantioselective enzymes are known [103, 106, 107, 114]. Of particular interest is the possibility to selectively hydrolyse 2-phenylproprionitriles, the core structure for ibuprofen and many other profens [103, 107, 114, 115]. This enables the enantioselective synthesis of the amides of ketoprofen and naproxen (Scheme 6.39). 

An additional substituent at C-2 does not alter the relative energies of the transition states thus, trisubstituted alkenes are accessible with a high degree of stereoselectivity. Examples arc the synthesis of the natural product nuciferol8 and other allylic oxygenated terpenes9 or 2-phenylthioallyl alcohols10. 

A small number of reactions has been developed that follow the concept of enzymatic tandem C-C bond formations,however,which do not proceed in abi-direc-tional but in a repetitive maimer from a single functional group, which relates such reactions to polymerization. However, only controlled, stereoselective examples of oligomerizations are considered to be relevant for the scope of this account. 

Fora stereoselective example where the relative stereochemistry with respect to a preexisting stereocenter in a six-membered ring see Dana, G. Weisbuch, F. Tetrahedron 1974,30,2879-2885. 

Only activated olefins, e.g. a,yS-unsaturated carbonyl compounds, enolethers, and enolacetates, are good radical acceptors in these reactions. The olefins have to be used in five- to tenfold excess. The overall transformation is completed by elimination of the thiophenyl radical. Thus, diphenyl disulfide, the source of thiophenyl radicals, can in principle be employed in substoichiometric amounts. The diaster-eoselectivity of vinylcyclopentane formation is difficult to control, and mixtures of diastereomers are often obtained. A highly stereoselective example is shown in Scheme 3. 

PLA is a fascinating material and there are big differences in the thermomechanical properties of the polymer depending on the tacticity formed. For example, atactic and heterotactic PLA are amorphous and have no defined melting point. Polymerisation of either the pure l- or d- stereoforms produce isotactic PLA with a Tg of ca. 50 °C and a T , of 170-180 °C. However, a 50 50 mixture of PLLA and PDLA has a melting point of ca. 220-230 °C, this enhancement is due to the formation of a stereocomplex between opposite chiral chains. It is also possible, and indeed preferred, to produce the stereocomplexed polymer from the isoselective polymerisation of rac-LA forming stereoblock PLA. There are many examples of zirconium, and indeed group-4 metals as a whole, in the literature for the polymerisation of rac-LA, the next section will focus on the stereoselective examples of ROP. 

These bulky aluminium aryloxides will also promote a variety of carbon-carbon bond forming reactions with a high degree of regio and stereoselectivity. Examples include Michael addition to a,/S-unsaturated ketones, Diels-Alder additions, and Claisen rearrangements. In the case of Diels-Alder reaction, ATPH promoted the exo-selective condensation of a,/6-unsaturated ketones with dienophiles. The Claisen rearrangement can be catalysed by ATPH and its more Lewis acidic 4-bromo derivative. A series of chiral aluminium aryloxides were also synthesized and have been applied to asymmetric Claisen rearrangements, aldol reactions, and aldehyde alkylations. ... 

---