Stereochemically pure

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

Although steroidal spiro oxiranes are difficult to obtain stereochemically pure by peracid epoxidations of exocyclic methylenes,the recently developed methylene transfer reagents, dimethylsulfonium methylide and di-methylsulfoxonium methylide in tetrahydrofuran, proved useful in the stereoselective transformation of steroid ketones to a- and -oxiranes, (87) and (88), respectively. ... 

The same nitrotetraphenylporphyrin 14 (M = Cu) undergoes bisalkylation by conjugative addition with malononitrile to give the stereochemically pure chlorin 16 with the thermodynamically favored trans arrangement of the introduced malononitrile residues.22... 

Active stereochemically pure compounds with structural data... 

Inactive stereochemically pure compounds (structural data are not possible)... 

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

Although acetylenic bonds are more reactive than C=C bonds, the reactions are often initiated by AIBN or UV radiation. Baldwin and Barden119 have used the latter method to treat a doubly labelled phenylacetylene with triphenyltin deuteride (Scheme 19). The addition of the triphenyltin deuteride was both regiospecific and gave a stereochemically pure product. A five-step synthesis (Scheme 20) converted this product into an optically pure trideuterophenylcyclopropane, which was used to study the thermal stereomutations that these compounds undergo. 

Alternatively, a chiral lithium amide was added regio- and diastereoselectively to an achiral 2,4-dienoate, and the 1,4-addition product formed could again be converted into the desired, stereochemically pure /J-lactam (equation 31)106. 

The unusual amino acid (S)-2-amino-(Z)-3,5-hexadienoic acid (269), which is a component of the toxic y-glutamyl dipeptide isolated from the defensive glands of the Colorado beetle [209], has been synthesized along Scheme 17, after two initial attempts had proved unsuccessful due to the instability of 269 towards various oxidation conditions [210]. Scheme 17 relies on the hydrolysis of an ortho ester to generate the required carboxylic acid. Thus, the L-serine aldehyde equivalent 270 was treated with ( )-l-trimethylsilyl-l-propene-3-boronate to give the addition product 271. Reaction of 271 with KH gave the stereochemically pure (Z)-diene 272. Mild acid treatment of 272 followed by... 

Alternatively, by performing a zirconium-catalyzed Negishi methylalumination on 1-hexyne, it is possible to produce stereochemically pure alkenylcopper species 62, which adds to enones in a 1,4-fashion, to give compounds such as 63 (Scheme... 

Substances such as carbohydrates, amino acids, and other small molecules available from natural sources are valuable starting materials for the synthesis of stereochemi-cally defined substances. Suggest a sequence of reactions which could effect the following transformations, taking particular care to ensure that the product would be obtained stereochemically pure. 

A Lewis acid-mediated reaction of 1 equiv of acrolein (as enone) with 2 equiv of an aldehyde gave access to stereochemically pure 4-hydroxy-1,3-dioxanes when the reaction was carried out in the presence of stoichiometric amounts of BU4NI (Equation 78). Interestingly, an iodomethyl group rather than the expected exocyclic double bond was formed in the product <19990L1383>. 

Sales of stereochemically pure herbicides were rising indicating that they give competitive market advantage over competitors still selling racemic products. 

If the allylic system is substituted, several isomeric products can be formed. Assuming a chair-like transition state, the stereochemical outcome of a metallo-Claisen rearrangement is controlled by the geometries of the vinyl and allyl moieties, so that, if the vinyl part 68 is stereochemically pure, three different products, syn/anti-73 or 74, can be formed, depending on the exact nature of the active allylic part 72 (Scheme 6)38. 

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