Stereochemical configuration preservation

Figure 2.11 Some plausible confoimations of (2/f, 4Y )-l-hydroxy-2,4-dimethylhex-5-ene. How many different torsional isomers might one need to examine, and how would you go about generating them [Note that the notation 2/f, 45 implies that the relative stereochemical configuration at the 2 and 4 centers is R,S - by convention, when the absolute configuration is not known the first center is always assigned to be R. However, the absolute conformations that are drawn here are S,R so as to preserve correspondence with the published illustrations of Stahl and coworkers. Since NMR in an achiral solvent does not distinguish between enantiomers, one can work with either absolute configuration in this instance.]...

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

A comparison of the cyclizations of meso- and J/-3,4-die thy 1-2,5-hexanediones showed that these compounds cyclize at unequal rates, and that the stereochemical configuration of unchanged dione is preserved during the reaction. These findings are at odds with the commonly accepted mechanism that involves the ring closure of a rapidly formed monoenol. 

Following initial solvolysis of the tosylate, addition of acetic acid to the carbocation generates an SN1 product. Please note that there is no preservation of the stereochemical configuration in this reaction. 

From the point of view of synthetic planning it is important to recognize that stereochemical configuration at sp carbon will not usually be preserved during... 

We discovered a complementary procedure for conversion of OMen to other functional groups. The ester P-OMen bond was shown to be cleaved in a stereoselective manner reductively [85,86]. The cleavage takes place with almost complete preservation of stereochemical integrity at phosphorus. The reducing agents are usually sodium or Hthium naphthalenide, lithium biphenyUde, and Hthium 4,4 -di-fert-butylbiphenyl (LDBB). The species produced is then quenched with an alkyl hahde or methanol to afford tertiary or secondary phosphines, respectively (Scheme 5b). Overall, the displacement reaction proceeds with retention of configuration. 

There are classes of natural products, such as erythromycins (14-membered ring), which contain macrocycles as building blocks and yet which still preserve conformational restrictions. Thus, it is possible to assign configurations using the arsenal of NMR methods. An example is the determination of the stereochemical position of a hydroxy group at C-9 in macrocycle 2 by a comparison of <5( H) and 3/HH values64. ... 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

The reductive desulfonylation of vinylic sulfones has been mainly carried out with sodium and aluminum amalgams, Sml2, and Mg in M,Al-dimethyl-formamide (DMF) in the presence of chlorotrimethylsilane (TMSC1). The configuration of the double bond is not necessarily preserved in the desulfonylation of vinylic sulfones when Na/Hg is employed. This reagent is the most widely used when there is no need for stereochemical control as shown in Eqs. 2139 and 22.40... 

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