When the HCl addition is complete, add another 10 mL of water and stir the mixture for 15 minutes. A second chiral center is being formed in this experiment and two diastereomeric compounds are created. Your product should be completely dry before going on next week to the next reaction.
Swirl the funnel gently before inserting its stopper. Entry 9 demonstrates that carbanions that are not resonance stabilized are also highly selective. The ester enolates illustrated in Entries 3 and 4 are considerably more selective when the lithium cation is exchanged for dicyclopentadienyl zirconium .
In these cases, transition structures similar to Figures 6. Drain off the lower aqueous layer and extract the remaining organic layer with two mL portions of water and then one 30 mL portion of saturated sodium chloride solution. As will be seen, amides of C2-symmetric amines can be excellent chiral auxiliaries in this process.
A more complex example involving extension of a steroid side chain similar to Scheme 6. The structures in Figure 6. Carefully replace the stopper and continue to stir the cooled flask for 20 minutes.
It was noted that propargylic anions rearrange via a transition structure that has significantly shorter bond lengths, and also a compressed allylic bond angle cf. Discard filtrate in the organic waste container. As seen in Scheme 6. Pyrrolidinyl amides undoubtedly form Z O -enolates, and the [2,3]-Wittig rearrangement of the E-alkene Entry 5 is highly selective .
Rinse the reaction flask with two 25 mL portions of diethyl ether and add the rinses to the separatory funnel. H2SO4 is highly corrosive Equations for all Reactions: Additionally, it is possible to introduce a stereogenic element elsewhere, such as a chiral auxiliary X of Figure 6.
Immediately stopper the flask and cool it in an ice bath. Entry 1 was the first example, reported in , of a highly stereoselective [2,3]-Wittig rearrangement, but comparison with entry 5 shows that only the Z isomer is selective.
If there is a substituent R1 at the allylic position, A1,2 and A1,3 allylic strain will play a role. The topicity of these examples can be analyzed by reference to Figure 6. While stirring, do not turn on the heater carefully add 0. If both R1 and R2 are not hydrogen, A1,2 strain will disfavor the left conformer.
The filtrate should be placed in the aqueous acidic waste container. Safely store the product in your drawer until next week. The latter effect amplifies A1,3 strain, and E selectivity is restored when the carbanionic carbon is propargylic Scheme 6. Similar transition structures having stereogenic carbanionic carbons are illustrated in Figure 6.Problem in Diastereoselectivity Purpose: The purpose of this experiment is to determine the stereochemical outcome of a reaction.
A second chiral center is being formed in this experiment and two diastereomeric compounds are created. Although diastereoselectivity is a broad term, in this section its description will be reserved for stereoselectivities from insertion reactions of diazo compounds that are bound to a.
Chapter 34 — Diastereoselectivity - The Felkin-Ahn model for carbonyl conformations and diastereoselective nucleophilic attack - The effect of electronegative atoms on carbonyl conformation Work these problems to.
View Lab Report - A problem in Disastereoselectivity Lab from CHEM #4 at Wayne State University. Experiment 4: A Problem in Diastereoselectivity You %(6).
Dave Evans taught me that you should generally analyze these types of problems by drawing out allyl conformations. I've drawn the nitronate to minimize 1,3-allylic strain. Now, the top face of the nitronate is largely blocked by the t-butyl carboxylate. The problem in this question is that it isn't specified whether the methyl groups are up or down etc.
So presumable what we're actually seeing is a resolution (i.e. we're only forming a small amount of product, but its optically pure because certain combinations just don't react at a sufficient rate).Download