Chiral reagent to create a chiral product

[Monkeydfdg64]

An organic synthesist seeks to identify an efficient stereoselective reagent with which to produce the enantiomer of the biologically active sphingosine molecule pictured in the passage. Which of the following would be the most logical choice to try?

Tert-butoxide
DMSO
Diethyl Tartrate
Propanol

When I first read this question I thought they were looking for a solvent for SN2 substitution? Does that logic work? Also I have never heard of Diethyl tartrate which is the stereoselective answer. Can someone just walk me through how having a chiral reagent would generate the enantiomer?

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[deleted859689]

Sn2 will reverse stereochemistry via back attack. Sn1 will result in racemic (mixed stereochemistry) mixture.

 

[Monkeydfdg64]

Ya that's what I thought creating the enantiomer of the product, but apparently they were looking for a stereoselective reagant.

 

[deleted859689]

Ya that's what I thought creating the enantiomer of the product, but apparently they were looking for a stereoselective reagant.

Some solvents can direct stereoselectivity, but theY usually have a chiral center as well. Only solvent there that has a chiral center is diethyl tartrate

 

[Monkeydfdg64]

So why can't this be created from SN2?

 

[BerkReviewTeach]

This is a particularly good example of excessive wording, a technique the MCAT writers have mastered. There is so much in that sentence to consume and process that you get caught up in it and fail to see it's an easy question at its core. The key phrase is "efficient stereoselective reagent" and that is all you need. No need to decipher the mechanism or whether the stereoselectivity is opposed by the enzyme (catalyst) or reactant.

The question is really, "which compound is chiral?" text-butoxide [(H3C)3CO-] has no chiral center, DMSO [(H3C)2SO] has no chiral center, and propanol has no chiral center. Choices A, B, and D are all gone. It's a bonus if you know that tartaric acid is a four carbon chiral compound derived from a sugar and that it has two chiral carbons. But even if you don't know that, you can still eliminate via process of elimination until it is the last answer standing.