Asymmetric Synthesis 1 Flashcards

1
Q

What are the properties of a chiral molecule?

A

are devoid of a plane of symmetry
are non-superimposable upon their mirror image
are optically active
rotate plane polarised light

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2
Q

What are the four methods to achieve asymmetry in a reaction?

A

chiral reagent
chiral catalyst
chiral pool
chiral auxiliary

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3
Q

What is the problem with using an chiral substrate instead of an achiral substrate?

A

an achiral substrate produces a racemic mixture
- must be separated/resolved

a chiral substrate produces one enantiomer in excess
- no need for separation

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4
Q

How is the transition state of reactions which involve achiral substrates?

A

achiral substrates produce transition states which are enantiomeric

  • are of equal activation energy
  • product distribution produces equal amounts of R and S isomers
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5
Q

How is the transition states of reactions which involve chiral substrates?

A

chiral substrates produce transition states which are diastereoisomers

  • are not of equal activation energy
  • products are formed in unequal amounts
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6
Q

Which synthetic method requires the starting material to be a single enantiomer?

A

chiral pool

  • start from natural products which are incorporated into the product
  • starting material is a single enantiomer
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7
Q

What are the steps involved in the chiral pool method?

A

must start with a single enantiomer
- which is incorporated into the final product

addition of a methyl group at the carboxylic acid
- forms an ester
addition of a side chain
- amino group attacks the side chain

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8
Q

How does synthesis of an isoxazolidinone occur?

A

uses the amino acid phenylalanine
- can use either the R or S enantiomer

the bulky benzyl group blocks the lower or upper face of the molecule
- forces the reactant to come in from the opposite side
= directs the stereochemistry of the reaction

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9
Q

What are the steps involved in the chiral auxiliary method?

A

diastereoselective reaction takes place

addition of the auxiliary to the starting material 
- is covalently bonded 
forms an enolate 
alkylation via a new reagent 
removal of the auxiliary
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10
Q

What is the purpose of the chiral auxiliary?

A

provides steric control
- reaction can only occur from the upper face as the phenyl group blocks the lower face
= ensures you only form one enantiomer

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11
Q

How does use of a chiral reagent cause asymmetric synthesis?

A

do not covalently bond to the substrate but instead interact with it

  • achiral reducing agents produce one enantiomer in excess over the other
    = NaBH4, LiAlH4
  • enantioselecive agents produce only one enantiomer
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12
Q

What are examples of asymmetric reactions that use a chiral catalyst?

A

sharpless epoxidation
noyori asymmetric hydrogenation

both use a metal catalyst with an asymmetrical ligand

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13
Q

What is enantiomer excess?

A

the proportion of the major isomer less than that of the minor isomer as a percentage

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14
Q

What are methods of determining enantiomer excess?

A

must involve a chiral environment

  • chiral HPLC = need a chiral column
  • chiral GC = need a chiral column

NMR of a mixture of optical isomer will not give enantiomeric excess
- enantiomers would have the same NMR
= must add a chiral reagent

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