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A CH20150 > Umpolung Reactivity > Flashcards

Umpolung Reactivity Flashcards

1
Q

What is natural polarity

A
  1. Certain functional groups have an innate polarity which dictates which synthons you can and can’t use
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2
Q

What is the innate polarity of ketone and what does it mean about synthons

A
  1. Delta negative on oxygen

2. So synthon which is positive on carbon matches natural polarity

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3
Q

What happens to natural polarity in a conjugated system

A
  1. Alternating delta + and - atoms

2. If synthon matches it is a good synthon

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4
Q

What does umpolung reactivity refer to and give example with ketone

A
  1. Reactivity that goes against the natural polarity
  2. In case of ketone this would be using a reagent corresponded to the synthon with a negative charge on the carbonyl carbon
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5
Q

Why is umpolung reactivity useful for ketones

A
  1. Allows you to build up any ketone by nucleophilic substitution
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6
Q

What is a dithiane and how can it be used

A
  1. 6 membered Cyclic with two sulfur atoms and carbon atom in between
  2. The carbon in between the two sulfur atoms has acidic protons on it
  3. So you can deprotonate and alkylate- you can do this twice
  4. Then unmask the ketone to give the desired product
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7
Q

What is used to unmask the ketone from a dithiane

A
  1. Hg(ClO4)2 + H2O
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8
Q

What else can dithiane be used for

A
  1. Protecting the latent carbonyl from nucleophilic attack

2. Similar to ketal but with sulfur instead of oxygen

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9
Q

What is the difference in reactivity between a ketal and dithiane

A
  1. The acidic position that can be deprotonated and alkylated - can’t do with acetal or ketal
  2. Different methods for installation and removal
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10
Q

What are the different methods of installation for acetal/ketal compared to dithiane

A
  1. Sulfur is much softer than oxygen, so installation and removal of a diathine is catalysed best by soft lewis acids (Hg2+)
  2. Installation and removal of an acetal/ketal is catalysed by Bronsted acids (H+)
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11
Q

Give 3 examples of Lewis acids

A
  1. TiCl4
  2. BF3
  3. Zn(OTf)2
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12
Q

What are the problems with using Hg and water to remove dithiane

A
  1. Mercury salts are toxic
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13
Q

Which methods are preferred for dithiane removal

A
  1. methods based on oxidation or alkylation of the sulfur are often used
  2. Silver salts
  3. Iodine, N-chlorosuccinimide, N-bromosuccinimide - but would add to alkenes
  4. Produce a ketone
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14
Q

What is another method of dithiane removal that doesn’t produce a ketone

A
  1. hydrogen over Raney-nickel

2. Very good at cleaving C-S bonds so doesn’t give ketone but instead removes the dithiane without trace

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15
Q

What is relationship between ketals and dithianes

A
  1. They are orthogonal meaning you can install or take off one without affecting the other
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16
Q

What does 1,3-dithiane correspond to

A
  1. It can be alkylated in the 2-position twice

2. Corresponds to a dianioninc carbonyl system

17
Q

How can a dithiane be used to access a 1,2 dicarbonyl

A
  1. Add BuLi to 2 position
  2. Then can add ketone which adds as alcohol
  3. Then remove dithiane with Hg(ClO4)2
  4. Then oxidise alcohol
18
Q

How can a dithiane be used to access a 1,4-dicarbonyl

A
  1. Michael addition to an enone
  2. Add nBuLi to dithiane
  3. Then add alpha, beta unsaturated carbonyl
  4. Then remove dithiane with AgNO3
19
Q

How can 1,3-dicarbonyls be accessed

A
  1. Addition of base to ketone to form enolate
  2. Add onto different ketone which becomes alcohol
  3. Oxidation by TPAP, NMO to form dicarbonyl
20
Q

How can 1,5-dicarbonyls be accessed

A
  1. A vinylogous aldol can be used- Michael addition of an enolate
21
Q

How can you access 1,6-dicarbonyl

A
  1. Ozonize a cyclic alkene with 6 carbons in it- O3, PPh3
22
Q

What would be retrosynthesis sequence from a 1,6-dicarbonyl

A
  1. 1,6-dicarbonyl (FGI, Ozonolysis) –> cyclohexene (FGI Diels-Alder)–> diene + dienophile
23
Q

What is ozonolysis

A
  1. Turns alkene into two carbonyls

A CH20150 (15 decks)

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