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Organic Chemistry > Retrosynthesis > Flashcards

Retrosynthesis Flashcards

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1
Q

total synthesis

A

assembling a product from scratch from a simple molecule

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2
Q

advantage of total synthesis

A

helps meet supply demand

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3
Q

disadvantage of total synthesis

A

more expensive than cultivating natural product

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4
Q

semi-synthesis

A

product from nature and synthesise into another product

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5
Q

retrosynthesis

A

devise synthetic route to target molecule by working backwards from that molecule

break into simple fragments

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6
Q

synthons

A

hypothetical compounds that would react together to form desired bond (if they existed)

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7
Q

how are enolates formed?

A

deprotonation of corresponding carbonyl

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8
Q

would a base be more or less strong if its corresponding anion is less stable?

A

stronger (wants to be deprotonated)

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9
Q

why is it important to pick the most appropriate base?

A

to avoid side reactions

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10
Q

polar aprotic solvent

A

[no H+ source]

weakly polar solvents

can coordinate to cations but leave naked, reactive anions

smaller charge separation -> generates less reactive enolates

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11
Q

C alkylation

A

small cations binding tightly to oxygen

oxygen derived leaving groups

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12
Q

O-alkylation

A

larger cations

halides

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13
Q

how to separate mixtures of kinetic and thermodynamic enolates?

A

TRAPPING

use trimethylsilyl chloride (TMS-Cl)

the 2 enolates have different b.p. and physical properties

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14
Q

activating groups

A

method of deprotonation at certain point - increases acidity of desired proton

via introduction of EWG (e.g. ethyl ester)

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15
Q

problem with direct alkylation of ketones

A

double alkylation + self-condensation

SOLUTION - use of enamines

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16
Q

where are enamines nucleophilic?

A

β-position

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17
Q

advantages of using enamines

A

no base used = no self-condensation

monoalkylation (intermediate = unreactive)

alkylation of unsymmetrical ketones = regioselective
-> major = less sub.

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18
Q

problem with grignards?

A

ambident electrophile -> electrophilic in > 1 place

have to consider selectivity

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19
Q

cuprate

A

soft nucleophilic species

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20
Q

where do cuprates react?

A

@ soft electrophilic site (β carbon)

EXCEPT - acid chlorides -> react once to displace chloride as opposed to Grignard’s which would react x2

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21
Q

aziridines

A

nitrogen equivalent to epoxides

22
Q

is ring-opening energetically favourable or unfavourable?

A

favourable -> relieves ring strain

23
Q

how can silyl ether protecting groups be removed?

A

treatment with acid or with source of fluoride (Si-F bond = v. strong)

*** can’t use sodium fluoride as it’s ionic and doesn’t dissolve in organic compounds

24
Q

THP ether

A

cleaved by acid but not fluoride

= ORTHOGONAL to silyl ethers

25
Q

carboxylic acid + LiAlH4

A

primary alcohol

26
Q

ester + LiAlH4

A

primary alcohol x2

27
Q

amide + LiAlH4

A

amine

28
Q

nitrile + LiAlH4

A

amine

29
Q

an alternative to using H- for carbonyl reduction

A

DIBAL-H

contains electrophilic aluminium atom (O coordinates to Al) - v. strong Lewis acid

30
Q

homologation

A

extension of carbon chain

(nitriles v. useful for this)

31
Q

Appel reaction

A

R - OH to R - Cl [PPh3 / CCl4]

NOT USED ANYMORE - CCl4 = ozone depleating compound = environmental issues

32
Q

when does diels-alder reaction work best?

A

when dienophile has EWG attached

33
Q

is diels-alder stereospecific or stereoselective?

A

stereospecific -> fixed due to mechanism

34
Q

how do you recognise diels-alder in retrosynthesis?

A

see six-membered ring with double bond in it

35
Q

how many disconnections are involved in diels-alder retrosynthesis?

A

2

36
Q

diels-alder -> what happens if we use an alkyne as the dienophile?

A

product = 6-membered ring with 2 double bonds in it

37
Q

how can we use the diels-alder reaction to assemble a cyclohexANE ring

A

transform alkene to alkane via hydrogenation (H2 gas / Pd catalyst)

38
Q

what would happen if we tried using Wittig reaction to make a molecule with an alcohol group in it?

A

Wittig reaction + alcohol group are NOT compatible

have to use OTBDMS protecting group

39
Q

disadvantage of Wittig reaction

A

byproduct = triphenylphosphine oxide (hard to remove)

40
Q

Horner- Wadsworth-Emmons reaction - byproduct

A

phosphorus byproduct = ionic (washed away in aq phase)

41
Q

HWE - which alkene is produced?

A

E

42
Q

why does the Wittig reaction produce a Z alkene?

A

phosphorus ylide = unstable ∴ oxaphosphetane intermediate is irreversible

2nd step = syn-oxaphosphetane intermediate = Z-ALKENE

due to orbital control - reactants approach perpendicular + align so that bulky sub. are apart

43
Q

why does the HWE reaction produce a E alkene?

A

phosphonate anion nucleophile = stabilised by EWG - formation = reversible

equilibrium leads to anti-oxaphosphetane intermediate = E-ALKENE

+ fragmentationof anti-oxaphosphetane intermediate is faster due to less sterically congested transition state

44
Q

why are alkynes useful in retrosynthesis?

A

proton at end of terminal alkyne = acidic enough for strong bases to deprotonate it

Nu sub. reactions can then occur

45
Q

natural polarity

A

functional groups have innate polarity which dictate which synthons can/can’t be used

46
Q

umpolung reactivity

A

goes against natural polarity

47
Q

umpolung reactivity in ketones

A

-ve charge on carbonyl carbon

useful in synthesis as ketone can be built up via Nu sub.

48
Q

dithianes - uses

A
  1. as umpolung reagent
  2. protecting group
49
Q

differences between dithianes and ketals

A

dithiane = sulphur vs ketal = oxygen (orthogonal)

[differences in reactivity]

  1. acidic position that can be deprotonated and alkylated (can’t with acetal/ketal)
  2. different methods of installation and removal
50
Q

installation of dithiane

A

soft Lewis acids (e.g. Hg 2+)

51
Q

installation of acetal/ketal

A

Bronsted acids (e.g. H+)

52
Q

removal of dithianes

A

METHOD 1 - mercury salt + H2O to push equilibrium back towards desired ketone
-> mercury = v. toxic (bad for environment

METHOD 2 - oxidation with silver salts (AgNO3 + EtOH; H2O)

METHOD 3 - reductive cleavage with hydrogen (Raney Ni + H2)
-> cleaves C-S but WON’T give you back ketone

Organic Chemistry (13 decks)

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