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SY (Option) > Advanced Retrosynthesis > Flashcards

Advanced Retrosynthesis Flashcards

1
Q

Draw the RSA and forward synthesis of the following ketone molecule.

A
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2
Q

What is chemoselectivity?

How can ketones be chemoselectively reduced?

How can alkenes be chemoselectively oxidised?

A

A chemoselective reaction is a reaction in which one functional group reacts in preference to another functional group(s).

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3
Q

What is regioselectivity?

A

A regioselectie reaction is a reaction that leads to the selective formation of one structural isomer (regioisomer).

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4
Q

How do you regioselectively 1,2-reduce an enone?

A

This is called a Luche reduction.

The NaBH4, in the presence of a Lewis acid, undergoes methanolysis where one or more Hs on the B are replaced by OMe. The resulting methoxy boro-hydrides are hard reducing agents.

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5
Q

How do you regioselectively 1,4-reduce an enone?

A

L-selectride is a bulky reducing agent and therefore favours the attack of the less sterically hindred 4-position.

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6
Q

How do you regioselectively form a kinetic enolate in an aldol addition?

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7
Q

How do you regioselectively form a thermodynamic enolate in an aldol addition?

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8
Q

What is stereoselectivity?

A

A stereoselective reaction is a reaction in which one enantiomer, one diastereoisomer or one double bond isomer is formed selectively over others.

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9
Q

What are the protecting groups for alcohols?

A
  1. esters
  2. silyl ethers
  3. benzyl ethers
  4. tetrahydropyranyl ethers (OR-THP)
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10
Q

How are alcohols protected by esters?

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11
Q

How are alcohols protected by silyl ethers?

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12
Q

What is the order of stability of silyl ethers?

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13
Q

How can silyl ethers regioselectively protect alcohols?

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14
Q

How are alcohols protected by benzyl ethers?

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15
Q

How do benzyl ethers provide chemoselective protection?

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16
Q

How are alcohols protected by tetrahydropyranyl ethers?

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17
Q

What are the protecting groups for carbonyls?

A

Just acetals.

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18
Q

How are carboxylic acids protected by acetals?

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19
Q

How do acetals chemoselectively protect carbonyls?

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20
Q

What are the protecting groups for carboxylic acids?

A

Just esters - methyl ester, tert-butyl ester, benzyl ester.

21
Q

How are carboxylic acids protected by esters?

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22
Q

How do esters provide chemoselective deprotection?

A
23
Q

What are amines protected by?

A

Just carbamates - Boc group, Cbz (or Z) group.

24
Q

How are amines protected by carbamates?

A
25
Q

How do carbamates provide chemoselective deprotection?

A
26
Q

What are the 10 guidelines for good C-C bond disconnections?

A
  1. Disconnect in middle of molecule
  2. Use convergent rather than linear strategy e.g. make two halves of TM separately then join together at end
  3. Disconnect at branch points
  4. Disconnect rings from chains
  5. Use two-group disconnections (need to recognise two FGI’s)
  6. Use symmetry elements if possible
  7. Disconnect to recognisable SM’s
  8. Analyse oxidation levels and possible FGI’s
  9. Look for key structural features
  10. Install most reactive functional group last
27
Q

Describe umpolung reactions.

A

The synthetic equivalents for umpolung reactions allow for a reversal of the normal reaction profile. This increases the synthetic opportunities for making C-C bonds.

28
Q

What are the normal and umpolung (reverse) reactivities of carbonyls?

A
29
Q

What are the synthetic equivalents of acyl anions?

A

1,3-dithianes.

They are formed from reaction of aldehydes/ketones with propane-1,3-dithiol. The anion formed on deprotonation of the dithiane reacts with a range of electrophiles and the dithiane can be converted into a carbonyl.

Electrophiles include haloalkanes, aldehydes, ketones and epoxides.

30
Q

Draw the reaction scheme of the formation of a 1,3-dithiane from an aldehyde.

A
31
Q

Draw the RSA and forward synthesis of the following molecule using a dithiane.

A
32
Q

What are the synthetic equivalents of enolate cations?

A

α-bromocarbonyls and ketene thioacetals.

33
Q

What is the Diels-Alder reaction?

A

A reaction forming two C-C bonds and a 6-membered ring with control over the stereochemistry (in a single step).

34
Q

What are the variations of the Diels-Alder reaction?

A
35
Q

What is the reactivity of the Diels-Alder reaction?

A
  • dienes need to adopt the s-cis conformation
  • normally the diene is electron-rich and the dienophile in electron-poor, to allow good overlap of dienophile LUMO and diene HOMO
36
Q

What is the stereochemistry of the Diels-Alder reaction?

A
  • the reaction is stereospecific - a cis-dienophile gives a cis-configured product
  • the (less stable) endo product is preferred in irreversible reactions - endo rule
  • the (more stable) exo product is preferred in reversible reactions
37
Q

What is the regioselectivity of the Diels-Alder reaction?

A
  • ortho (1,2-) and para (1,4-) products are favoured, explained by orbitals interactions
38
Q

What products form via a Diels-Alder reaction of achiral reagents?

A

This reaction with form racemic products.

39
Q

How can products of a Diels-Alder reaction be made enantiomerically enriched?

A

By using a chiral auxiliary.

40
Q

What other chiral auxiliaries can be used?

A
41
Q

What are IMDA reactions?

A

Intramolecular Diels-Alder reactions. These are useful for the simultaneous construction of two rings. A dienophile with an EWG is not necessarily required.

42
Q

What are the advantages of IMDA reactions?

A
  • close proximity of reacting groups (higher effective concentration)
  • lower entropy of activation (increased reaction rates/milder conditions)
  • lower degrees of freedom in TS (higher regio-/stereoselectivity)
43
Q

Draw the IMDA reaction RSA of this molecule.

A
44
Q

Draw the IMDA reaction RSA of this molecule.

A
45
Q

What are tethered reactions?

A

A removable tether can temporarily link a diene with a dienophile. A tether needs to be easily introduced, stable to key reaction conditions and easily removed.

e.g. silicon tether

46
Q

Draw the RSA of the following molecule, using a silicon tether.

A
47
Q

What are polymer-supported reactions?

A

Many D-A reactions have been carried out on a solid support e.g. diene attached to solid polymer-support and then reacted with excess dienophile (+ Lewis acid) to drive the reaction to completion.

The product is detached from the solid support and isolated following a simple filtration. A large number of compounds can be produced very quickly.

48
Q

Give an example of a D-A reaction on a polymer-support.

A

SY (Option) (4 decks)

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