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aa Biomolecules > Lecture 5 - Carbohydrates 2 > Flashcards

Lecture 5 - Carbohydrates 2 Flashcards

1
Q

How is a hemiacetal formed

A
  1. Protonated aldehyde + alcohol
  2. Intramolecular cyclisation
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2
Q

What is observed for orientation of substituted Me, OR, OMe, OAc, Cl at anomeric carbon

A
  1. A preference for alkoxy (OR) and acetyl groups to reside in the axial position was noticed
  2. Alkyl substituted cyclohexanes prefer equatorial orientation over axial
  3. Alkyl substituted tetrahydropyrans show this same preferences
  4. D-glucopyranose shows preferences for an equatorial orientation but surprising amount is axial
  5. If OH is converted to OMe at the anomeric carbon however it then shows preference for the axial
  6. Substitution with more electronegative groups (OAc/Cl) change the observed ratio to a greater extent - anomeric effect
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3
Q

What is responsible for the anomeric effect

A
  1. Hyperconjugation stabilises axial conformation
  2. Bond between anomeric carbon and O in the ring is shorter than expected as some C=O bond character
  3. Lone pair of O in ring donates into empty antibonding orbital of OR bond causing bond to elongate
  4. Only in axial, not right orientation when in equatorial (beta) position
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4
Q

Why are protecting groups needed for sugars

A
  1. Only functional group on a sugar are -OH groups
  2. There are 3 different hydroxyl environments with different steric accessibility- primary, secondary and anomeric alcohol
  3. Needs to selectively activate and protect alcohols
  4. In glucose all secondary alcohols are equivalent (either all axial or equatorial)
  5. The anomeric position determines if alpha or beta sugar
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5
Q

What is used to protect the anomeric alcohol and how is it added

A
  1. Methyl protecting group
  2. MeOH added with HCl
  3. Goes to alpha position- axial
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6
Q

How is the anomeric alcohol deprotected

A
  1. H+, H2O
  2. Acidic so can’t do acidic chemistry unless you want PG to be removed
  3. Easy to take off and put on- mild conditions
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7
Q

Describe features of protecting the C6 -OH group

A
  1. Primary alcohol so more sterically available and nucleophilic
  2. Mono-protection possible
  3. Big bulky groups
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8
Q

What can be used to protect the C6 -OH group

A
  1. Trityl (triphenylmethyl) PG
  2. Tosylate
  3. Silyl ether
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9
Q

Describe Trityl (triphenylmethyl) as a PG for the C6 -OH group

A
  1. Added using trityl chloride -(Ph)3CCl and a weak base (Et3N) in an SN1 type reaction-
  2. Weak base is used to mop up H+ once trityl has attached not other way round
  3. Typically remove with weak acid (HCl, TsOH, BF3)
  4. But can be tricky to remove in practice
  5. Then Methoxytrityl ethers are used instead - make more acid labile so easier to remove
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10
Q

Describe how tosylate can be used as a protecting group for C6 -OH and how it can be used to selectively carry out a reaction on C6 -OH

A
  1. Add TsCl + base e.g Et3N to mop up H+
  2. Can then Add Ac2O to acetylate all the other -OH
  3. Then could use NaI + acetone to replace tosylated C6 with I -SN2
  4. Good way of only carrying out reaction on C6 -OH
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11
Q

Describe how silyl ether can be used as a protecting group

A
  1. R3SiX with tertiary amines
  2. E.g t-BuMe2SiCl and pyridine
  3. Very stable protecting group
  4. Removal with TBAF, BF3KF, Pyridine-HF- Unusual removal is good as doesn’t interact with other chemistry
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12
Q

What can be used to protect every OH in a sugar

A
  1. Acyl or benzoyl
  2. Add Ac2O and pyridine
  3. Protects every OH
  4. Removed using NaOMe in MeOH
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13
Q

What can be added to a sugar with al OHs protected with AcO group to change just the anomeric OH

A
  1. HBr- Change to Br
  2. BnNH2 - change to OH
  3. H2NNH2-HOAc- change to OH
  4. S-nucleophile - Change to SH
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14
Q

What is the neighbouring group effect

A
  1. Stereochemical outcome of glycosylation reactions may be affected by type of protecting group at position 2 of glycosyl donor
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15
Q

Give example of neighbouring group effect

A
  1. A participating group e.g. carboxyl group present –> Beta- glycoside
  2. Anomeric carbon reacts with Position 2 =O protecting bottom face
  3. Therefore HO-R comes in from top face- Beta-glycoside
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16
Q

What position of a glycoside is favoured if there are no participating groups at position 2

A
  1. Prefers to attack at bottom face and produce alpha-glycoside due to anomeric effect
17
Q

Describe difference in cyclohexane and tetrahydropyran in equatorial bs axial

A
  1. In cyclohexane ring sterics dominates and the equatorial position dominates
  2. In tetrahydropyran ring a heteroatom stabilises the axial position via hyperconjugation
18
Q

What can be used to prevent the neighbouring effect

A
  1. Benzyl protecting group
  2. No stabilisation by neighbouring group coordination so get favoured axial alpha-glycoside product
19
Q

What are good methods for protecting 1,2 and 1,3 diols

A
  1. Cyclic acetal formation
  2. Acetone/H+
  3. Benzaldehyde/H+
  4. Can be used in regioselective protection of polyol compounds
20
Q

What size rings do acetyl and benzaldehyde prefer to form

A
  1. Dimethyl (acetone) derived acetals tend to form 5 membered rings - kinetically favourable
  2. benzylidene (benzaldehyde) derived acetals tend to form 6 membered rings -thermodynamically favourable
21
Q

What is best for protection of 1,3 diols

A
  1. Benzaldehyde to form benzylidene
22
Q

Describe Benzaldehyde as a protecting group

A
  1. Prefers to give 6-membered ring with Ph group equatorial
  2. To add use an acid catalyst e.g. TsOH, PPTS or ZnCL2
  3. Stable to basic conditions
  4. Mild acid for removal or H2, Pd/C
23
Q

What can the 1,3-diol intermediate formed with benzaldehyde be used for

A
  1. A range of selectively protected carbohydrate derivatives
24
Q

What is the best method for protecting 1,2 diols

A
  1. Isopropylidene- reaction with acetone
  2. Add acid e.g H2SO4
  3. Steric hindrance gives the furanose form
  4. Cyclic hemiacetal ring adopted by a given ring structure is not constant
25
Q

What is formed if acetone and H2SO4 is added to glucose

A
  1. Furanose form due to steric hindrance of C-3 and C-4 OH groups preventing formation of 5-membered acetyl
  2. C-3 is free
  3. Others protected with acetal formation
26
Q

What happens if acetone and H2SO4 are added to D-galactose

A
  1. Produces pyranose product
  2. Orientation of C-3 and C-4 hydroxyl groups in galactose (both on same side) permits formation of a less strained 5-membered acetal
  3. Leaves C-6 free
27
Q

How can you leave a free C-3 Oh group in glucose

A
  1. First add benzaldehyde and acid
  2. Protects groups 4 and 6
  3. Then acc acetone and aicd
  4. Protects groups 1 and 2
  5. But don’t need to do both steps could just add acetone to glucose- reverts back to pyranose structure once PG is removed
28
Q

How can you add and remove Benzyl groups

A
  1. Add using BnBr, NaH - OBn groups
  2. Remove using Pd/C, H2
  3. Used for removal of neighbouring effect
29
Q

Why is Me often added to sugars

A
  1. Not commonly used for protection as difficult to deprotect
  2. However often used for mass spec analysis
30
Q

What are 2 methods to add Me

A
  1. Dimethyl sulphate (Me2SO4), in aq NaOH
  2. Methyl Iodide, NaH, THF
31
Q

Describe the dimethyl sulphate methyl addition ot a sugar

A
  1. Methylates every OH
  2. Used as a way of isolating and confirming conformation
  3. Doesn’t come off - so not protecting group just characterisation method
32
Q

Describe addition of methyl iodide

A
  1. CH3I is very toxic and rapidly absorbed by the skin

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