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aa Biomolecules > Lecture 6- Carbohydrates 3 > Flashcards

Lecture 6- Carbohydrates 3 Flashcards

1
Q

What is a general glycosylation reaction

A
  1. Glycone (sugar) + aglycone (alcohol) –> glycoside
  2. Reaction at anomeric position
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2
Q

What happens when you link two sugars together

A
  1. Glycosyl acceptor (Nucleophile) gives electrons from OH to glycosyl donor (electrophile) Carbon
  2. Opposite terminology then may be expected
  3. Forms a glycosidic bond
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3
Q

What do all glycoside reactions involve

A
  1. Oxonium intermediate
    • charge on O in ring if no neighbouring group participation
  3. Or on O coming of C1 if there is neighbouring group participation
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4
Q

Which is major product if OH is added with no neighbouring participation

A
  1. Alpha is major due to anomeric effect
  2. Hyperconjugation or donation of lone pair into sigma* bonding orbital of OR group
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5
Q

What are the three methods used to generate a carbocation at position 1

A
  1. Fischer glycosylation
  2. Koenigs-Knorr Method
  3. Thioglycosides
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6
Q

Describe Fischer Glycosylation

A
  1. Formation of a glycoside by reaction of a sugar with an alcohol in the presence of an acid catalyst
  2. Carbohydrate is typically unprotected
  3. The alcohol is commonly the solvent
  4. SN1 style reaction
  5. Produces alpha carbon due to anomeric stability
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7
Q

How is Fischer glycosylation altered to form beta bond instead

A
  1. Fully acetyl protected sugars are used
  2. Get neighbouring group effects and more selective beta bond formation
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8
Q

Describe the Koenigs-Knorr glycosylation

A
  1. First must make glycosyl halide (typically Br) from fully protected acetyl sugar
  2. Silver salts (Ag2CO3 or Ag2O) are used as the promoter - both insoluble
  3. Driving force is formation of AgBr
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9
Q

How do you generate a thioglycoside

A
  1. Prepared from acetylated sugar
  2. Add strong LA e.g. BF3.OEt2 which cleaves of Ac
  3. RSH (R=Ph, Tol, Et)
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10
Q

How are thioglycosides used in glycosylation

A
  1. Use an electrophilic thiophilic promotor to generate S+ - activated
  2. This is cleaved to generate O+ in ring
  3. Then add R’OH
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11
Q

Give examples of electrophilic thiophilic promotors

A
  1. Hg salts: HgSO4, HgCl2
  2. Triflate salts: AgOTf, MeOTf, TMSOTf, Cu(OTf)2
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12
Q

What is another way to activate the thioglycoside

A
  1. Oxidise to sulphoxide with mCPBA
  2. Treat with triflic anhydride or TMS triflate to further activate
  3. Add aglycone to get glycoside
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13
Q

What is advantage of thioglycoside method

A
  1. Starting material is stable and protected
  2. Needs to be activated
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14
Q

What is iterative synthesis

A
  1. Start with 1 carbohydrate and add to it
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15
Q

What is chemoselective glycosylation

A
  1. Start with reactive thioglycoside and add less reactive ones on
  2. Starting material is all protected except for one OH
  3. Only activate more reactive thioglycoside - form glycosidic bond between C1 of more reactive and whichever contains free OH of less reactive
  4. Carries on
  5. Last one added has no SR group - terminal glycloside
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16
Q

What are disadvantages of chemoselective glycosylation

A
  1. Need to determine which thioglycoside is less reactive
  2. Time consuming and have to do prelim experiments
  3. Probably can’t get much longer than tetraglycoside
17
Q

Describe orthogonal glycosylation

A
  1. Uses mixed linkers
  2. Start with C-F leaving group at C1 and add activator selective for F-
  3. React with thioglycoside then add activator selective for SR
  4. Then C-F one
  5. Then Thioglycoside etc
  6. Can keep going as long as correct protective sugars
  7. Better than chemoselective
18
Q

What is another method of iterative glycosylation

A
  1. Pre-active the system- generate carbocation before addition to flask
  2. Depends how stable species is however
  3. Easy to get iterative glycosylation
  4. Don’t need to worry about selective activation
  5. But can get side reactions when it gets longer
19
Q

What can methylation be used for with oligosaccharides

A
  1. Useful to help determine the structure of complex oligosaccharides
20
Q

How do you methylate oligosaccharides

A
  1. Treat with MeI or dimethyl sulphate ((MeO)2SO2) to fully methylate all free OH groups (ones not involved in glycosidic bond)
  2. Break glycosidic bonds to give mixture of monosaccharides using 100 degrees, HCl/H2O
  3. Analyse by MS, NMR. Use other derivatisation methods to show location of glycosidic linkages

aa Biomolecules (9 decks)

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