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O+B > Retrosynthetic Analysis and Group Interconversions > Flashcards

Retrosynthetic Analysis and Group Interconversions Flashcards

1
Q

Retrosynthetic analysis

A

the process of breaking down a target molecule into readily available starting materials by means of imaginary breaking of bonds and by conversion of one functional group to another

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2
Q

disconnection

A

imagined cleavage of a bond to break the molecule into possible starting materials

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3
Q

functional group interconversion (FGI)

A

process of converting one functional group into another e.g. substitution, addition, oxidation or reduction

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4
Q

synthons

A

idealised fragment (cation/anion) resulting from a disconnection

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5
Q

synthetic equivalent

A

reagent carrying out the function of a synthons

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6
Q

latent polarity

A

imaginary pattern of alternating positive and negative charges used to assist in the identification of disconnections and synthons

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7
Q

reductions
ketones
esters + carboxylic acids

A

ketones –> secondary alcohol (NaBH4 or LiAlH4)

esters/carboxylic acids –> primary alcohol (LiAlH4)

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8
Q

nones

A

undergo conjugate addition reactions with appropriate nucleophiles
soft = conjugate e.g. dialkylcuprates
or NaBH4/CuI MeOH - copper makes soft E+ so MeOH can reduce
hard = 1,2 addition e.g. alkyl lithium

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9
Q

conjugated unsaturated ketones e.g. dienones

A

addition may occur further
e.g. Ph2CuLi to a dienone addition occurs at furthest alkene first then a second addition to alkene and reduced carbonyl group

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10
Q

LiAlH4

A 
reduces:
ester and acyl chlorides to primary alcohols
ketones to secondary alcohols 
tosylates and alkyl bromides to alkanes
nitriles to primary alkanes
amides to amines
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11
Q

NaBH4

A 
reduces:
ketones to secondary alcohols
aldehydes to primary alcohols 
in presence of CuI - 1,4-reduction of enone 
in presence of CeCl3 - 1,2-reduction 
doesn't reduce:
amides
(usually) esters, alkyl halides
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12
Q

DIBALH (dis-butyl aluminium hydride)

A

selective reduction of an ester to an aldehyde
reduction of nitrile to aldehyde
-78 C

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13
Q

Oxidation: Jones reagent

A

Jones reagent : CrO3, H2SO4, H2O
secondary alcohol to ketone
primary alcohol to carboxylic acid

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14
Q

Oxidation: PDC (pyridinium dichromate)

A

protonated pyridine and Cr2O7-
primary alcohol to aldehyde
intramolecular (5-membered ring intermediate)
or intermolecular using pyridinium

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15
Q

using protecting group on ketones

A

acetals
protection = R”OH/ H+
deprotection = H2O/H+

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16
Q

protecting groups for alcohols

A

acetals
silly ethers
esters
ethers

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17
Q

protecting alcohol during Grignard reaction

A

Grignard reagents destroyed by water and acid functional groups
protection = TBDMSCl (tertbutyldimethylsilyl chloride)
deprotection = tBu4NF (TBAF) (strong Si-F bond)

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18
Q

RLi

A

1,2-addition

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19
Q

R2CuLi

A

1,4-addition

20
Q

RMgBr

A

1,2-addition

21
Q

RMgBr/CuI

A

1,4-addition

22
Q

NaCH(CO2Et)2

A

1,4-addition

23
Q

LiAlH4

A

1,2-addition

24
Q

NaBH4/CuI

A

1,4-addition

25
Q

NaBH4/CeCl3

A

1,2-addition

26
Q

RNH2

A

1,4-addition

27
Q

RSNa

A

1,4-addition

28
Q

RONa

A

1,4-addition

29
Q

Baeyer-Villiger OXidation

A

oxidation of ketones to esters and cyclic ketones and lactones
in general substituted between alpha carbon and carbonyl
involves alkyl group migration
reagent = MCPBA (nucleophile)

30
Q

Beckmann rearrangement

A

overall insertion of nitrogen next to the carbonyl group to form an amide
NH2OH forms oxime (C=NHOH)
add H2SO4 (migration of C-C bond)
base - amide formed
group that migrates is trans to the OH group, mixture of geometric isomers of oxime and amides

31
Q

hydration of double bonds

A

least substituted: 1. BH3, THF 2. H2O2, NaOH

most substituted: 1. Hg(OAc)2 2. NaBH4

32
Q

addition of HBr to double bonds

A 
least substituted (old) : radical mechanism, HBr, H2O2
most substituted (fresh): ionic mechanism, HBr
33
Q

electrophilic aromatic substitution

A

ortho/para: X = Me, NHCOCH3, OH etc

meta: X = NO2, CN, CO2Me etc

34
Q

addition of a nucleophile to an epoxide

A

most substituted: favoured under acidic conditions (combination of products but major is most sub) e.g. 1. HCl 2. MeOH
least substituted: favoured under basic conditions e.g. NaOMe

35
Q

regioselective alkylation of ketones

A

also use of examines and beta-keto esters
base
kinetic enolate - least sub (favoured when using low temp and hindered base e.g. LDA)
thermodynamic enolate - most sub

36
Q

1,2- and 1,4- dicarbonyl retrosynthesis

A

mis-matched/dissonant pattern of latent polarities requires reversal of polarity usually associated with the carbonyl group for one of the precursors - unpolung
e.g. RO=C- –> dithiane + strong base
CH3O=C- –> HCCH (alkyne) + base
formyl anion goes to NO2CH3 + base (Henry reaction)
RCOHCH2+ –> epoxide
RC=OCH2+ –> RC=OCH2Br

37
Q

unpolung

A

used to describe cases in which a synth of opposite polarity to that normal associated with a required functional group must be used

38
Q

use of dithianes in synthesis

A 
HSRSH/H+ (formation)
sulphur creates acidic proton because EWG 
BuLi/THF -78C deprotonates 
\+ electrophile 
hydrolysis of dithiane e.g. Hg2+, H2O 
CaCO3
MeI, H2O (?)
39
Q

use of nitriles in synthesis of alpha hydroxy acids (OH group on alpha carbon of CO2H)

A

aldehyde + NaCN –> alpha-hydroxy nitrile + H+/H2O –> alpha-hydroxy acid

40
Q

use of nitroalkanes in synthesis of alpha-hydroxy aldehydes

A

nitroalkane + aldehyde –> Henry reaction to give nitroalcohol
1. NaOH 2. H+ (Nef Reaction) –> alpha-hydroxy alcohol

41
Q

use of nitroalkanes to prepare 1,4 - disubstituted products

A

nitroalkane + enone (+iPrNH) —> conjugate addition –> NaOMe = nitronate
further - ozonolysis (O3, MeOH) = 1,4-dicarbonyl

42
Q

dehydration of alcohols

A

H+/H2O
alcohols are easy to prepare : RMgBr + RCHO, reduction of carbonyls
dehydration may give a mixture, difficult to separate
rearrangements commonly occur

43
Q

elimination of alkyl halides

A

base to give alkene
mixture of products often obtained - difficult to separate
major product - more substituted C=C because more stable

44
Q

reduction of alkynes

A

geometry of double bond controlled by selective reduction conditions

first: H2, Lindlars cat (5% Pd-CaCO3, Pb(OAc)2, quinoline) gives cis product
second: 1. Na, liquid NH3 2. H2O gives trans product

45
Q

preparation of alkyne

A

substituting H
1. base e.g. BuLi, RMgX - deprotonation
2. electrophile e.g. alkyl halide, aldehyde, epoxide
addition of aldehyde gives secondary alcohol
addition of epoxide, less hindered end attacked so OH one bond away
position of double bond is fixed

46
Q

wittig

A

R2CO + Ph3P=CH2
phosphonium slide (from salt and base)
versatile
chemoselective - aldehydes and ketones only
regiospecific
PhP=O by-product may contaminate product and be difficult to remove
resonable control of double bond geometry

47
Q

3 types of slid

A

reactive - R= alkyl, requires strong base to be formed, (Z) alkenes predominate
moderated = R=Ph or vinyl. some stabilisation charge. mixtures
stabilised - R=EWG, requires weak base, predominantly E alkenes. only aldehydes

O+B (5 decks)

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