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Organic Photochemistry
Introduction to Photochemistry
Classifications of Photochemical Reactions
Application of Photochemistry in OrganicSynthesis
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Energies
100 kcal/mol= 4.3 eV= 286 nm= 35000 /cm (near UV)nano= 10-9
286 kcal/mol= 12.4 eV= 100 nm= 100000 /cm (far UV)
Typical Bond Energies
C-H = 110 kcal/mol
C-C = 80
C=C = 150
C=O = 170
Uv light 150 -40 nm wavelength, so this is sufficient energy to break bonds knock
electrons out of bonding orbitals (electronic excitation).
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Chemically useful light is generally in the range of 200-400 nm
Often employ filters to regulate the wavelength of the radiation
T
n
T*
T
n
T*
T
n
T*
T
n
T*
T
n
T*
ground state (S0) n-T* (S1) n-T* (T1) T-T* (S1) T-T* (T1)
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A Jablonski diagram, named after the Polish physicist AleksanderJaboski, is a
diagram that illustrates the electronic states of a molecule and the transitions
between them.
The states are arranged vertically by energy and grouped horizontally by spin
multiplicity.
Radiative transitions are indicated by straight arrows and nonradiative transitions
by squiggly arrows.
The vibrational ground states of each electronic state are indicated with thick
lines, the higher rotational states with thinner lines.
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Physical Processes Undergone by Excited Molecules
So + hv --- S1 Excitation
S1v -- S1 + heat Vibrational Relaxation
S1 ----- So + hv Fluorescence
S1 ---- So + heat Internal Conversion
S1 --- T1 Intersystem Crossing T1
v -- T1 + heat Vibrational Relaxation
T1v -- So + hv Phosphorescence
T1 --- So + heat Intersystem Crossing
S1 + A (So) --- So + A (S1) Singlet-Singlet Energy Transfer
T1 + A (So) -- So + A (T1) Triplet-Triplet Energy Transfer
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Why Use Photochemistry
Overcome large kinetic barriers in a short amount of time
Produce immense molecular complexity in a single step
Form thermodynamically disfavored products
Allows reactivity that would otherwise be inaccessible by almostany other synthetic method
The reagent (light) is cheap, easily accessible, and renewable
Drawback Reactivity is often unpredictable
Many substrates are not compatible
Selectivity and conversion are sometimes low
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Chemical Processes undergone by Excited Molecules
(A-B- ) A-B. + . imple leavage
(A-B- ) E + F Decomposition
(A-B- ) A- -B Intramolecular Rearrangement
(A-B- ) A-B- ' Photoisomerization
(A-B- ) A-B- -H + R. Hydrogen Atom AbstractionRH
(A-B- ) (AB )2 Photodimerization
(A-B- ) AB + A* PhotosensitizationA
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1) -Cleavage (Norrish type I reaction). In solution the radicals undergofurther reactions to give products.
hv
0 n
C
h
h
hv
h
h
h
h
C
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An intramolecular example:
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Chang, S.-Y.; Huang, S.-L.; Villarante, N. R.; Liao, C.-C. Eur. J. Org. Che
m.2006, 4648-4657.
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Chemoselective Photorearrangement of Diazinobarrelenes.
Deuterium Labeling Study
Cheng, A.-C.; Chuang, G.; Villarante, N.; Liao, C.-C. J.Org. Chem. 2007, 72,
6 0- 6 7
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Substituent Effects on the Bridging Modes of Photochemical
Rearrangements of Pyrazino-, quinoxalino-, and
benzoquinoxalinobarrelenes
Chen, A.-C.; Lin, S.-Y.; Villarante, N.R.; Chuang, G.J. Tetrahedron, 2008, 62, 8 08-8 21
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Recent Advances in the Chemistry of
Masked o-Benzoquinones
Nelson R. Villarante
Department of ChemistryNational Tsing Hua University
Hsinchu 300, TAIWAN
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2
Generation of Masked o-
Benzoquinones
Oxidant : TTN = Thallium(III) nitrate [Tl(NO3)3]
BTIB = Bis(trifluoroacetoxy)iodobenzene [PhI(OCOCF3)2]
DAIB = Diacetoxyiodobenzene [PhI(OAc)2]
OMe
O
[Oxidant]
'OO
O '
OMe
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Self-dimerization of MOBs
The formation of a dimer occurs via Diels-Alder reaction between two molecules of MOB
The reaction is highly siteselective
regioselective
stereoselective
Out of eight-possible isomers, only a single isomer is observed
Ref: 1) Anderson, G.; Berntsson, P. Acta Chem. Scand. B 1975, 29, 48.
2) Liao, C.-C.; Chu, C.-S.; Lee, T.-H.; Rao, P. D.; Ko, S.; Song. L.-D.; Shiao, H.-C.
J. Org. Chem. 1999, 64, 4102.
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Dimerization (DI) vs Diels-Alder
Reaction (DA)
RateDI DI
RateDA DA Dieno ile
RateDI DIRateDA DA Dieno ile
as ed - enzo inone
X
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Reactions of MOBs
Ref: Liao, C.-C.; Peddinti, R. K. Acc. Chem. Res. 2002, 35, 856.
O
OR1
OR2R
XOMe
OMe
O
R
O
XR
OMeMeOH
R
X
OMe
OMeO
O
OMeOMe
R
C60
CH2=CH
CH=C
HX
CH2
=CHX
R1=R2=CH3R1 =R
2 =CH 3
+
h, OBn, h, ePh
X=CHO, COMe, CO2CH3
X
Rn
X
O
OMe
OMe
Rn
X
O
OMe
OMe
R1=R2=CH
3
R1=R2=CH3
X=CH2,
O, NBz,
X
R1=R2=CH3
X=O, NH
O
R1=R2=CH3
OMe
Rn
O
O
OMe
OMe
O
OMe
OH
R
DAIB
MeOH, 0
C
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Reactions of MOBs
Ref: Liao, C.-C.; Peddinti, R. K. Acc. Chem. Res. 2002, 35, 856
O
OR1
OR2O
OMe
O
R
O
XH
O OMeH
O
OMe
O
X
R1
= CH3R2 = CH2CH=CH2
R2 = CH2CH=CHCH=CHX
R1 = CH3
+R1 = R2 = CH3
R3 = OX( )n
n = 1 - 3X = H or TBS
R4 = H
O
OMeOMe
OX
( )n
O
OMeOMe
OX
( )n
+
R1 = R2 = CH3
R4 =
X = TBS
R3 = H
O
MeOMeO
OX
O
MeOMeO
OX
+
OX
4 eq. SmI2/-78oC
THF/MeOH
O
OX
O
H
OXH
OXO
OBz
H
H
H
.. O
BzO
R4
R3
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Diastereoselective Intermolecular Diels-Alder Reactions of
MOBs
O
O
O
OMeOMe
R1
R2
R3
R4
O
OMe
Xc
O
O
MeO
OMe
Xc
O
+
R3 R3R4
R2
R1 R1
R2
R4
R1 R2 R3 R4 t/h de/%* Yield/%
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
H Ketal
H Ketal Br
H H COMe
H H CO2Me
H H TMS
H H Br
H H H OMe
H Me Br
H Me TMS
1 2a2a' 71 53
1 2b/2b' 71 57
48 2c/2c' 67 66
48 2d/2d' 73 65
36 2e/2e' 62 8372 2f/2f' 81 68
24 2g/2g' 83 98
48 2h/2h' 82 95
48 2i/2i' 78 77
48 2j/2j' 74 95
48 2k/2k' 1 98Me H Br
CH2Cl2, r
OMe
Ketal =O
O* eter ined by 1H NMR (400 MHz) analysis.
2a-k( ajor) 2a-k'( inor)
H
H
H
H
H
H
H
H
H
H Br Ketal H
1a-k
Products
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Syntheses of Natural Products
O
O
O
(+)-3F-Angeloyloxy-furanoeremophilane
O
OR1
OR2R
NNH
MeOH H
H
MeO2C OAr
OMe(+)-Reserpine
Chem. Commun. 1996,1537
HO
CO2HClerodane diterpenic acid
Synlett1998, 912
HO
CO2H
HO
HO
O
O
H
HMeO2C R'
CO2Me
O
N
OH
HO
H
H
H
(+)-Bilosespenes A and B
(+)-Magellaninone
(+)-Eremopetasidone
(+)-Pallescensin B
(+)-Forsythide aglucone dimethyl ester
Clerodane diterpenic acid
Angew. Chem. Int. Ed.
2002,41, 4090
Org. Lett. 2003,24, 4741
Org. Lett. 2001,3, 263
Tetrahedron Lett. 1989,30, 2255
Chem. Commun.1999,117
Tetrahedron Lett. 1996,37, 6869
H
(+)-Capnellene
HO2C
Pure Appl. Chem.
2005,7,1221
Pure Appl. Chem.
2005,7,1221
H
H
H H
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Current Synthetic Targets
O
OR1
OR2RHO
H
(+)-Conidiogenone
O
OH
O
(+)-Eudesmadieneolide
O
O
OH
(+)-Drechslerine D
OH3CO2C
O
H OHH3COCO
O
O
CO2CH3OH
OO
OHO
Azadirachtin
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3
Production of1O2 by
Photochemical MethodSen
hR1Sen*
3Sen*
ISC3
O2 1O2
Photosensitizers
ye: ose engal ( ), Tetraphenylporphyrin (TPP)
O O
Cl
Cl
Cl
Cl
I
NaO
I I
IO
ONaN
N
N
N
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Electronic Configuration of Ground State and
Excited State O2
2p
(3Wu)
(1 g)
(3Wu)
(1 u)
Reference:
http://www.rsbs.anu.edu.au/ResearchGroups/PBE/Oxygen/O2_1_%20ElectronicConfig.htm
157 KJmol-194 KJmol-1
Ground State Excited State Excited State
( 37g- ) (1(g
- ) (17g+)
(0.9772 eV) (1.6266 eV)
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ON2t-Bu
t-Bu
h, O2
MB, CH2Cl2- 8 oC
h, O2
8 %
6%
Ryang, H-S.; Foote, C. S. J. Am. Chem. Soc.1981, 103, 495
OO
O
O2, CH2Cl2
OO
O
OO
95%
Wood, J. L.; Graeber, J. K.; Njardarson, J. T.Tetrahedron, 2003, 59, 8855
O
O
N2
O
t-Bu
t-Bu
O
O
O
O
t-Bu
t-Bu
[4+2]
+OO
OO
3 26
OO OO
2.1 1.5
OO OO
14
Ene
Ene
+
+
OO
2.
3
Ene
atusch, .;Sch idt, G.Angew.
hem. Int. Ed.1988, 27, 717
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TPP / CHCl3, O2,hR
0oC,2h
O
O
OMe
OMe
O
Me
O
O
OMe
OMe
OMe
S
H2N NH2
rt, CDCl3 OH
HO
OMe
OMe
OMe
3b 82%)
3a 87%) 5a 80%)
TPP / CHCl3, O2,hR
0o
C,3h
RB / MeOH, O2,hRO
O
OH
Me
OMe-25oC,4h
PPh3
6a 69%)
OOMe
OOH
Me
O
4a
RB / MeOH-dropof
AcOH, O2,hR
-10oC,5h
O
O
OHOMe
6b 72%)
O
OMe
OMe
Me
O
OMeOMe
Me
2a
2b
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l3
R
4
i RB / M OH O hR
C 4 h
ii PPh3
OO
OM
OH
6c (69%)
O
O
OM
OM
O
3c (35%)O
OM
OM
2c
OO
OM
OOH
4c(Yield is not
yet determined)
:
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OO
O
R
(i) RB / OH, O , R
- 5
C, 4
(ii) OH
R t
R t l
R i r
OO
O
OO
OO
O
(46%)
(68%)
OO
O
(41%)
O
O
O
2
8i
8
8
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46
O
O
OOH
e
O e
O
O
O e
O e
Oe
Condition 1 eOH, 0 oC
Condition 2: R / eOH, O2, hR
Recovery ofstarting aterial
OR
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Plausible Mechanisms for the Observed Products
OOMe
O
R3
O
O
O
O
O
OO
OMe
OMe
1O2
R3
R1
R1
R3
R1
4+ 2]
R4
R4
O
R3
O
O
O
OR1
R4
OMe
R4
O
O
OMe
OMe
OR1
R3 R4
O
OOHO
OMe
R3
R1 R4 + H2O
-MeOH
ISOLATED
PPh3
AcOHO
O
OMe
R3
R1
O
R4 = H
O
OHO
OMe
R3
R1 R4
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4
A Short Synthesis of (+/-)-Untenone A
References
1. Saito, .; Takeuchi,R.; Kamino, T.; Kuramochi, K.; Sugawara, .; Sakaguchi, K.; Kobayashi, S. Bioorg.Med. Chem. Lett.
2004,14, 1975.
OH
OMe
CHO
i C15H31MgBr
ii H4Cl s t.
OH
OMe
C15H31HO
Pt/H2
80%
99%
OH
OMe
C16H33
DAIB
MeOH, 0
C
95%
OMe
OMeO
C16H33
i PP / CHCl3, O2, R,
4
ii PPh3
78%
O
OH
C16H33
O
OMe
5 (+/-)-U tenoneA
ref. 1
ref. 1
C16H33
CO2Me
C16H33
OHO2C
H
H
MeO2C
CO2Me
OH
C16H33
O
O
O
(+/-)-ManzamenoneA
(+/-)-Plakev lin
1 2 3
4
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Diels-Alder Reactions of Nitroso Compounds
Hart,H.; Ramaswami, S. K.; Willer,R. J. Org. Chem. 1979,44,1.
R
O
O
PhH2CO NHOH
O
N
OBr
Cbz
O
O
R = BrR = H
MeO2
C
NO
Cl
NH2+
Cl-O
O
O
OMOM
OMOM
PhH2CO NHOH
O
N
O
Br
Cbz
MOMO
MOMO
NO
Ph
H OHOH
O
NPh
Elango, S.; Yan, T.-H. J. Org. Chem. 2002,67, 6954 Hudlicky, T.; Olivi,H. . J.Am. Chem. Soc. 1994,116, 5108.
Yamamoto,Y.; Yamamoto,H. Eur.J. Org. Chem. 2006,ASAP.Martin, S. .; Tso,H.-H.Heterocycles 1993,35, 85.
OR2
R3
N
O
Me Me
Me
Me
O
Ph
O
N
Me Me
Me
Me
O
+
1 : 1
Ph
R1 = R4 =H
O
NPh
R1
R4
N
O
Me
Me
O
Ph
R2= R3 =Me
R1 = R2=
R3= R4 =Me
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Hetero-Diels-Alder Reactions of
Masked o-Benzoquinones with Nitroso Compounds
OMe
OH
R1R2
R3
OMe
OMe
OR1
R2
R3
PhI(OAc)2/Bu4NIO4
MeOH-CH2Cl2, 0oC
RNHOH(4a,b)
MeOH-CH2Cl2,
N OR O
NOMe
OMe
OR1R
2
R3
R
+
R=Boc orCb0 oC or 50 oC
Te p/oCa Ti e/hbAdduct (Yield/ )c
a)Reaction te perature. b)Reaction ti e after the addition ofRNHOH.c)Yields ofpure and isolated adducts.
R1 R2 R3MOB
1a-i
2a-i
3a,b5a-i6a-i
for3a,4a,5a-i: R = Bocfor3b,4b,6a-i: R = Cb
with 3a with 3b
2b Me H H 0 1 5e(69) 6e(81)
2c H Me H 0 1 5c(93) 6c(95)
O
O2d H H 0 1 5d(92) 6d(96)
2a H H H0
1 5a(90) 6a(96)
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OMe
OHR1
R
R3
OMe
OMe
O
R1R
R3
PhI(OAc)2/Bu4NIO4
MeOH-CH2Cl2,0oC
RNHOH(4a,b)
MeO
H-CH2Cl2,
N OR O
NOMe
OMe
OR1R
2
R3
R
+
R=Boc orCbz0 oC or50 oC
Temp/oCa Time/hbAdduct (Yield/ )c
a)Reaction temperature. b)Reaction time after the addition ofRNHOH.c) Yields ofpure and isolated adducts.
R1 R2 R3MOB
1a-i
2a-i
3a,b5a-i
6a-i
for3a,4a,5a-i: R = Bocfor3b,4b,6a-i: R = Cbz
with 3a with 3b
O
O2i H H 50 12 5i(71) 6i(92)
2g H H Me 50 3 5g(84) 6g(90)
2h H H Br 50 12 5h(91) 6h(90)
2e H CO2Me H 0 1 5e(71) 6e(81)
2f H H CO2Me 0 1 5f(70) 6f(74)
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Diastereoselective Hetero-Diels-Alder Reactions of
Masked o-Benzoquinones with Nitroso Compounds
OMe
OH
R1R2
R3
OMeOMe
O
PhI(OAc)2/Bu4 IO4
MeOH CH2Cl2, 10oC MeOH CH2Cl2, 10
oC
+
2a d
1a d
3g
O O OMe
OMe
O
O
O
MeOMeO
O
OO
O
+
5g-8g (major) 5g'-8g'(minor)
R1R2
R3
R1R2
R3
R1 R2
R3
O HOH
O
4g
O
O
Entry
1
2
3
4
Temp/o
-10
-10
-10
-10
Time/h
24
24
24
24
Products
5g/5g'
g/6g'
7g/7g'
8g/8g'
Yield/%
96
92
92
96
de(%)a
90
80
88
90
a. etermined y PL (Shim-peak L -SILcolumn)
1
Me
2
Me
3
O
O
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5
Towards the Total Synthesis of (+/-)-Valienamine
2
(+/-)-Valienamine
OMe
OHO
O
O
NOMe
OMe
O
O
O
Cbz
O
NOMe
OMeO
O
Boc
OH
H
CbzNHOH DAIB,Bu4NIO4 DIBALH, THF
O
HN
OOH
H
Ohydrolysis
85%
97%MeOH
1 2 3
4
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ON
OH
OMe
OMOM
Cbz
ON
OH
Cbz
O
Entry
12
3
4
56
7
8
9
10
Reagent
6N HCl, THF
6N HCl, CH2Cl2
5% Oxalic acid, THF
70%AcOH, THF
(CH3)2S : MeOH= 1 : 5PTSA,Acetone
BF3.OEt2,Bu4NI, CH2Cl2
MeSiCl3, NaI, CH3CN
TMSI, CH2Cl2
50% TFA : CH2Cl2 = 1 : 2
Product (11)
37%
Trace
Recovery ofSM
Recovery ofSM
Recovery ofSMTrace
Messy
Messy
Messy
60%
OMOM
OH
O
N
O
OMOM
OMe
Cbz1. IBD, MeOH,0oC
O
NOMOM
OMe
Cbz
OH
DIBALH,
THF-78 oC
81%
96%2. Bu4NIO4, CbzNHOH
CH2Cl2,0oC
8 9 10
10 11
A Sh T l S h i f C d i A 1
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A Short Total Synthesis of Conduramine A-1
O
NCbz
OH
H
O
NCbz
OH
O
L-Selectride,
THF-78 oC
NH
OH
OH
OH
Cbz
SmI2, THF
NH2
OH
OH
OH
76% 74%
NaOMe,MeOH
quant.
11 12 13
ConduramineA-1
NH
OH
OH
OH
Cbz
13
NH
OH
Cbz
14
O
OAcetonepTSA
HC(OMe)3
OH
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Tetrodotoxin
O OH
N
NH
O
()-tetrodotoxin (TTX)
OHHO
HO
H
HOH
NH2
O
isolated from the ovaries
and liver of the Japanesepuffer fish the tora fugu(tiger puffer)
total syntheses
( )-TTX: (a) Kishi, Y.; Aratani, M.; Fukuyama, F.; Nakatsubo, F.; Goto, T.; Inoue, S.;Tanino, H.; Sugiura, S.; Kakoi, H. J. Am. Chem. Soc.1972, 94, 217. (b)Kishi, Y.; Fukuyama, F.; Aratani, M.; Nakatsubo, F.; Goto, T.; Inoue, S.;Tanino, H.; Sugiura, S.; Kakoi, H. J. Am. Chem. Soc.1972, 94, 21 .
()-TTX: Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc.2003, 125, 87 8.
()-TTX: Hinman, A.; Du Bois, J. J. Am. Chem. Soc.2003, 125, 11510.
( )-TTX: Sato, K.; Akai, S.; Sugita, N.; Ohsawa, T.; Kogure, T.; Shoji, H.; Yoshimura, J. J. Org. Chem.2005, 70, 74 6.
i i
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Retrosynthetic Analysis
O
O
OAcTBSO
OAc
HN
H
NHAc
NAc
O
O
OAc
AcOO OH
N
NH
O
tetrodotoxin
OHHO
HO
H
HOH
NH2
O
O
OHTBSO
OH
HN
O-t-Bu
O
O
MeO
OMe
O
OO
N
O
O
OMe
OMe
O
O
t-Bu-O
O
OR
O
OH
TBSO
O
TBSO
O O
OR
O
OH
OMeOMe
OTBS
OMeTBSO
O O
OR
O
OH
OH
CHO
OMe
vanillin
H Di l Ald R i f MOB i h B NO
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Hetero Diels-Alder Reaction of MOBs with BocNO
O
R1
O
O O
R2
OMeOMe
OH
OMeR
1O
O O
R2
N
O
O
OMe
OMeO
O
R2
OR1
t- u-O
O
DAIB
MeOH, rt
n-Bu4NIO4,BocNHOH
solvent, temp, 5 h
R1 R2Entry
1
2
3
4
5
1a
1a
1b
1c
1d
1a-d 2a-d 3a-d
TBS
TBS
H
H
H
Me
Me
CH2OMe
CH2OBn
CHO
Product (Yield/%)
2a(95)
2b(96)
2c(95)
2d(94)
Solvents Temp./oC
55
85
55
55
55
MeOHCH2Cl2
ClCH2CH2Cl
MeOHCH2Cl2
MeOHCH2Cl2
MeOHCH2Cl2
Product (Yield/%)
3a(46) / 2a(48)
Phenol
3a(0) / 2a(95)
3b(0) / 2b(70)
3c(0) / 2c(91)
3d(0) / 2d(75)
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The iels- lder adducts obtained from MOB chemistry were realized as powerful
intermediates for the preparation of various molecular skeleta.
New synthetic methodologies were successfully applied for the syntheses ofbiologically intriguing and pharmacologically important natural products.
The potentiality of MOB chemistry was further validated by the reactions with heterodienophiles in an effort to provide methodologies in the domain of heterocyclicchemistry.
The substituent effect and solvent effect on the reaction between singlet oxygen andMOBs will be studied in detail.
Nitroso compounds have emerged as important hetero dienophiles for MOBs; whichdelivered iels- lder adducts with excellent selectivities and yields. Theapplicability of the methodology is exemplified by the synthesis of conduramine- 1;studies towards the synthesis of related conduramines are underway.
Conclusions
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Acknowledgements
National Tsing Hua University
National Science Council
Ministry of Education, Taiwan
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