最新哥伦比亚教授有机化学课件Olefination教学讲义PPT

Epoxidation of trans-2-Butene Problem 7.17 (p 285)
R RCO3H
R
S +
S
50%
50%
syn addition to trans-2-butene gives racemic mixture of chiral diastereomer
Epoxidation of cis-2-Butene Problem 7.17 (p 285)
S
R
RCO3H
R
S
meso
syn addition to cis-2-butene gives meso diastereomer
Stereospecific reaction
of two stereoisomers of a particular starting material, each one gives different stereoisomeric forms of the product related to mechanism: terms such as syn addition and anti addition refer to stereospecificity
S
R
Br2
R
S
meso
anti addition to trans-2-butene gives meso diastereomer
Bromine Addition to cis-2-Butene Fig. 7.15 (p 285)
R Br2
R
S
+
S
50%
50%
anti addition to cis-2-butene gives racemic mixture of chiral diastereomer

5. Reductive Elimination
O RO O
Cl Cl Cl
Zn, AcOH
Troc = trichloroethoxycarbonyl
H O RO O
ZnCl Cl Cl
Cl -
Cl
-[C O 2]
ROH
6. -elimination
O R2N O
M ild base
O R2N O
Pd,
m o rp h o lin e
RO
or dimedone
ROH
RO PdL2
Can also use (Ph3P)3RhCl and acid
16
Protective Groups: Orthogonal Sets of Protecting Groups
6
Protective Groups: Orthogonal Sets of Protecting Groups
1. Cleavage by basic solvolysis
O
OH
RO Me
ROH
O RO Me krel 1
O Cl
RO
760
O Cl
RO Cl
16,000
O Cl
RO Cl
Cl
100,000
3
Protective Groups: Background and General Considerations
Tactical considerations to consider for each protecting group selected in a synthesis:
It should be easily and efficiently introduced. It should be cheap and readily available. It should be easy to characterize and avoid the introduction of new stereogenic centers. It should not afford so many spectroscopic signals that it hides key resonances for the substrate. It should be stable to chromatography. It should be stable to a wide range of reaction conditions. It should be removed selectively and efficiently under highly specific conditions. The by-products of deprotection shoud be easily separated from the substrate.

Me
Ph3P H
O
H
-[Ph3P=O] Me
Z-alkene 4
W ittig Olefination: Background and Principles
Stereoselectivity with stabilized ylides
Me
Ph3P
O
O
Ph3P
OEt
Ph3P
Me
se m is tab ilize d
Peterson Olefination D. J. Ager, Synthesis 1984, 384. Julia (Julia-Lythgoe) Olefination B. M. Trost, Bull. Chem. Soc. Jpn. 1988, 61, 107.
1
PPh3 + X R1
drives reaction
Ph
Ph P Ph
O
R1
oxaphosphatane
Ph Ph Ph
P
O
R1
betaine
G. W ittig and G. Schollkopf, Chem . Ber. 1954, 87, 1318. 2
W ittig Olefination: Background and Principles
Incredibly stable; not moisture sensitive, can be chromatographed Price for stability is lower reactivity: reacts well with aldehydes, slowly with ketones
TBSO
Me H Me O

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