Page 228 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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514 Organic Chemistry, Synthesis

HO
F F
HO
CH 3 (86)

HO O Cyclodextrins can have very regular cavity sizes. The cav-
CH 3 HO
ity will incorporate aromatics to a uniform depth, often
tilting the substrate away from the axis of the truncated
conical cyclodextrin, most probably to maximize van der
(83) Waals contact. The limited size of the cavity results in sub-
strate selectivity. It has also been possible to selectively
This material, with the correct configuration at C-11, may functionalize cyclodextrins as illustrated by the prepara-
be efficiently converted to 11-hydroxy-progesterone, an tion of a cyclodextrin substituted with imidazole units:
intermediate in a commercial synthesis of hydrocortisone
O
acetate. Johnson has shown that excellent asymmetric in-
duction in polyene cyclizations is possible by treating ac-
etals derived from (2s,4s)-pentanediol with Lewis acids:

SiEt
SO 2 SO 2
O O
O
(84) KI
O O
O

HO ClSO 2 SO 2 Cl
N N

B. Guest–Host Inclusion Enzyme Models I I N N
(87)
Enzymes have unique catalytic, regulatory, and transport N NH
properties. The development of reagents that mimic these
properties would be a significant synthetic advance. In
early work it was possible to prepare model compounds
that could duplicate simplistically some of the transport
After the development of methods to prepare these
properties but with little substrate selectivity. Currently
molecules, the preparation of compounds more closely
enzyme models are being prepared which not only have
resembling enzymes was possible. Typical of these are cy-
the ability to recognize the substrate but also can effect
clodextrins bearing a histamine-zinc complex and found
site specific reactions.
to function as anhydrase models or polyamine metal
To promote substrate selectivity several types of inclu-
complexes which act as carboxylic hydrolases:
sion host molecules have been developed. Remarkable
successes in chiral recognition have been achieved with N
cyclic polyether compounds known as crown ethers: N N
N
N N (88)
H 2 N M
O O O NH 2 N
(85)
O
O O O
C
O
Cyclodextrins, oligosaccharides forming a distorted There are numerous other examples of biomimetic syn-
torus whose cavity may be occupied by other molecules, thesis. This narrow selection has been chosen to illustrate
have also been explored as selective host molecules: the progress and potential of this area.