Books
in black and white
Main menu
Home About us Share a book
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

The diels alder reaction selected practical methods - Fringuelli F.

Fringuelli F. The diels alder reaction selected practical methods - John Wiley & Sons, 2002. - 350 p.
ISBN 0-471-80343
Download (direct link): thedielsalderreaction2002.pdf
Previous << 1 .. 83 84 85 86 87 88 < 89 > 90 91 92 93 94 95 .. 123 >> Next

The study was extended to other dienes and dienophiles [16d, e]. Some examples and comparisons are reported in Scheme 6.2. With respect to the organic solvent, the aqueous reaction requires milder conditions and the reactio-nis faster and more selective. It is significant that the use of cosolvents such as methanol, dioxane and tetrahydrofuran results in a reduction of reaction rate.
Table 6.4 Endo/exo diastereoselectivity of Diels-Alder reactions in water and organic media
.COR 20-25 C
COR
(endo)
(exo)
Endo / Exo
Medium
R = OMe
^Me
None (excess of diene)
Isooctane
Ethanol
1-Butanol
Formamide
N-Methylacetamide
Water
83:17
87:13
82:18
90:10
74:26
69:31
84:16
96:14
79:21
89:11
256
Diels-Alder Reaction in Aqueous Medium
MeO
tifcto
H 0
CHO
xo2r
Chaparrinone
co2r
(endo)
CO2R
(exo)
R Medium t(h) endo/exo Yield (%)
Et PhH 288 46:54 52
Et H20 168 56:44 82
H PhMe 168 41:59 46
H H2O 17 60:40 85
Na H20 5 75:25 100
Scheme 6.1
O
MeO.
OMe
O OMe
MeO
II II
0 'co2r 'co2r
R Medium t(min) T(C) Yield(%)
Me
Na
PhH
H20
720
12
80
r.t.
69
93
AcOo,
0Ac co2r
R Medium t(h)
AcO
(endo) (exo)
7(C) endo/exo yield(%)
Me PhMe
Na H20
96
24
110
60
33:66
67:33
84
67
+
+
2
+
Scheme 6.2
Diels-Alder Reaction in Unconventional Reaction Media
257
The nitroso moiety of the N-acylnitroso function is a powerful dienophile and therefore N-acylnitroso compounds are trapped rapidly, especially in an intramolecular reaction, with a diene allowing the Diels-Alder reaction to occur also in water, although N-acylnitroso compounds are short-lived in aqueous medium.
N-Acylnitroso compounds 4 are generated in situ by periodate oxidation of hydroxamic acids 3 and react with 1,3-dienes (e.g. butadiene) to give 1,2-oxazines 5 (Scheme 6.3). The periodate oxidation of 4-O-protected homo-chiral hydroxamic acid 6 occurs in water in heterogeneous phase at 0 , and the N-acylnitroso compound 7 that is generated immediately cyclizes to cis and trans-1,2-oxazinolactams (Scheme 6.4) [17a, b]. When the cycloaddition is carried out in CHCl3 solution, the reaction is poorly diastereo-selective. In water, a considerable enhancement in favor of the trans adduct is observed.
H
Y OH
O
iof
Y Nv
Y '
O
Y O
O
Y = R, OR, NR^2
Scheme 6.3
3
OMOM
O
0 C, 1 min
MOMO H
(cis)
Medium
trans/cis
Yield (%)
CHCl3
H2O
63:37
82:18
75
93
Scheme 6.4
P^NIO4
+
O
6
The acylnitroso approach has been used for the enantioselective syntheses of ()-swainsonine and ()-pumiliotoxin [17d] (Scheme 6.5).
258
Diels-Alder Reaction in Aqueous Medium
Lubineau and coworkers [18] have shown that glyoxal 8 (Rj = R2 = H), glyoxylic acid 8 (Rj = H, R2 = OH), pyruvic acid 8 (Rj = Me, R2 = OH) and pyruvaldehyde 8 (Ri = H, R2 = Me) give Diels-Alder reactions in water with poor reactive dienes, although these dienophiles are, for the most part, in the hydrated form. Scheme 6.6 illustrates the reactions with (E)-1,3-dimethyl-butadiene. The reaction yields are generally good and the ratio of adducts 9 and 10 reflects the thermodynamic control of the reaction. In organic solvent, the reaction is kinetically controlled and the diastereoselectivity is reversed.
Scheme 6.5
J O
r + A
JL Ri COR2
8 9 10
R1 R2 t (h) 9/10 Yield (%)
H OH 1.5 64:36 97
H Me 48 53:47 96
H H 60 50:50 36
Me OH 48 33:67 74
Scheme 6.6
These results have been used to prepare key starting compounds 11 and 12 for the enantioselective synthesis of 3-deoxy-D -manno-2-octulosonic acid
Diels-Alder Reaction in Unconventional Reaction Media
259
(KDO) 13 [19a] and for a concise synthesis of ketodeoxyheptulosonic acid derivatives 14 [19b], respectively (Scheme 6.7).
HO
HO
O
+ A
H CO2Na
1. H2O
2. MeOH/H
3. Ac2O/Py 54 %
AcO AcO
CO2Me
HO
O
O
1. H2O, 100 C
2. H+/CH2N2 1
(endo-si + exo-si = 76 %) 11
OH
O
CO2Me
12
HO
HO
HO
OH
OAc
CO2H
AcO
OH
OH 13 (KDO)
2
14
Scheme 6.7
Previous << 1 .. 83 84 85 86 87 88 < 89 > 90 91 92 93 94 95 .. 123 >> Next