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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
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2. DDQ

Yield (%) D/E
R1 R2 R3 PhHa HP* US5 PhH HP US
1 H H H 45 67 65 2 6 3.5
2 H H Me 11 56 10 20
3 H H SiMe3 18 62 57 0.3 0.3 0.2
4 H Me Me 53 76 1.2 3.3
5 H O-COPh CO2Me 76d 2.5
6 H O-CMe2OCH2 721 4
7 H O-(CH2)2 -O 18 75 65 1 2.5 8
8 H CO2Me Me 66 8
9 H OSMDTB Me 71* 12*
10 O-CH2-O Me 15 73 66 1 7 5
11 OSMDTB Me Me 73h 30h
a Reflux, 6-12 h;
* 10-11 kbar, 0.75-2h;
5 Neat, 45 C, 2h;
d At 8 C;
In MeOH, 0 C, 8h, yield 50%, A/B = 5.5; f At 25 C; in MeOH, 3 h, yield 59 %, A/B =10;
* In MeOH, 45 C, 1.5 h; h: In MeOH, 35 C, 1 h
The Diels-Alder reaction between 1-dimethylamino-4-methyl-1-azadiene 39 and 5-hydroxynaphthoquinone 40 was investigated by Luche, Jenner and coworkers [31] in order to obtain some functionalized derivatives of naturally occurring 4-methylated-1-azaanthraquinones. The reactions were performed under sonochemical, thermal and HP conditions in solution and neat conditions. Some results are illustrated in Scheme 4.8. The primary adduct 41 was not isolated due to a rapid elimination of dimethylamine, and the adduct 42 was obtained as a single regioisomer. Ultrasonic irradiation in PhMe accelerated the Diels-Alder reaction but produced 30% aminoquinones 43 which is the result of an addition-oxidation of dimethylamine to 40. Unlike in the Diels-Alder reactions of o-quinone 37 with vinylcyclohexenes 38, methanol and neat reaction conditions were not a good reaction medium for this sonochemical cycloaddition. The study was enlarged to include azanaphthoquinones as dienophiles.
156
Ultrasound-Assisted Diels-Alder Reaction
-ffNMe.
r.t.
2 '
OH O 43
NHMe2
OH O 40
r.t.
N
NMe2
39
NHMe2
42
Medium Conditions t(h) 42(%) 43(%)
PhMe US 6 52 30
PhMe --- 24 48 traces
PhMe HP 6 48 traces
MeOH US 6 14 traces
MeOH --- 6 15 traces
Neat US 6 15 traces
Neat --- 6 14 traces
O
O
Scheme 4.8
Spanish researchers [32] also noted a considerable improvement upon sonica-tion of Diels-Alder reactions of 1-dimethylamino-3-methyl-1-azadiene 44 with a variety of electron-deficient dienophiles by using diene as solvent or in acetonitrile (Scheme 4.9). Ultrasound irradiation which allows mild reaction conditions gave good to excellent yields.
The sonochemical effect, the importance of solvent and the mechanism of US-assisted Diels-Alder reaction were recently critically investigated [33-35].
Caulier and Reisse [33] studied the influence of US on the yield and diaster-eoselectivity of the reaction between cyclopentadiene and methyl vinyl ketone in various organic solvents. Yield and endo/exo diastereoselectivity increased with US in halogenated solvents (CHCl3, CH2Cl2, CH2Br2) whereas they were not affected in non-halogenated solvents (CH3OH, PhMe). The authors give evidence that the cycloaddition is not affected by US, but US promotes the in situ generation of hydrogen halide which acts as catalyst. The observed sonochem-ical effect would be indirect and this would also occur in other cases described in the literature [36]. In many cases the US acts by mechanical effects (the same result could be obtained by very efficient stirring) or by generating radicals or molecules that act as catalysts.
Diels-Alder Reaction Facilitated by Physical and Chemical Methods
157
NMe2
A = Acrylonitrile; B = Benzoquinone; C = Naphthoquinone; D = 5,8-Quinolinequinone; E = Methyl vinyl ketone; F = Dimethyl acetylene dicarboxylate; G = Methyl acrylate;
H = Dimethyl fumarate
Scheme 4.9
Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (^-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels-Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0C with 1% FeCl3. The US-promoted Diels-Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak.
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