org.rxn.mech15a

[ Pobierz całość w formacie PDF ]

variety of computational methods incorporating different ways of accounting for
electron con�guration, and the mechanistic course of Lewis acid-catalysed cycloaddi-
15 Molecular Rearrangements 537
HO But But OH O But But O
But But But But
Me Me Me Me
Me Me Me Me
But But But But
HO But But OH O But But O
(282)(283)
[O]
O But
O But But O
But O
But Me
But But
Me
Me Me
But
Me
But
Me Me
But But
Me But
O But
O But But O
But O
(284)
(285)
F3C
F
P
P CF2 P
F3C
F
C P
R2N
R2N
(286)
F3C F F3C F
P C P C
P C P C
F NR2 F NR2
(288)
(287)
SCHEME 64
538 Organic Reaction Mechanisms 1997
Ar Ar
N Me N Me
H H
Me O O Me O
O
(289)
O
O
Ar Ar
N N
H Me H Me
Me O Me O
(290)
SCHEME 65
tions and ene reactions has been determined321 using high-pressure kinetics. The
transition structures for the ene reactions of cyclopropene with ethylene, propene, and
cyclopropene have been located322 with ab initio calculations. The ene reaction between
maleimides and allyl-substituted aromatics has been investigated,323 and the in�uence
of a protected alcohol group adjacent to the ene or enophile component on
diastereoselectivity in both thermal and Lewis acid-catalysed ene reactions of a series
of 1,6-dienes has been studied.324 The results have indicated that the effect can be
considerable, and this new �nding has been exploited in a synthesis of epijasmonoid
natural products. It has been reported325 that propargylic-p-toluenesulfenates (291), a-
substituted by a 4,5-unsaturated side-chain, rearrange thermally to 1-tosylmethylcyclo-
pentene derivatives (293) with prior isomerization into the corresponding allenic
sulfones (292).
O(O)SAr SO2Ar
SO2Ar
Me
Me Me
R1, R2 = Me
Me
Me Me
R1 R1
Me
R2 R2
(291)(292) (293)
A new approach to spirolactams involving the thermal reaction of N-unsaturated alkyl
b-carboxamidoenamines in which the enamine is the hetero-ene component has been
reported.326 N-Alkylnitrilium salts have been shown to undergo ene reactions with
electron-rich alkenes to afford either 2-azoniaallene salts (where the nitrilium salt acts
15 Molecular Rearrangements 539
R1
R1 R3
H R3
N+ R3
N+ R3
R2
R2
SbCl6 SbCl6
SCHEME 66
as ene and the alkene as enophile) or 1-azoniapenta-1,4-diene salts (where the alkene
reacts as the ene and the nitrilium salt as the enophile).327 A previously unknown ene
reaction of iminium ions and alkynes has been found to occur when complex anions
with low nucleophilicity328 are used as counterions. This new reaction has provided
stereo- and regio-selective access to substituted allylamines (see Scheme 66). Another
new type of thermal intramolecular concerted ene reaction involving allenylsilane
imines has been developed329 [see (294)!(295)] and has proved to be useful in the
H
SiMe2Ph
H
SiMe2Ph
N
NHCH2Ph
H
Ph
(294) (295)
enantioselective syntheses of a number of pentacyclic 5,11-methanomorphanthridine
Amaryllidaceae alkaloids. Diastereoselective syntheses of 1-hydroxy-substituted
benzo[b]quinolidines and 11-hydroxy-substituted azepino[1,2-b]isoquinolines via
hetero-ene cyclization processes have been described,330 and an enantioselective
synthesis of the potential substance P antagonist 4-isopropyl-3-(2-methyoxybenzyl-
amino)-2-phenylpiperidine (297) has been reported331 in which the key reaction is a
diastereoselective Lewis acid-catalysed hetero-ene reaction of aldehyde (296).
O Me
OMe
Ph
H
Me
HN Me
N
Ph
Ph
Me
(296)
HN
(297) [ Pobierz całość w formacie PDF ]

Linki