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Indolizine, 18+2 -cycloaddition with alkyn

Bipyridinium ylides 133, generated in situ from 4,4-bipyridinium diquaternary salts 133, undergo 1,3-dipolar cycloaddition with activated alkynes under micro-waves, on KF-alumina in the absence of solvent, to give 7,7-bis-indolizines 134 in 81-93% yield (Scheme 9.40) [91]. The same reactions, when performed using benzene as a solvent under classical heating, yielded 7,7-bis-indolizine derivatives in yields of only 50-60% [92],... [Pg.319]

The attacks of heterocyclic A -oxides, e.g. of pyridine, quinoline, isoquinoline, phenanthridine, etc., on activated alkynes (RC CR R = R = COOMe R = Ph, R = COOEt R = Ph, R = CN) pose similar problems . An acyclic intermediate has been postulated but is rarely detected. Some of the possibilities are illustrated in equation (126) . If the open intermediate is formed, then the paths to the ylid and the 2-substituted quinoline in equation (126) seem simple enough, but several possible mechanisms can lead to the 3-substituted products . Other workers regard the reaction of the nitrone (or azomethine oxide) with alkyne as simple cycloadditions - which yield 2,3-dihydro-l,2-oxazoles since these are often unstable, only decomposition products may be found (equation 127). The construction of the indolizine skeleton initiated by a similar process has been reviewed (equation 128). ... [Pg.354]

Indolizines have been made via carbene addition to 2-vinylpyridine or dipolar addition of pyridinium ylides. 3-substituted indolizines have been obtained in a one step procedure from reaction of chlorocarbenes with 2-vinylpyridine (94CC509). Microwave irradiation has been used in 1,3-dipolar cycloaddition of pyridinium dicyanomethylide with alkynes <94H(38)785>. [Pg.217]

Acceptor-CH-substituted pyridinium-N-betaines 15 (accessible in situ by deprotonation of the corresponding N-alkylpyridinium salts 14) undergo 1,3-dipolar cycloaddition with activated alkynes and alkenes as dipolarophiles. With alkynes, the cycloadducts (16/19) dehydrogenate spontaneously to indolizines, which are either of the 1,2,3-trisubstituted type 17 or (indicating a regioselective cycloaddition) of the 1,3-disubstituted type 18. With olefinic substrates, the presence of an oxidant for additional dehydrogenation of the primary cycloadduct (20 -> 17) is required [219] ... [Pg.154]

The first example of a stable pyridinium yhde of type 80 is the pyridinium phenacylide (80, A = CH2COPh), obtained from the N-phenacylpyridinium ion (79, A = CH2COPh) by deprotonation with Na2C03 [97]- The reactivity of the pyridinium betaines is determined by their electron distribution (80a-c). Thus, they can be smoothly alkylated or acylated at the N-substituent ( 81) as 1,3-dipoles, they undergo dipolar cycloadditions with activated alkynes or alkenes [98] for example, the sequence 80 —> 82 83 establishes an efficient principle of indolizine synthesis (cf. p. 154). [Pg.360]

The utilization of solid-support pyridinium salts in the synthesis of bicyclic pyridines has been reported. Yue et al. synthesized 1,2,3,7-tetrasubstituted indolizines using poly(ethyleneglycol)bound pyridinium salts <06JHC781>. The PEG-bound pyridinium salts 53 were reacted with alkenes or alkynes in the presence of Et3N, via 1,3-dipolar cycloaddition, to give polymer-bound indolizines 54 and 55, respectively. Liberation of the heterocycle with KCN/MeOH afforded 1,2,3,7-tetrasubstituted indolizines 56 and 57 in good to excellent yield. [Pg.322]

Reactions of indolizines with electron deficient alkenes and alkynes were studied. Cycloaddition reactions with dimethyl alkynedicarboxylate yield cycl(2.2.3)azine derivatives (22)-(24), adducts (25), and (2.3.4)cyclazines (26) <84H(22)705>. [Pg.241]

Cycloaddition of 4,4 -bipyridinium ylides 218, generated in situ from 4,4 -bipyridine and substituted phenacyl bromides 216 via 4,4 -bipyridinium diquatemary salts 217, with activated alkynes has been achieved under solvent-free conditions and in the presence of basic catalyst KF on alumina within 7-10 min by MWI to give 7,7 -feir-indolizines 220 in 81-93% yields via intermediate 219 (Scheme 46). On heating in benzene (or in benzene and A-methylpyrrolidone (NMP)), the reaction times were longer (3 h) and the yields were lower (50-59%) (00SL1013). [Pg.31]


See other pages where Indolizine, 18+2 -cycloaddition with alkyn is mentioned: [Pg.122]    [Pg.670]    [Pg.60]    [Pg.121]    [Pg.206]    [Pg.376]    [Pg.370]    [Pg.211]    [Pg.892]    [Pg.169]    [Pg.245]    [Pg.396]    [Pg.399]    [Pg.27]   
See also in sourсe #XX -- [ Pg.1175 ]




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Alkynes 2+2]-cycloadditions

Alkynes cycloaddition

Alkynes cycloaddition with

Cycloaddition with

Indolizine

Indolizines

Indolizines. cycloaddition

With alkynes

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