Indolizine, structure

Tri(indolizinyl)methanes (16) and (17) were prepared from HC(OEt)3 and indolizines. Structure (17) is a three-bladed propeller lacking local Cj-axes. Compounds of this type are chiral and eight stereomeric conformations are possible <87CB(120)239>. 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

Indolizine, 3-benzoyl-6-bromo- l-(2-pyridyl)-structure, 4, 445 In4olizine, 3-chloro-synthesis, 4, 470 Indolizine, 3-cyano-cyclazine synthesis from, 4, 460 Indolizine, 8-cyano-synthesis, 4, 469... 

The X-ray analysis of compound 2 (Figure 2) was used to confirm its structure and revealed that only the carbomethoxy group at C-l lies on the plane of indolizine nucleus as already evidenced for other 1,2,3-trisubstituted indolizidines <2000CHE1192>. 

The dipeptide isosteres with indolizin-3-one structure 271 were also synthesized using similar strategies <1998JOC7463>. 

The crystal structure of dimethyl indolizine-l,6-dicarboxylate shows that the indolizine unit is almost planar. The two carboxylate groups are coplanar with the indolizine moiety <2006AX(E)4080>. 

Robinson and Sugasawa (8) selected dibenzo[b,g]pyrrocoline for the parent ring system, and this name was adopted for the alkaloid family. The numbering is shown on structure 3 although Robinson used a different pattern. The systematic name for the ring system is indolo[2,l-a]isoquinoline, but di-benzo[ib,g]indolizine also appears in the literature. 

The HMO resonance energy per electron (REPE value5,48) using an appropriate reference structure was found to be 0.040 / (cf. pyrrole 0.039 / , indolizine 0.027 /3).s... 

There is no structural information from diffraction data on simple isoindoles, the majority of studies on the ring system pertaining to phthalimide and its derivatives (72AX(B)415). Until recently the situation was almost as bad for the indolizine system, with structures for... 

Quantum chemical calculations of indolizine and its aza derivatives have been reviewed (77HC(30)117). Attempts to correlate the results with various physical and chemical properties have not always been crowned with success. (A critical discussion of semi-empirical molecular orbital methods is given in Section 3.01.7.) A qualitative discussion using single structures may be appropriate. 

X-Ray data on the crystal structure of the indolizine (7) and the [2.3.4]cyclazine (8) showed that the indolizine ring system is nearly planar. Bond distances of (7) and (8) are given in Table 1. 

The indolizine nucleus is readily oxidized. No N- oxides have been isolated so far. In the course of many oxidations, ring fission occurs. The reaction has therefore been used in the past for structural elucidation (B-66MI30800). Thus l-(4-nitrophenyl)indolizine on treatment with perhydrol and acetic acid gave picolinic acid N- oxide and 4-nitrobenzoic acid (Scheme 7). 

Substituted vinylindolizines sometimes polymerize spontaneously and also copolymerize with styrene. Polymers of similar structure have been prepared by formation of indolizines on the polymer chain using methods outlined in Sections 3.08.3 and 3.08.6 (see Scheme 36 for an example). Moreover, indolizine dyes such as (214) have been bound to an ethyl acrylate/acrylic acid copolymer by heating to give dyes that do not migrate in photographic colour film emulsions. 

The indolizines constitute the core structure of many naturally occurring alkaloids, such as (-)-slaframine, (-)- dendroprimine, indalozin 167B and coniceine. There are a number of different routes to the synthesis of indolizines and they are most commonly synthesised by sequential N-quaternisation, intramolecular cyclocondensation reactions or the cycloaddition reaction of /V-acyl/alkyl pyridinium salts. 

Attempts to confirm the structure of 32 by conversion into the methyl indolizine and comparison with an authentic sample were inconclusive. However, rearrangement of 27 with acetic acid or phenol gave an indolizine triester formulated as 33 which could be converted into 34. This last product had markedly different physical properties from 32 and its decarboxylation product was identical with 3-methylindolizine [Eq. (9)]. 

Some indolizines (90-92) have been prepared because of their structural similarity to naturally occurring indoles tryptophan, gramine, and heterauxin, respectively. 

In another example of oxidation without ring fission, the major product obtained from the reaction between methyl 2-pyridylacetate and ethyl-a-bromopropionate was not the expected indolizine 151, but a compound whose chemical and physical properties suggested the structure 152 (Scheme 20). When the reaction was repeated under... 

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