Dihydroindolizines

A useful method for the synthesis of 1-unsubstituted 2-arylindolizines is provided by the 1,5-dipolar cyclization of pyridinium ylides derived from 181, in the presence of the oxidant tetrakis(pyridine)cobalt(ll) dichromate (TPCD), which oxidizes the intermediate dihydroindolizine 182 (Scheme 43). The study demonstrated that the presence of an aryl group on the double bond was necessary for the reaction to occur. 

Dihydrogenated tallowalkylmethylamines, melting point, 2 521t Dihydrogen phosphate, 24 43 Dihydroindolizines, photochromic materials, 6 600... 

In a similar fashion, hydroformylation of N-allyl-pyrrols leads to 5,6-dihydroindolizines via a one-pot hydroformylation/cyclization/dehydration process (Scheme 27) [81,82]. The cyclization step represents an intramolecular electrophilic aromatic substitution in a-position of the pyrrole ring. This procedure was expanded to various substrates bearing substituents in the al-lyl and in the pyrrole unit. 

General Procedure for the Hydroformylation/Electrophilic Substitution. Synthesis of 5,6-dihydroindolizines. A solution of 1-allylpyrroles (leq) and Rh4(CO)i2 (lmol%) in toluene was introduced by suction into an evacuated stainless-steel reaction vessel. CO (60 bar) was introduced, the autoclave was then rocked, heated to the desired temperature and H2 (60 bar) was introduced rapidly. When the gas absorption reached the value corresponding to the fixed conversion, the reaction mixture was siphoned out. The degree of conversion and the product distributions were determined by GC and GC-MS, by using acetophenone as an internal standard. 

Scheme 27 Synthesis of dihydroindolizines via hydroformylation/aromatic substitution... 

Photochromic systems that have been examined in both of these approaches include spiropyrans, spirooxazines, diarylethenes, dihydroindolizines and azoben-zenes. A schematic of a disk structure is shown in Figure 1.14. To produce the... 

Treatment of allylpyridinium salts with weak bases gave ylides (152) an intramolecular 1,5-cyclization then yielded dihydroindolizines (153), which are prone to oxidation to (154). More than 90% of (153) is formed if R1 is phenyl and R2 is benzoyl or acetyl. If R is H or p-anisyl the indolizine (154) is the major product. 1,5-Cyclizations of ylides of the type (152) with a different substitution pattern have been investigated. This type of reaction seems to be somewhat unpredictable since the 1,5-cyclization is accompanied by 1,3-dipolar cycloadditions (Section 3.08.6.2.2) to a varying extent (75JCS(Pl)575). 

Recently, the reaction of DMAD with pyridinium arylsulfonyl methylides has been used as a simple route to indolizines.63 The intermediate dihydroindolizines are readily aromatized by 1,4-elimination of p-toluenesulfinic acid. 

Esters of various substituted acrylic acids have also been used, giving, on reaction with pyridinium methoxycarbonyl methylides, various 2,3-dihydroindolizine-l,3-dicarboxylates. These were stable and not... 

A novel reaction of nitro-substituted 2,3-dihydroindolizines (113) was the claimed conversion into substituted indoles on refluxing with base in an organic solvent.181... 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

Fig. 7 Light-driven switch represented by dihydroindolizine 12, betaine 13, and protonated betaine 14 (A = thermal activation).

Chapter 6 (Photochromism of dihydroindolizines and related systems). The dihydro-, tetrahydro- and hexahydroindolizine compounds involving one, two or three nitrogen atoms make up another family of compounds that exhibits a generally thermoreversible photochromic system based on 1,5-electrocyclization reactions (Scheme 6). Photobleaching is sometimes superimposed on the thermal back reaction. 

The photochromism of dihydroindolizines (DHIs) - a new class of photo-chromic molecules - was discovered only in 1979.1,2 These molecules are among the few new photochromes discovered in the past 20 years.3 Their synthetic access has been exploited in detail and their photochemical and photophysical parameters have been studied in a broad sense.3 7 The potential applications are vast and compare favorably with those of the known photochromes.8 The chemistry and photochemistry of the DHIs have been described previously in several surveys.3-7 This review will serve as a short introduction to the field. It will as well include typical preparation modes for the class of dihydroindolizines as well as their aza-homologs, typical five-membered heterocycles. A new class of photochromies derived from these photochromic heterocycles (DHIs) that allows for supramolecular interactions is presented. Some typical applications that are close to commercially useful systems are discussed. 

The characteristic structural feature of the dihydroindolizines-based molecules is a five-membered ring, typically a cyclopentene anion. The substitution of the carbanion in this molecule by a heteroatom generates heterocyclic five-membered rings. 9-15 These can undergo a ring opening to zwitterionic species or neutral heteropentadienes. The process can be induced photochemically and is reversed either thermally or photochemically. Thus a new photochromic system based on a five-atom, six-electron (4n + 2)k system is created. This system can in principle be... 

Reactions of the easily obtainable spirocyclopropenes 10 with aza-hetero-cycles or azines such as pyridines, pyridazines, quinolines, isoquinolines, azaphe-nanthrenes, and others afford spiro (1,8a) dihydroindolizine 7 in good yields (50-84%) (Scheme 3). The intermediate in this synthesis is the colored betaine 9. Typical dihydroindolizines have been made via this route.1 7 16-18... 

The UV spectra of spiro[l,8a]dihydroindolizine 7 show two typical maxima, one at 240-250 and the other at 360-410 nm. The colored form (betaine) 9, absorbs in the range of 500-700 nm. Similar absorption ranges are observed for the... 

Table 6.2.A Uv-visible data of selected Spiro[l,8a]dihydroindolizines (DHI) 7 prepared by cyclopropene route (a) or pyrazole route (b). Solvent CH2C12/Ether and half life (T ) at...

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