Indolizine ring opening

Palladium-carbon/oxygen Indolizine ring opening... 

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

The only reaction reported between 3//-pyrrolizine (1) and a carbene (or carbenoid) is that involving n-butyllithium and dichloromethane.135 One of the products is formed by cycloaddition (Section III,B,6), one by ring opening (Section III,B,7), and the third according to Eq. (4). The latter two products are viewed as derived from the carbenoid 272, and the proposed route to indolizine (273) is shown in Eq. (4). 

An unusual synthesis of ( )-septicine (866) from dimethyl squarate (878) exploits rearrangement of a 4-(l-pyrrolo)cyclobutenone 879 to the indolizine-5,8-dione 880, presumably via a ketene intermediate formed by electrocyclic ring opening of 879 (Scheme 113) (584). Partial reduction of 880 yielded the 4-hydroxypyridone 881, the triflate ester of which was deoxygenated with a palladium(II)-formic acid system to yield the l,2,3,5-tetrahyi oindolizin-5-one 882. Mild reduction with aluminum hydride completed the synthesis of ( )-866 in an excellent overall yield of 27% from 878. 

Six articles cover general aspects of heterocyclic chemistry 1,3-dipolar cycloreversions, syntheses with arylnitrenes and a-metallated isocyanides, and photo-oxygenation of nitrogen heterocycles,while others deal with more specialized subjects, i.e. preparation and use of halogeno-lactones, aspects of the chemistry of furan, 1-hydroxy-indoles, ring-opening of azoles by the action of amines, " the use of 2-chlorobenzoxazolium (1) and other heterocyclic onium salts for dehydration and condensation reactions," the synthesis of monosub-stituted tetrathiafulvalenes (2), cycloadditions of azoles containing three heteroatoms,sydnone imines (3), the conversion of acyl-benzofuroxans into nitro-indazoles (cf. p. 199), and advances in the chemistry of pyrrolizidine " and indolizine." ... 

The same principle of sequential cyclopentene-opening RCM resulting in the formation of a dihydropyrrole ring was the key step in Blechert s novel approach to the polyhydroxylated indolizine alkaloid (-)-swainsonine (378) via RRM of 375 (Scheme 73) [157]. 

Cyclic amino acids 139, when heated in acetic anhydride, probably form initially mesoionic oxazolium 5-oxides (munchnones) subsequent 1,3-dip olar cycloaddition of 1,2-dicyanocyclobutene, loss of carbon dioxide, and opening of the cyclobutane ring lead to dinitriles 140 (80JHC1593). Pyridone 141 is the by-product (together with an indolizine) of the mono-cyclic pyridone dicarboxylate and acrylic ester (73JHC77). 

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