Pyridine, dihydro, reaction with alkenes

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

Additional general reactions are the synthesis of 11 -acyldipyridoimidazolium salts such as (174 R = Ac, Bz) by condensation of l-acylmethyl-2-chloropyridinium salts with pyridine and substituted pyridines under mild conditions <76CB3646, 77LA1692), and a 1,3-dipolar cycloaddition of a nitrone (196) with heterocyclic alkenes. A derivative (198) of the parent system (35) has been obtained by the reaction of (196) with 5,6-dihydro-2-pyranone (197) (Equation (10)) <93T3857>. A similar reaction occurs with 2,3-dihydropyran yields (172), a perhydro derivative of the parent system (34) <82JOC230, 90JCS(P1)2593>. 

Bennasar extended his research on 2- and 3-indolylacyl radicals to intramolecular cyclizations to yield 2,3-fused indoles [112], Under nomeductive conditions (n-Bu6Sn2, hv), radical 201 underwent a cascade addition-oxidative cyclization sequence with a number of alkene acceptors including dimethyl fumarate (45%), methyl 1-cyclohexenecarboxylate (53%), methyl crotonate (71%), vinyl sulfone (22%), and the a,p-unsaturated lactam ester, 2-oxo-5,6-dihydro-2H-pyridine-l,3-dicarboxylic acid dibenzyl ester (41%) to form cyclopenta[h]indol-3-ones 202. Reaction of 201 with acrylonitrile and methyl acrylate, however, generated cyclo-hepta[h]indoles, the products of bis-addition-cyclization sequences. 

As a general principle, dihydro- and tetrahydropyridines can be converted to pyridines by dehydrogenation and elimination reactions. Considering the retrosynthetic operations b and d-f the hydropyridines 144-146 should be obtained from (4 + 2)-cycloadditions ofazadienes with activated alkynes or alkenes, and likewise from 1,3-dienes with imines. 

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