The copper(I)-catalyzed regioselective [3 + 2] cyclization of pyridines toward alkenyldiazoacetates leading to functionalized indolizine derivatives is reported. A broad range of pyridine derivatives (including quinoline and isoquinoline) is compatible with this cyclization reaction. The process represents the first successful example of metal-catalyzed cyclization of a π-deficient heterocyclic system with alkenyldiazo compounds.

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Polysubstituted furan derivatives are regioselective obtained from (bis-alkynyl)methyl carboxylates in the presence of catalytic amounts of copper(I) salts. This multistep process is consistent with the intermediacy of a copper(I) (2-furyl)carbene complex which is intercepted by suitable trapping reagents.

In the last few years the role of diazo compounds in organic synthesis, particularly in cyclization reactions via metal carbenoids (metal: ruthenium, copper, rhodium, etc.) has been prominent. [1] By taking advantage of the ease with which these metal carbenes collapse into the corresponding symmetrical alkenes (homocoupling), we and others have been able to access nonsymmetrical alkenes by the selective heterocoupling of diazoacetate esters with copper(I) [2] and ruthenium(II) [3] carbene complexes [Eq. (1)]. Interestingly, the metal-catalyzed selective cross-coupling reaction between two different diazo substrates has been reported recently [Eq. (2)]. [4] At this point, we became intrigued as to whether discrimination between appropriately selected diazo compounds, for instance simple diazo and vinyldiazo systems, could be attained. Firstly, we found that [Cu(MeCN) 4 ]BF 4 catalyzes the reaction between ethyl vinyldiazoacetate and ethyl diazoacetate at room temperature. Surprisingly, the expected cross-coupling conjugated diene was not formed, but diethyl 2-methylcyclobutene-1,3-dicarboxylate was obtained in moderate yield as the sole heterocoupling product [Eq. (3)].This initial result seems to encompass, among others, two relevant features: 1) it represents a novel [3+1] coupling between diazo compounds, wherein two CÀC bonds rather than one C=C bond are formed, and 2) it might provide direct access to a relevant, uncommon structure. Therefore, herein we report our preliminary studies on the synthesis of cyclobutene structures by copper(I)-catalyzed cyclization of vinyldiazoacetate esters and diazo compounds (Table 1).First, [Cu(MeCN) 4 ]BF 4 (5 mol %) was added to a solution of ethyl 2-diazo-3-methylbut-3-enoate 1 a (R 1 = Me) and ethyl diazoacetate 2 a (R 2 = CO 2 Et, R 3 = H) in CH 2 Cl 2 at room temperature. After stirring the reaction mixture for two hours the solvent was removed and the resulting mixture was subjected to column chromatography, affording the cyclobutene 3 aa in 33 % yield along with ethyl maleate and ethyl fumarate (entry 1). The yield could be increased to 45 % using tert-butyl diazoacetate 2 b (entry 2). The cyclization of 1 a with