The 1,3-dipolar cycloaddition reaction of several unsymmetrically substituted munchnones and sydnones with methyl propicíate has been examined. The initially formed cycloadducts readily extrude carbon dioxide to produce five-membered heteroaromatic ring compounds. The reaction of sydnones with methyl propicíate produced a mixture of regioisomeric pyrazoles. The analogous [3 + 2] cycloaddition reaction of munchnones with methyl propicíate proceeds with formation of mixtures of both possible regioisomeric pyrroles. The structural assignment of the isolated adducts is based on spectroscopic data. The distribution of products depends on the nature and location of the substituent groups present on the heterocyclic ring. The observed regioselectivity is discussed on the basis of MO-perturbation theory.During the last decade a new impulse has been given to research in the field of heterocyclic chemistry when it was found that various mesoionic compounds undergo 1,3-dipolar cycloaddition with different dipolarophiles.1'19 Of the known mesoionic heterocycles, the structure, physical properties, and reactions of sydnones have drawn the closest scrutiny.20 These mesoionic compounds can be readily prepared by cyclodehydration of N-nitroso-a-alkyl amino acids (1) with reagents such as acetic anhydride.The resulting compounds contain a mesoionic aromatic system (2) which can only be depicted with polar resonance structures.21 Sydnones undergo smooth cycloaddition with acetylenes to give pyrazoles (4) in high yield.22 *'24The reaction involves a 1,3-dipolar cycloaddition of the sydnones, behaving like a cyclic azomethine imine, to the corresponding acetylene followed by carbon dioxide evolution and aromatization. 4Huisgen and co-workers have also described the cycloaddition behavior of the "munchnones", unstable mesoionic A2-oxazolium 5-oxides of type 6 with azomethine ylide structures.26"29 Their reactions closely parallel those of the related sydnones. The reaction of munchnones with acetylenic dipolarophiles constitutes a pyrrole synthesis of broad scope.30"36 1,3-Dipolar cycloaddition of acetylene to the A2
The Lewis; acid catalyzed reaction of methyl chloropropiolate or dimethyl acetylenedicarboxylate with alkenes leads to ene reactions and/or stereospecific [2 + 21 cycloaddition. Ethylaluminum dichloride was found to be a very effective catalyst since it is a strong Lewis acid and an HCl scavenger. With trisubstituted alkenes the ene reaction is regiospecific. A hydrogen is transferred from the alkyl group trans to the alkenyl hydrogen. The regio-and stereoselectivity of the ene reactions of methyl chloropropiolate, methyl propiolate, and dimethyl acetylenedicarboxylate are described. From cyclohexenes, pseudoaxial hydrogens are transferred exclusively.The relative reactivity of alkenes was found to be 1,l-disubstituted > trisubstituted >> monosubstituted and 1,2-disubstituted. The ene adducts and cyclobutenecarboxylates derived from methyl chloropropiolate undergo substitution reactions with organocuprates and can be hydrolyzed to 0-keto esters and substituted dimethyl glutarates, respectively. (5) Viehe, H. G. (12) Aluminum chloride catalyzed Diels-Alder reactions of methyl acrylate are second order: Inukai, T.; Kojima, T. J. Org. Chem. 1967,32, (13) Puchkova, V. V.; Gur'yanova, E. N.; Laba, V. I.; Prilezhaeva, E.
Ethynyl ,D-tolyl sulfone undergoes EtAlC12-catalyzed ene reactions with alkenes to give 1,4dienyl p-tolyl sulfones in good yield.We have recently described the Lewis acid catalyzed ene and [2 + 21 cycloaddition reactions of ethynyl esters and ketones with alkene^.^^^ Depending on the substitution pattern of the alkene, good yields of ene adducts and/or cyclobutenes are obtained at 25 "C. The adducts which result from these reactions have been used for the syntheses of the A1 component of the female sex pheromone of the California red scale4 and of novel insideinside bicyclic ~ompounds.~ We report here tbe analogous reactions of ethynyl sulfones with alkenes (Scheme I). The reaction introduces a functionalized two-carbon fragment, a vinyl sulfone, which is a versatile synthon.6 The results of EtA1C12-catalyzed reactions of ethynyl p-tolyl sulfone (1) with alkenes are shown in ' Table I. These reactions proceed at 25 OC in benzene or other aromatic solvents. The reaction proceeds best for highly substituted, i.e., electron rich, alkenes. Good yields of ene adducts are obtained from all alkenes with at least one disubstituted carbon. Monosubstituted alkenes give low yields of ene adducts while l12-disubstituted alkenes give low yields of a mixture of ene adduct and cyclobutene. A similar propensity for l12-disubstituted alkenes to give cyclobutenes was observed with methyl propiol.ate2 and 3-butynoneS3This reaction is very susceptible to changes in conditions. Use of CH2C12 as solvent or AlC13 as Lewis acid results in complex mixtures of products. The stabilization of the 1.EtAlC12 complex by aromatic solvents and the ability of EtAlC12 to act as a proton scavenger minimize the side reactions. However,, even under optimal conditions, methylenecyclohexane is slowly isomerized to l-methylcyclohexene so that inixtures of 5,6, and 7 are formed. In all cases just under 1 equiv of EtA1Cl2 is used since the Table I. Adducts from the EtAlC1,-Catalyzed Reactions of Ethynyl p-Tolyl Sulfone (1 ) with Alkenes 0 9 ( 1 l ) b 8 (20)b (1) Fellow of the Alfred P. Sloan Foundation, 1979-1981. (2) Snider, B. B.; Rodini, D. J.; Conn, R. S. E.; Sealfon, S. J. Am. Chem. SOC. 1979,101,5283. Snider, B. B.; Roush, D. M.; Rodini, D. J.; Gonzalez, D.; Spindell, D.Consists of 80% 7 and 20% of a 3 : l mixture of 5 and 6.obtained. X = p-tolylsulfonyl.product akenyl sulfone is more basic than 1 and complexes preferentially to EtAlCl,.' A 19% yield of 14 and cyclohexyl benzene were also
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