The 1, 3-dipolar cycloaddition of trimethylsiyl diazomethane with α, β-unsaturated esters was examined. The resulting 1-pyrazolines isomerize to regioisomeric 2-pyrazolines (a or b) or undergo desilylation (c). Acrylates yield only b or c. β-substituted dipolarophiles may yield all three types of products. This work demonstrates that the distribution of 2-pyrazoline products is highly dependant on the relative configuration of the substituents on the 1-pyrazoline intermediate.Trimethylsilyldiazomethane has been most frequently used as a source of carbene for cyclopropanation reactions 1 and has enjoyed only limited synthetic utility in 1, 3-dipolar cycloadditions. In fact, Seyferth reported that among a series of dipolarophiles examined, only acrylonitrile reacted with TMS diazomethane to produce a cycloadduct in a synthetically useful yield. 2, 3 Recently, with its commercial availability, TMS diazomethane has become somewhat more popular as a synthetic reagent, particularly in the preparation of novel amino acid analogs. 4,5 1, 3-Dipolar cycloadditions of diazoalkanes and alkenes yield 1-pyrazolines. In most instances, when the dipolarophile is an α,β-unsaturated ester, HOMO-LUMO interactions dictate that the regioselectivity is such that the carbon atom of the diazoalkane attacks the β-carbon of the ester (Scheme 1). 1-Pyrazolines tend to be unstable and isomerize to the 2-pyrazolines with the regioselectivity of the isomerization dependent on the substituents. When the dipolarophile is an α,β-unsaturated carbonyl compound, isomerization typically yields the conjugated 2-pyrazoline. 6, 7A survey of the recent literature indicates that the regioselectivity of the double bond isomerization of TMS-substituted 1-pyrazolines does not always follow the typical course and is, at first glance, unpredictable. Carreira and Kanemasa have reported the 1, 3-dipolar cycloaddition of TMSCHN 2 with camphorsultam and oxazolidinone derivatives, respectively. 4, 5, 8, 9 . These 1-pyrazoline products undergo proteodesilylation to yield 2-pyrazolines (1 and 2) or loss of the proton α to TMS (3). On the other hand, Barluenga et al. 10 recently described the cycloaddition of the menthol ester of trans-cinnamic acid to produce the conjugated 2-pyrazoline, with retention of the TMS group (4).reink@fiu.edu. † Permanent address; Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. In an effort to develop a mechanistic rational for the cycloaddition/ isomerization process, which could account for the products obtained, we undertook a systematic survey of 1, 3-dipolar cycloadditions between TMS diazomethane and α, β-unsaturated esters. The dipolarophiles vary in terms of the size of the ester group and the substituents at the β-carbon. The results of these studies are summarized in Table 1. Because 1 and 2-pyrazolines oxidize readily to pyrazoles, the cycloaddition products were immediately converted to the Cbz (benzyloxycarbonyl) derivatives of the 2-pyrazolines for characterization. However, we were able to ob...

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Two syntheses for the production of an unsubstituted azakainoid are described. The 1,3-dipolar cycloaddition of diazomethane with trans-dibenzyl glutaconate yields a 1-pyrazoline, which may be reduced directly to the pyrazolidine. An unexpected trans-cis isomerization is observed during Hg/Al reduction of the 1-pyrazoline N=N bond. Alternatively, when TMS diazomethane is used as the dipole, the resulting 2-pyrazoline obtained after desilylation may be reduced with NaCNBH3 to provide the trans azakainate analog exclusively. The synthesis of an unsubstituted isokainoid via Michael addition is also described. Glutamate receptor binding assays revealed that the azakaniod has a moderate affinity for unspecified glutamate receptors. Membrane depolarization of Aplysia neurons upon application of the azakainoid demonstrates that it is an ionotropic glutamate receptor agonist.

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