Tandem Dipolar Cycloaddition-Mannich Cyclization as an Approach to Tricyclic Nitrogen Heterocycles

Arylation of enelactams using TIPSOTf: reaction scope and mechanistic insight

Triisopropylsilyltrifluoromethanesulfonate can be effectively used for the arylation of a wide range of enelactams. The multinuclear NMR study provided deep insights into the reaction mechanism.

Enantioselective Total Synthesis of Cannabinoids-A Route for Analogue Development

A practical synthetic approach to Δ⁹-tetrahydrocannabinol (1) and cannabidiol (2) that provides scalable access to these natural products and should enable the generation of novel synthetic analogues is reported.

Vinylogous reactivity of enol diazoacetates with donor-acceptor substituted hydrazones. Synthesis of substituted pyrazole derivatives

A regiospecific synthesis of multifunctional pyrazoleshas been developed from a cascade process triggered by Rh(II)-catalyzed dinitrogen extrusion from enol diazoacetates with vinylogous nucleophilic addition followed by Lewis acid catalyzed cyclization and aromatization.

Domino reactions of rhodium(II) carbenoids for alkaloid synthesis

In this tutorial review, the rhodium(II)-catalyzed domino reactions of alpha-diazo carbonyl compounds are summarized. The article will emphasize some of the more recent synthetic applications of the rhodium carbenoid cyclization/cycloaddition domino cascade for alkaloid synthesis. The many structurally diverse and highly successful examples of both oxa and azapolycyclic ring formation cited in this tutorial review clearly demonstrate that the domino cyclization/cycloaddition cascade of metallo carbenoids has evolved as an important strategy for the synthesis of nitrogen containing natural products.

The Domino Way to Heterocycles

Sequential transformations enable the facile synthesis of complex target molecules from simple building blocks in a single preparative step. Their value is amplified if they also create multiple stereogenic centers. In the ongoing search for new domino processes, emphasis is usually placed on sequential reactions which occur cleanly and without forming by-products. As a prerequisite for an ideally proceeding one-pot sequential transformation, the reactivity pattern of all participating components has to be such that each building block gets involved in a reaction only when it is supposed to do so. The development of sequences that combine transformations of fundamentally different mechanisms broadens the scope of such procedures in synthetic chemistry. This mini review contains a representative sampling from the last 15 years on the kinds of reactions that have been sequenced into cascades to produce heterocyclic molecules.

2-Diazoacetoacetic acid, an efficient and convenient reagent for the synthesis of alpha-diazo-beta-ketoesters

The formation of various alpha-diazo acetoacetic esters can be obtained in a single transformation with good to excellent yields using readily available 2-diazoacetoacetic acid.

Reaction of vinylcarbenoids with benzaldehydes: formation of vinylcarbonyl ylides followed by ring closure to oxiranes and dihydrofurans

Rh(2)(OAc)(4)-catalyzed reaction of vinyldiazo compound 1a in the presence of p-methoxybenz- 2a, mesit- 2b, and p-chlorobenzaldehyde 2c gave a mixture of isomeric vinyloxiranes 3a-c and 4a-c, and sterically unstable (E)-dihydrofurans 5a-c, but not stable (Z)-dihydrofurans 6. However, the reactions with p-nitro- 2d and 2,4-dinitrobenzaldehyde 2e gave (Z)-dihydrofurans 6d,e along with 3d, 4d, and 5d,e. The reaction in the presence of maleic anhydride and dimethyl fumarate gave single 1,3-dipolar cycloadducts 11 and 13, respectively, indicating that a single conformer, the sterically unstable endo-aryl-endo-cyanostyryl carbonyl ylide 14 (15), is initially formed in the reaction of 1 with 2. It was concluded that the endo-vinyl-exo-aryl vinylcarbonyl ylides 17a-c arising from 2a-c undergo disrotatory cylization to exclusively produce 5, whereas the ylides 17d,e arising from 2d,e undergo partly symmetry-forbidden conrotaory cyclization to sterically stable trans-diaryldihydrofurans 6d,e as well as the symmetry-allowed process to 5d,e.

Cycloaddition-Rearrangement Sequence of 2-Amido Substituted Furans as a Method of Synthesizing Hexahydroindolinones

A convenient synthesis of various substituted hexahydroindolinones has been achieved by an intramolecular Diels-Alder cycloaddition (IMDAF) reaction of 2-amido substituted furans. The initially formed [4 + 2] cycloadduct undergoes nitrogen-assisted ring opening followed by deprotonation of the resulting zwitterion to give the rearranged ketone. The stereochemical outcome of the IMDAF cycloaddition has the sidearm of the tethered alkenyl group oriented syn with respect to the oxygen bridge. The reaction rate and product yield were found to be markedly dependent upon the electronic properties of the alkenyl pi-bond. 2-[2-(tert-Butoxycarbonylfuran-2-yl-amino)ethyl]acrylic acid methyl ester was synthesized from 3-chlorocarbonyl-but-3-enoic acid methyl ester. Thermolysis of the carbomethoxy activated furanamide occurred at 80 degrees C to produce a rearranged hexahydroindolinone. When Me(3)Al or (MeO)(3)Al was used as a Lewis acid to promote the cycloaddition, a rearranged alcohol was obtained. The initially formed [4 + 2] cycloadduct undergoes ring opening in the presence of the Lewis acid, and the resulting aluminum intermediate delivers the substituent group from the same face as the neighboring oxygen to ultimately furnish a rearranged cis-alcohol. In contrast to this result, a mixture of diastereomeric methoxy alcohols was isolated when the IMDAF cycloaddition was carried out in methanol. The major isomer corresponds to the trans-diastereomer that results from trapping of the iminium ion from the less crowded face of the pi-bond.

Utilization of the Intramolecular Cycloaddition-Cationic pi-Cyclization of an Isomünchnone Derivative for the Synthesis of (+/-)-Lycopodine

A new annulation sequence leading to the tetracyclic skeleton of the Lycopodium family of alkaloids is effected by using the tandem cycloaddition-cationic pi-cyclization reaction of an isomünchnone dipole as the key strategic element. Synthesis of the required alpha-diazo imide precursor involved treating 5-methylcyclohex-2-en-1-one with the organocopper reagent derived from 3-methoxybenzyl chloride in the presence of chlorotrimethylsilane. Ozonolysis of the resulting silyl enol ether followed by a Wittig reaction and conversion to the desired alpha-diazo imide was carried out using standard malonylacylation and diazotization procedures. Treatment of the alpha-diazo imide with rhodium(II) perfluorobutyrate afforded a transient 1,3-dipole which subsequently cycloadded across the tethered pi-bond. The resulting cycloadduct was treated with BF(3).2AcOH to give a rearranged tetracyclic compound derived from a Pictet-Spengler-type cyclization of an N-acyliminium ion. The rearranged product was subsequently converted into a key intermediate previously used for the synthesis of (+/-)-lycopodine.