Tandem double intramolecular [4 + 2]/[3 + 2] cycloadditions of nitroalkenes

Access to Spirooxindole-Fused Cyclopentanes via a Stereoselective Organocascade Reaction Using Bifunctional Catalysis

The present study reports an asymmetric organocascade reaction of oxindole-derived alkenes with 3-bromo-1-nitropropane efficiently catalyzed by the bifunctional catalyst. Spirooxindole-fused cyclopentanes were produced in moderate-to-good isolated yields (15-69%) with excellent stereochemical outcomes. The synthetic utility of the protocol was exemplified on a set of additional transformations of the corresponding spirooxindole compounds.

The oxazolomycin family: a review of current knowledge

Oxazolomycin A and neooxazolomycin were firstly isolated in 1985 by the group of Uemura et al. from the Streptomyces sp. bacteria.

Synthesis of PDE IVb Inhibitors. 3. Synthesis of (+)-, (-)-, and (±)-7-[3-(cyclopentyloxy)-4-methoxyphenyl]hexahydro-3H-pyrrolizin-3-one via reductive domino transformations of 3-β-carbomethoxyethyl-substituted six-membered cyclic nitronates

Simple three-step asymmetric and racemic syntheses of GlaxoSmithKline's highly potent PDE IVb inhibitor 1 were developed. The suggested approach is based on reductive domino transformations of 3-β-carbomethoxyethyl-substituted six-membered cyclic nitronates, which are easily accessed by a stereoselective [4 + 2] cycloaddition of an appropriate nitroalkene to vinyl ethers. In vitro studies of PDE IVb inhibition by enantiomeric pyrrolizidinones (+)-1 and (-)-1 were performed.

Tandem double intramolecular [4+2]/[3+2] cycloadditions of nitroalkenes: construction of the pentacyclic core structure of daphnilactone B

An asymmetric synthesis of the ABCD ring system of daphnilactone B is described. The synthesis features a tandem, double intramolecular, [4+2]/[3+2] cycloaddition of a highly functionalized, enantiomerically enriched nitroalkene to generate a pentacyclic nitroso acetal. The cycloaddition establishes six contiguous stereogenic centers including the critical CD ring junction that bears two quaternary stereogenic centers. Hydrogenolysis of the nitroso acetal followed by amide reduction and cyclization provided the AB rings. The methyl substituent on the A ring was installed in the correct configuration via hydrogenation of an exocyclic olefin in the final step.

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.

Tandem double intramolecular [4+2]/[3+2] cycloadditions of nitroalkenes. The fused/bridged mode

A new class of tandem [4+2]/[3+2] cycloadditions of nitroalkenes is described in which both pericyclic processes are intramolecular. Two subclasses of intra [4+2]/intra [3+2] cycloadditions have been explored in which the dipolarophile is tethered at either C(5) or C(6) of the nitronate. For both families of precursors, the cycloadditions occur in good yield and are found to be highly regio- and stereoselective. This method converts linear polyenes to functionalized polycyclic systems bearing up to six stereogenic centers.

An Atom-Economical Approach to Conformationally Constrained Tricyclic Nitrogen Heterocycles via Sequential and Tandem Ugi/Intramolecular Diels−Alder Reaction of Pyrrole1,2

An efficient approach to rigid tricyclic nitrogen heterocycles via sequential and tandem Ugi/intramolecular Diels-Alder (IMDA) cycloaddition of pyrrole is described. The one-pot Ugi four-component condensation (4CC) reaction was used as the key transformation to prepare trienes with a carboxamide substituent on the tether. The use of acrylic acid (21) and N-propyl- and N-benzylmaleamic acids (24b and 24C) as the acid components provided trienes 22, 25b, and 25c, respectively, which upon heating at 120 degrees C for 12 h yielded the corresponding [4 + 2] cycloaddition products. In the case of maleic acid derivative 24a, heating the reaction mixture at 60 degrees C for 6 h promoted the cycloaddition reaction and provided the desired product 26a in 78% yield. In contrast, fumaric acid monoethyl ester (27a) and 3-acetyl- and 3-(4-methylbenzoyl) acrylic acids (27b-c) directly yielded the corresponding Ugi/IMDA cycloaddition products 29a-c in high yields at room temperature without any trace of initially formed trienes 28a-c. The IMDA cycloaddition reactions proceed with excellent stereoselectivity with the formation of five stereogenic centers and three rings.

Intramolecular Diels-Alder and tandem intramolecular Diels-Alder/1,3-dipolar cycloaddition reactions of 1,3,4-oxadiazoles

The scope of intramolecular Diels-Alder and a novel tandem Diels-Alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles is disclosed. In the cases examined, the tandem cycloadditions construct three new rings with formation of four new C-C bonds and set all six stereocenters about a central six-membered ring in a single step including three contiguous and four total quaternary centers without a trace of a second diastereomer.