Franca Zanardi

The Vinylogous Aldol and Related Addition Reactions: Ten Years of Progress†

Rephrasing Fusons original formulation, the principle of vinylogy explains how the influence of a functional group may be relayed to a distant point in the molecule when these two sites are connected to by conjugated unsaturated linkages, such as double and triple bonds or aromatic moieties. This principle has been applied, over the years, to the majority of polar carbon-carbon bond-forming reactions of various repute, including the venerable Michael addition reaction, where the electrophilic -CdX site (1,2-addition) is “usurped” by a remote conjugated -RCdCR-CdX position (1,4-addition). The aldol addition reaction and the related Mannich process, both fundamental pillars of organic synthesis, have not escaped this fate, and both of their vinylogous extensions have emerged as extremely valuable synthetic methodologies.

The Synthetic utility of furan-, pyrrole- and thiophene-based 2-silyloxy dienes

The aim of this review is to highlight the utility of a remarkable triad of 2-silyloxy diene synthons derived from furan, pyrrole and thiophene in organic synthesis. These heterocycles, in reacting with a number of carbonyl-related compounds (aldehydes, imines, heteroatom-stabilized carbenium ions), act as vinylogous nucleophile modules giving rise to a myriad of functionality-rich aldol-type constructs. These, in turn, represent invaluable synthetic platforms onto which a limitless number of functional elements and chosen chirality may be introduced. Eleven syntheses, amongst the most appealing of 1991–1999, have been chosen to illustrate the potentiality of silyloxy diene chemistry.

Bifunctional Cinchona Alkaloid/Thiourea Catalyzes Direct and Enantioselective Vinylogous Michael Addition of 3‐Alkylidene Oxindoles to Nitroolefins

3-Alkylidene oxindoles (methyleneindolinones), be they natural or man-made substances, occupy a preeminent position among the various classes of chemically and medicinally relevant small-molecule scaffolds. Their plural functional architecture featuring a lactam carbonyl flanked by a highly substituted exocyclic double bond renders them enabling intermediates to be elaborated into a myriad of useful nitrogen heterocycles of varied complexity. For example, 3-alkylidene oxindoles can be viewed as electrophilic Michael acceptors, which react with carbon-centered anions to give bsubstituted oxindoles of type A (Scheme 1a). In addition, they can act as electron-poor components in synchronous (Scheme 1b) or stepwise (Scheme 1c) cycloadditive functionalizations, thus opening the way to a wide range of highly valuable 3,3-spirocyclic structures of type B or C. Whereas these protocols have been largely pursued and formed the basis of many synthetic achievements, an “umpolung” option could also be envisaged (Scheme 1d), and capitalizes on the vinylogous pro-nucleophilic character of the alkyl group attached at the b-position of the ylidene. By reacting with the proper acceptors, these nucleophiles furnish olefinic oxindoles of type D, which are functionalized at the most distant point of the molecule (Cγ).

Total synthesis of both enentiomers of trans-β-hydroxyppecolic acid

Abstract trans-β-Hydroxypipecolic acids of both l - and d -series, l -1 and d -1, have been straightforwardly prepared in 14% and 15% yields, respectively, starting from glyceraldehyde imines d -7 and l -7 as useful three-carbon chirons. The key feature of these parallel syntheses lies on the highly diastereoselective character of the initial coupling manoeuver between silyloxy furan TBSOF and imines 7, which ultimately accounts for the relative, and hence absolute configuration of the target pipecolic acids.

anti -selective, catalytic asymmetric vinylogous Mukaiyama Mannich reactions of pyrrole-based silyl dienolates with n-aryl aldimines

Pyrrole-based silyl dienolates undergo asymmetric vinylogous Mukaiyama Mannich reactions with a series of N-aryl aldimines in the presence of the Hoveyda-Snapper amino acid-derived silver(I) catalysts. The Mannich products-α,β-unsaturated δ-amino-γ-butyrolactams-are typically obtained in high yields, excellent γ-site selectivities and anti-diastereoselectivities, and up to 80% enantioselectivity.

Discovery of Subnanomolar Arginine-Glycine-Aspartate-Based αVβ3/αVβ5 Integrin Binders Embedding 4-Aminoproline Residues

The embodiment of 4-aminoproline residues (Amp) into the arginine-glycine-aspartate (RGD) sequence led to the discovery of a novel class of high-affinity alpha Vbeta 3/alpha Vbeta 5 integrin binders [IC 50 h (alpha Vbeta 3) 0.03-5.12 nM; IC 50 h (alpha Vbeta 5) 0.88-154 nM]. A total of eight cyclopeptides of type cyclo-[-Arg-Gly-Asp-Amp-], 5- 12, were assembled by a standard solid-phase peptide synthesis protocol that involved the C2-carboxyl and C4-amino functionalities of the proline scaffolds, leaving the N (alpha)-nuclear site untouched. Functionalization of this vacant proline site with either alkyl or acyl substituents proved feasible, with significant benefit to the integrin binding capabilities of the ligands. Notably, six out of eight cyclopeptide inhibitors, 5- 7 and 9- 11, showed moderate yet significant selectivity toward the alpha Vbeta 3 receptor. The three-dimensional structure in water was determined by NMR techniques and molecular dynamics calculations. Docking studies to the X-ray crystal structure of the extracellular segment of integrin alpha Vbeta 3 complexed with reference compound 1 were also performed on selected analogues to highlight the structural features required for potent ligand binding affinity.

Design, Synthesis, and Biological Evaluation of Novel cRGD–Paclitaxel Conjugates for Integrin-Assisted Drug Delivery

The efficacy of taxane-based antitumor therapy is limited by several drawbacks which result in a poor therapeutic index. Thus, the development of approaches that favor selective delivery of taxane drugs (e.g., paclitaxel, PTX) to the disease area represents a truly challenging goal. On the basis of the strategic role of integrins in tumor cell survival and tumor progression, as well as on integrin expression in tumors, novel molecular conjugates were prepared where PTX is covalently attached to either cyclic AbaRGD (Azabicycloalkane-RGD) or AmproRGD (Aminoproline-RGD) integrin-recognizing matrices via structurally diverse connections. Receptor-binding assays indicated satisfactory-to-excellent α(V)β(3) binding capabilities for most conjugates, while in vitro growth inhibition assays on a panel of human tumor cell lines revealed outstanding cell sensitivity values. Among the nine conjugate ensemble, derivative 21, bearing a robust triazole ring connected to ethylene glycol units by an amide function and showing excellent cell sensitivity properties, was selected for in vivo studies in an ovarian carcinoma model xenografted in immunodeficient mice. Remarkable antitumor activity was attained, superior to that of PTX itself, which was associated with a marked induction of aberrant mitoses, consistent with the mechanism of action of spindle poisons. Overall, the novel cRGD-PTX conjugates disclosed here represent promising candidates for further advancement in the domain of targeted antitumor therapy.

Vicarious Silylative Mukaiyama Aldol Reaction: A Vinylogous Extension

A vinylogous, silylative, and direct variant of the venerable Mukaiyama aldol reaction has been developed. Exploiting N-Boc-pyrrol-2(5H)-one as the conjugate donor, several aldehyde and ketone acceptors were scrutinized under the guidance of suitable dual Lewis acid-Lewis base activators to provide a varied repertoire of functionality-rich alpha,beta-unsaturated-gamma-amino-delta-silyloxy carbonyl structures, in useful yields and often with an exquisite level of diastereoselection.

Lewis Acid Assisted Vinylogous Mannich and Mukaiyama Aldol Reactions: A Route to Densely Hydroxylated Indolizidine Alkaloid Analogues

The hydroxymethyl-substituted indolizidines 6 and 7, representative members of a ring-B-expanded alexine–australine subclass, are readily accessible by starting with furan-based silyloxydiene 12 and hydroxymethyl hemiaminal 11, through a synthesis sequence involving a scantily exploited vinylogous version of the Mannich reaction. The key iminium electrophilic acceptor 11 is, in turn, available through a vinylogous intermolecular Mukaiyama aldolization process between pyrrole-based silyloxydiene 8 and (S)-glyceraldehyde 9.

TOTAL SYNTHESIS OF BOTH ENANTIOMERS OF TRANS-2,3-CIS-3,4-DIHYDROXYPROLINE

Abstract Both enantiomers of trans-2,3-cis-3,4-dihydroxyproline, 4 and 5, have been stereoselectively synthesized from 2,3-O- isopropylidene- d -glyceraldehyde 1, by taking advantage of a divergent and parallel synthetic strategy, utilizing N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsiloxy)pyrrole (TBSOP) as the common four-carbon synthon.