Gloria Rassu

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 syntheses of N-boc-protected 3′-deoxy-4′-azathymidine and 4′-azauridine

Abstract Novel modified nucleosides 4, 11, ent-11, and 17, wherein the furanose ring oxygen is replaced by nitrogen, have been synthesized by reacting azasugars 3, 10, ent-10, and 15 with silylated uracil or thymine bases.

The four-carbon elongation of three-carbon chiral synthons using 2-(trimethylsiloxy)furan: highly stereocontrolled entry to enantiomerically pure seven-carbon α,β-unsaturated 2,3-dideoxy-aldonolactones

Abstract The BF3-promoted addition of 2-(trimethylsiloxy)furan to three carbon synthons derived from D- and L-glyceraldehyde, D- and L-serinal, and imines thereof furnishes C7 α,β-unsaturated 2,3-dideoxy-aldonolactone derivatives in high yield, with very high level of diastereoselection. In all the cases, compounds having 4,5-threo:5,6-erythro relative stereodisposition preferentially emerge from the reactions, accompanied by only marginal amounts of 4,5-erythro:5,6-erythro epimers. An empirical rule for the rapid assigment of the configuration at C-4 (γ-carbon) of γ- substituted unsaturated and saturated γ-butyrolactones is given, based upon the sign of the optical rotation values.

Total synthesis of 1,5-dideoxy-1,5-iminoalditols

Abstract Enantiomerically pure 1,5-dideoxy-1,5-imino-D-glycero-D-allo-heptitol (10) has been synthesized in ca. 9% overall yield by utilizing 2,3-O-isopropylidene-D-glyceraldehyde-N-benzylimine (1) as a chiral source and 2-(trimethylsiloxy)furan (2) as a homologative reactant. The opening move was the preparation of properly protected seven-carbon butenolide 4, followed by diastereoselective anti, cis-dihydroxylation of the lactone double bond and furanose-to-azapyranose ring expansion. This generated a piperidine intermediate 7, the stereochemistry of which was secured by a single crystal X-ray analysis of its diacetate 9.

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.

Homochiral α,β-unsaturated γ-lactams: Versatile templates

Abstract The enantiomerically pure crystalline α,β-unsaturated γ-lactams 4 and 5 have been synthesized by utilizing 2,3-O-isopropylidene- D -glyceraldehyde (3) as chiral source and novel N-tert-butoxycarbonyl-2-(tert-butyldimethylsiloxy)pyrrole (TBSOP) as four carbon homologative reagent. Unsaturated lactam 4 has been selectively elaborated into hydroxylated pyrrolidinones 7, 8, and 11 by stereocontrolled procedures involving conjugate addition of organocuprates, cis-dihydroxylation, and α-alkylation via hydrogenation and enulate formation. The absolute stereochemistries of 4 and 5 have been secured by single crystal X-ray analyses.