Vincenzo Zambrano

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).

Uncatalyzed, Diastereoselective Vinylogous Mukaiyama Aldol Reactions on Aqueous Media: Pyrrole vs Furan 2-Silyloxy Dienes

The first uncatalyzed, diastereoselective vinylogous Mukaiyama aldol reaction is reported, between pyrrole/furan-based dienoxy silanes and aromatic aldehydes on salty water/methanol medium, at almost human body temperature, under ultrasonic irradiation. With pyrrole dienes the reaction is anti-selective, while that of furan dienes is syn-selective. The dual role of water as both reaction medium and promoter is highlighted.

Effect of temperature in a closed unstirred Belousov–Zhabotinsky system

Complex periodic and aperiodic behaviours are reported in an unstirred Belousov-Zhabotinsky oscillatory reaction performed at temperatures varying between 0°C and 8°C. A route to chaos following a Ruelle-Takens-Newhouse (RTN) scenario is identified. Thus, temperature effects on the coupling between chemical kinetics, diffusion and convection, seem to be responsible for the observed RTN scenario. In this Letter we demonstrate that the temperature is a bifurcation parameter for the sequence period-1 → quasiperiodicity → chaos.

Organocatalytic, Asymmetric Eliminative [4+2] Cycloaddition of Allylidene Malononitriles with Enals: Rapid Entry to Cyclohexadiene-Embedding Linear and Angular Polycycles†

A direct aminocatalytic synthesis has been developed for the chemo-, regio-, diastereo-, and enantioselective construction of densely substituted polycyclic carbaldehydes containing fused cyclohexadiene rings. The chemistry utilizes, for the first time, remotely enolizable π-extended allylidenemalononitriles as electron-rich 1,3-diene precursors in a direct eliminative [4+2] cycloaddition with both aromatic and aliphatic α,β-unsaturated aldehydes. The generality of the process is demonstrated by approaching 6,6-, 5,6-, 7,6-, 6,6,6-, and 6,5,6-fused ring systems, as well as biorelevant steroid-like 6,6,6,6,5- and 6,6,6,5,6-rings. A stepwise reaction mechanism for the key [4+2] addition is proposed as a domino bis-vinylogous Michael/Michael/retro-Michael reaction cascade. The utility of the malononitrile moiety as traceless activating group of the dicyano nucleophilic substrates is demonstrated.

Synthesis of a Small Repertoire of Non-Racemic 5a-Carbahexopyranoses and 1-Thio-5a-carbahexopyranoses (‡)

A short and practical entry to optically pure 5a-carbahexopyranose and 1-thio-5a-carbahexopyranose representatives is described. Besides a few functional group and protecting group manipulations, the synthetic scheme counts on two fundamental carbon−carbon bond-forming reactions, namely (i) a regio- and stereoselective aldol addition between heterocyclic (silyloxy)diene donors (6 or 13) and D-glyceraldehyde as the acceptor (7) and (ii) a chemo- and stereoselective silylative cycloaldolization involving bifunctional aldehydes (10, 16, and 21). The 1H NMR based configurational and conformational assignment of target structures 1−5 and bicyclic intermediates 11, 12, 17, 18, and 22 is discussed. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

A short entry to novel C(2)-methyl branched 4a-carbafuranoses

Abstract A concise, diastereoselective synthesis of 2- C -methyl-4a-carba-β- d -lyxofuranose 13 and 2- C -methyl-4a-carba-β- d -arabinofuranose 14 , two novel representatives of the branched-chain carbasugar family, is presented. The construction is based on the sequential execution of two strategic carbon–carbon bond-forming reactions, a vinylogous crossed aldol addition ( 1 + 2 → 3 ), and a rare silylative cycloaldolization ( 8 → 9 + 10 ).

Exploiting the Distal Reactivity of Indolyl Methylenemalononitriles: An Asymmetric Organocatalyzed [4+2] Cycloaddition with Enals Enables the Assembly of Elusive Dihydrocarbazoles.

An unprecedented technique for the in situ generation of indolyl ortho-quinodimethanes from 2-methylindole-based methylenemalononitriles by amine-mediated remote C(sp(3) )-H deprotonation was developed. These intermediates were efficiently trapped by diverse enals to provide a rapid entry to 2,9-dihydro-1H-carbazole-3-carboxyaldehyde structures through a formal asymmetric [4+2] eliminative cycloaddition governed by a α,α-diphenylprolinol trimethylsilyl ether catalyst.