Nizar Haddad

STEREOSELECTIVE CYCLOPROPANATION WITH A SULTAM CARBENOID

Abstract Stereoselective cyclopropanation of alkenes with the Oppolzers sultam carbenoid was examined for the first time. The addition reaction proceeds in high yields on substituted alkenes and provides an easy access for stereoselective preparation of di- and tri-substituted cyclopropanes. The synthetic utility of the litle reaction was demonstrated in the stereoselective preparation of 1.

Studies towards total synthesis of borrelidin, stereoselective synthesis of the polysubstituted macrolidic part☆

Abstract Enantioselective synthesis of the polysubstituted macrolidic part of borrelidin ( 1 ) is presented. Six out of the nine stereogenic centers in 12 and 13 were achieved in >99.2 %ee in seven steps from meso -diol 4 . The synthesis is based on enantioselective formation of bis-epoxide 10 followed by regioselective alkylation of the epoxide functionalities.

About the stereochemistry of intramolecular [2+2] photocycloadditions

The stereoselectivity in [2+2] photocycloaddition was studied on System I. Compounds 9, 12a, and 15a cyclize in high yield and the selectivity is higher than 94%.

Studies towards total synthesis of borrelidin, regioselective methylation of bis-epoxides and structure determination☆

Abstract Regioselective methylation of bis-epoxides 9 were examined. Two examples of high regioselectivity and yield were achieved. Structure determination of the four possible regioisomers provide useful information for structure determination of structurally related flexible molecules.

Topological and Steric Effects in Mechanism of Intramolecular [2+2] Photocycloadditions

Abstract It was found that in [2+2] intramolecular photocycloaddition the isomer rationin system II is different from system I. Compounds 1 and 20 cyclize with high stereoselectivity to give in high yield 3 and 21 respectively. The mechanistic consequences are discussed.

Trapping of 1,4-diradical intermediate in intramolecular [2+2] photocycloaddition

Abstract For the first time a 1,4-diradical intermediate 10 was trapped in 43% yield in an intramolecular [2+2] photocycloaddition of olefins to cyclohexenone. The results support the assumption that the first bond is formed to carbon-β of the enone system.

Highly efficient trapping of short-lived 1,4-diradicals, the order of first bond formation in the intramolecular photocycloaddition of 3-(4′-pentenyl)-cyclohex-2-enones

Abstract Complete trapping of 1,4-diradical intermediates, formed in the intramolecular [2+2] photocyclo-addition of 13, provide direct evidence for the exclusive formation of the first bond at the C(β) of the cyclic enone. This result is found in full agreement with the previously studied compounds 1.

Total synthesis of (+)-ligudentatol via Photoaddition-Fragmentation-Aromatic Annulation sequence

Abstract First synthesis of the naturally occurring compound (+)-ligudentatol is presented. It demonstrates the synthetic utility of the “Photoaddition-Fragmentation-Aromatic Annulation” sequence, recently developed via the intramolecular photocycloaddition of dihydropyrones to alkenes.

Experimental evidence for the “twisted” approach in intramolecular 2+2 photocycloadditions

Abstract Separation and structure determination of the previously trapped products 5 is described, the results present the first experimental evidence for the “twisted” approach. An important temperature effect was found.

Diradical intermediates in intramolecular [2+2] photocycloaddition

Abstract Products formed on irradiation of substituted cyclohexenones 1 and 6 have provided information on the structure of the diradical intermediates involved in [2+2] photocycloaddition. It was found that compound 11 cannot be used for studying the structure of diradicals intermediates in intermolecular [2+2] photocycloaddition.