Chi Ching Mak

Synthesis and characterization of optically active, homochiral dendrimers

The first synthesis and optical activity measurement of homochiral, monodisperse dendrimers is reported. Optically active (2R, 3R)-tartaric acid is used to construct the zero and first-generation optically active, homochiral dendrimers 1 and 2 in good yields. The relationship between the optical rotation of all these molecules and the number of chiral tartaric acid units resided in them is explored.

A transition-metal mediated regioselective synthesis of phenyl quinones via sequential benzannulation and cross coupling reactions

Abstract A synthetic route for the regioselective synthesis of polysubstituted quinones by the benzannulation and subsequent cross coupling reactions has been developed. Alkenyl chromium carbene complexes undergo regioselective benzannulation with 2-(trimethylsilyl)-1-phenylethyne with oxidative workup to yield silyl quinones which upon iodination with ICI produce iodoquinones. Subsequent Stilles and Suzukis cross coupling reactions of these iodoquinones yield polysubstituted quinones.

Synthesis and chiroptical properties of layerblock dendrimers

A series of optically active, tartaric acid-based, monodisperse dendrimers 1 - 4 was prepared. The central core and branching junctures of these dendritic compounds were made up from phloroglucinol units and the surface sectors used were 4-tert-butylphenoxy moieties. The optically active elements, which were derivatives of either (D)- or (L)-tartaric acid, served as the chiral linker branches of these dendrimers. For this series of structurally flexible, low generation optically active dendrimers, the observed molar optical rotation was proportional to the number of chiral units residing within the dendritic matrix, with the chiroptical property of the (D)-tartrate unit cancelling that of the (L)-tartrate unit on a one to one mole basis. Circular dichroism studies, however, revealed that this cancellation effect was more effective when both the (D)- and (L)-chirons were situated within the same layer.

Facile preparation of optically active dendritic fragments containing multiple tartrate-derived chiral units

The synthesis of a series of (l)-tartrate-derived optically active, homochiral, dendritic fragments 1 – 5 by an iterative, convergent approach was reported. The chiroptical property of these chiral fragments was directly proportional to the number of chiral units.

Synthesis and properties of a new class of polyether dendritic fragments: Useful building blocks for functional dendrimers

Abstract Polyether dendritic sectors 1–4 from the first to the fourth generations, respectively, were prepared by a convergent approach from readily available 4- tert -butylphenol, phloroglucinol and 1,3-dibromopropane. Using a three-step iterative synthetic sequence involving mono- O -alkylation of [Gn]-OH with excess 1,3-dibromopropane, bis- O -alkylation of the resulting compound with 5-benzyloxyresorcinol 5 followed by hydrogenolysis with palladium catalyst, [G(n+1)]-OH of the next higher generation could be prepared in good yields. These dendritic sectors have good solubility properties and are inert in both acidic and basic media, and do not show any redox behaviour from −2.0 to +1.9 V. Hence they are useful dendritic building blocks for the preparation of functional dendrimers.

Facile construction of acid-base and redox stable polyether-based dendritic fragments

Abstract Polyether based dendritic fragments ot up in the fourth generation can be synthesized in good yields from phloroglucinol and 1,3-dibromopropane These dendritic fragments are stable in acid-base or redox conditions and can be linked to various functional core groups to form functional dendrimers which can he served as models to study the influence ot the dendritic structure on the physical, chemical and electrochemical properties of the interior core.

Synthesis and structure–optical rotation relationships of homochiral, monodisperse, tartaric acid-based dendrimers

Optically active, homochiral dendrimers of both the zeroth generation 1 and first generation 2 have been synthesized from (2R, 3R)-tartaric acid and phloroglucinol by an iterative, convergent method. These chiral dendrimers comprise 4-tert-butylphenoxy as the surface groups and phloroglucinol as the branching junctures. The chiral element, which is a derivative of L-tartaric acid, serves as the connecting unit between the surface group and the branching juncture, or between two branching junctures. Homochiral (2S,3S)-(–)-2, 3-O-isopropylidene-1, 4-di-O-tosyl-L-threitol 3 was first connected to the surface moiety via mono-O-arylation with 4-tert-butylphenol to generate the optically active mono-O-tosyl ester 4. Excessive O-alkylation of the branching juncture, 5-benzyloxyresorcinol 5, with 2 molar equivalents of the mono-O-tosyl derivative 4, followed by activation of the benzyl-protected phenol moiety at the focal point, led to dendritic ‘wedge’7. Treatment of the phenol 7 with a further molar equivalent of intermediate 4 gave the zeroth generation dendrimer 1 in 36% overall yield from the di-O-tosyl ester 3. By application of the same reaction sequence, the phenol 7 could similarly be transformed into the first generation dendrimer 2 in 13% overall yield from homochiral diester 3. Investigation on the optical rotation properties of these dendritic compounds showed that their specific rotations remained essentially constant and that the molar rotations were roughly proportional to the number of chiral tartrate units in the molecule.

Preparation and structure–chiroptical relationships of tartaricacid-based layer-block chiral dendrimers

Two optically active, diastereoisomeric, first generation layer-block dendrimers 1 and 2 have been prepared by a convergent synthetic procedure. These chiral, layer-block dendrimers utilize 4-tert-butylphenoxy moieties as the surface groups and phloroglucinol as the branching junctures. Two different chiral units, which are derivatives of (D)- and (L)-tartaric acid, serve as the chiral linkers between the surface group and the branching juncture, or between two branching junctures. The first layer-block dendrimer 1 has an outer chiral layer made up of six (L)-tartrate derived units and an inner chiral layer of three (D)-tartrate derived units. The second layer-block dendrimer 2 has an outer shell consisting of three (D)- and three (L)-chiral units and an inner shell of three (D)-chiral units. The molar rotation of these structurally flexible, low generation dendritic compounds is proportional to the number of chiral tartrate units in excess, with the chiroptical effect of a (D)-tartrate derived chiral unit cancelling that of an (L)-tartrate derived unit on a 1∶1 basis. Circular dichroism studies, however, reveal that this cancellation effect is more effective when both the (D)- and (L)-chirons are situated within the same layer.

Synthesis of allylsilanes from the [2+1] insertion reaction of alkenyl Fischer carbene complexes with silanes

Alkenyl Fischer carbene complexes undergo mild insertion reactions with organosilanes to give allylsilanes in good yields.

Synthesis and chiroptical properties of optically active layer-block dendrimers

The syntheses of two different homochiral, layered-block dendrimers 1 and 2 are reported; the chiral units used in the construction of these two first-generation dendrimers are derivatives of (D)- and (L)-tartaric acid; for these low generation dendrimers, the optical rotation strength of the (D)-tartaric derived chiron cancels out that of its antipodal (L)-tartaric derived chiral unit.