Catalytic, Z-Selective, Semi-Hydrogenation of Alkynes with a Zinc-Anilide Complex.

The reversible activation of dihydrogen with a molecular zinc anilide complex is reported. The mechanism of this reaction has been probed through stoichiometric experiments and density functional theory (DFT) calculations. The combined evidence suggests that H2 activation occurs by addition across the Zn-N bond via a four-membered transition state in which the Zn and N atoms play a dual role of Lewis acid and Lewis base. The zinc hydride complex that results from H2 addition has been shown to be remarkably effective for the hydrozincation of C═C bonds at modest temperatures. The scope of hydrozincation includes alkynes, alkenes, and a 1,3-butadiyne. For alkynes, the hydrozincation step is stereospecific leading exclusively to the syn-isomer. Competition experiments show that the hydrozincation of alkynes is faster than the equivalent alkene substrates. These new discoveries have been used to develop a catalytic system for the semi-hydrogenation of alkynes. The catalytic scope includes both aryl- and alkyl-substituted internal alkynes and proceeds with high alkene: alkane, Z:E ratios, and modest functional group tolerance. This work offers a first example of selective hydrogenation catalysis using zinc complexes.

Biocatalytic Dynamic Kinetic Resolution for the Synthesis of Atropisomeric Biaryl N-Oxide Lewis Base Catalysts.

Atropisomeric biaryl pyridine and isoquinoline N-oxides were synthesized enantioselectively by dynamic kinetic resolution (DKR) of rapidly racemizing precursors exhibiting free bond rotation. The DKR was achieved by ketoreductase (KRED) catalyzed reduction of an aldehyde to form a configurationally stable atropisomeric alcohol, with the substantial increase in rotational barrier arising from the loss of a bonding interaction between the N-oxide and the aldehyde. Use of different KREDs allowed either the M or P enantiomer to be synthesized in excellent enantiopurity. The enantioenriched biaryl N-oxide compounds catalyze the asymmetric allylation of benzaldehyde derivatives with allyltrichlorosilane.

Enzymatic desymmetrising redox reactions for the asymmetric synthesis of biaryl atropisomers.

Atropisomeric biaryls carrying ortho-hydroxymethyl and formyl groups were made enantioselectively by desymmetrisation of dialdehyde or diol substrates. The oxidation of the symmetrical diol substrates was achieved using a variant of galactose oxidase (GOase), and the reduction of the dialdehydes using a panel of ketoreductases. Either M or P enantiomers of the products could be formed, with absolute configurations assigned by time-dependent DFT calculations of circular dichroism spectra. The differing selectivities observed with different biaryl structures offer an insight into the detailed structure of the active site of the GOase enzyme.

Towards the enantioselective synthesis of (-)-euonyminol--preparation of a fully functionalised lower-rim model.

The development of a stereoselective total synthesis of ß-dihydroagarofuran 4 is described. This compound contains the same oxygenation pattern on its 'lower-rim' as found in the natural sesquiterpene (-)-euonyminol (1) and it is expected that the route described should be applicable to the synthesis of that complex natural product. (-)-Euonyminol is found as the core scaffold of a series of complex macrodilactone sesquiterpenoids isolated from the Celastraceae which possess interesting biological activities (e.g. anti-HIV activity). The synthetic route builds upon an epoxidative asymmetric desymmetrisation of meso-diallylic alcohol 10 that we have reported previously. It features a lactate Ireland-Claisen rearrangement to establish the quaternary stereocentre at C11 (27→28a) and an unusual dealkylative intramolecular epoxide-opening by the C11 methyl ether to establish the tetrahydrofuranyl C-ring of the ß-dihydroagarofuran skeleton (35→36).

N to C aryl migration in lithiated carbamates: alpha-arylation of benzylic alcohols.

We report a new mode of reactivity displayed by lithiated O-benzyl carbamates carrying an N-aryl substituent: upon lithiation, the N-aryl group is transferred cleanly from N to C. An arylation of the carbamate results, providing a route to alpha,alpha-arylated secondary or tertiary alcohols. We also report density functional theory calculations supporting the proposal that arylation proceeds through a dearomatizing attack on the aromatic ring, a significantly lower energy pathway than the 1,2-acyl transfer observed with related N-alkyl carbamates.

Synthesis of the fused polyether core of hemibrevetoxin B by two-directional ring-closing metathesis.

The tetracyclic fused polyether core of the marine natural product hemibrevetoxin B has been prepared in an efficient manner by using a strategy in which ring-closing metathesis (RCM) reactions were employed for ring synthesis. Simultaneous construction of the A and D rings was accomplished by double two-directional RCM of a tetraene. [reaction: see text]

Cis and trans selective 1,4-addition of a lithium dithioester enolate to 4-O-TBS-2-cyclohexenone.

The 1,4-addition of the lithium enolate of methyldithioacetate (LMDTA) to (+/-)-4-O-TBS-2-cyclohexenone (3) can be varied from being highly 3,4-trans selective to being highly 3,4-cis selective simply by varying the reaction temperature. This stereodivergency allows expedient syntheses of the corresponding trans and cis methyl esters 6t and 6c and derived bicyclic ketolactones 7t and 7c.