ABSTRACT
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.
ABSTRACT
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.
Subject(s)
Galactose Oxidase/metabolism , Oxidoreductases/metabolism , Aldehydes/chemistry , Aldehydes/metabolism , Biocatalysis , Catalytic Domain , Circular Dichroism , Galactose Oxidase/chemistry , Galactose Oxidase/genetics , Models, Molecular , Mutation , Oxidation-Reduction , Oxidoreductases/chemistry , StereoisomerismABSTRACT
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 (27â28a) and an unusual dealkylative intramolecular epoxide-opening by the C11 methyl ether to establish the tetrahydrofuranyl C-ring of the ß-dihydroagarofuran skeleton (35â36).
Subject(s)
Models, Chemical , Sesquiterpenes/chemistry , Sesquiterpenes/chemical synthesis , Chemistry Techniques, Synthetic , Furans/chemistry , Oxygen/chemistry , Propanols/chemistry , Stereoisomerism , Substrate SpecificityABSTRACT
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.
Subject(s)
Lithium/chemistry , Methylamines/chemistry , Urea/chemistry , Models, Molecular , Molecular StructureABSTRACT
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]
Subject(s)
Ethers/chemistry , Heterocyclic Compounds, 4 or More Rings/chemical synthesis , Marine Toxins/chemical synthesis , Cyclization , Models, ChemicalABSTRACT
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.
