Malcolm D. McLeod

Chemoenzymatic methods for the enantioselective preparation of sesquiterpenoid natural products from aromatic precursors

The enantiomerically pure cis-1,2-dihydrocatechols 2, which are generated by enzymatic dihydroxylation of the corresponding aromatic, engage in regio- and stereo-controlled Diels-Alder cycloaddition reactions to give a range of synthetically useful bicyclo[2.2.2]octenes. Certain examples of the latter type of compound have been used as starting materials in the synthesis of the sesquiterpenoids (−)-patchoulenone and (−)-hirsutene.

Structure-activity relationships of synthetic progestins in a yeast-based in vitro androgen bioassay

The recent identification of tetrahydrogestrinone (THG), a non-marketed designer androgen used for sports doping but previously undetectable by established mass spectrometry-based urine drug screens, and its production by a facile chemical modification of gestrinone has raised concerns about the risks of developing designer androgens from numerous marketed progestins. We therefore have used yeast-based in vitro androgen and progesterone bioassays to conduct a structure-activity study assessing the intrinsic androgenic potential of commercially available progestins and their derivatives, to identify those compounds or structures with the highest risk of forming a basis for such misapplication. Progestins had a wide range of androgenic bioactivity that was not reliably predicted for individual steroids by their progestin bioactivity. 17alpha-Hydroxyprogesterone and 19-norprogesterone derivatives with their bulky 17beta-substituents were strong progestins but generally weak androgens. 17alpha-Ethynylated derivatives of testosterone, 19-nortestosterone and 18-methyl-19-nortestosterone such as gestrinone, ethisterone, norethisterone and norgestrel had the most significant intrinsic androgenicity of all the commercially marketed progestins. Facile chemical modification of the 17alpha-ethynyl group of each of these progestins produces 17alpha-methyl, ethyl and allyl derivatives, including THG and norbolethone, which further enhanced androgenic bioactivity. Thus by using the rapid and sensitive yeast bioassay we have screened a comprehensive set of progestins and associated structures and identified the ethynylated testosterone, 19-nortestosterone and 18-methyl-19-nortestosterone derivatives as possessing the highest risk for abuse and potential for conversion to still more potent androgens. By contrast, modern progestins such as progesterone, 17alpha-hydroxyprogesterone and 19-norprogesterone derivatives had minimal androgenic bioactivity and pose low risk.

The Total Synthesis of (–)-Tetrahydrolipstatin

Careful control during the bromolactonization of β,γ;-unsaturated acid (4) was required to regioselectively afford the trans-β-lactone (3) as the major diastereomer. Radical debromination of (3) followed by a three-step sequence of reactions afforded the lipase inhibitor (–)-tetrahydrolipstatin (1).

Stereocontrolled aldol additions to α-methylene-β-alkoxy aldehydes: Application to the synthesis of a C13C25 segment of bafilomycin A1

Abstract A boron-mediated, syn -aldol coupling between ethyl ketone 8 and aldehyde 9 , followed by directed hydrogenation at C 16 and acetonide hydrolysis, gives the C 13 C 25 segment 6 of bafilomycin A 1 .

The Sharpless asymmetric aminohydroxylation reaction: optimising ligand/substrate control of regioselectivity for the synthesis of 3- and 4-aminosugars

An investigation of the factors responsible for the sense and magnitude of regioselectivity in the Sharpless asymmetric aminohydroxylation (AA) has been conducted. Theoretical investigations of ligand-osmium binding geometry and experimental investigations of the Sharpless AA reaction on a series of functionalized pent-2-enoic acid ester substrates demonstrate that the opposite regioselectivity afforded using PHAL and AQN ligands results from a change in substrate orientation with respect to the catalyst. Two distinct ligand binding domains within the catalyst have been proposed that undergo attractive interactions with the substrates. Selective access to each of the four potential regio- and stereo-isomeric AA products could be achieved through the appropriate choice of ligand and substrate. These results have been applied toward the efficient stereoselective synthesis of naturally occurring and regioisomeric 3- and 4-aminosugar derivatives.

Escherichia coli glucuronylsynthase: an engineered enzyme for the synthesis of β-glucuronides

The glycosynthase derived from E. coli beta-glucuronidase catalyzes the glucuronylation of a range of primary, secondary, and aryl alcohols with moderate to excellent yields. The procedure provides an efficient, stereoselective, and scalable single-step synthesis of beta-glucuronides under mild conditions.

Studies in macrolide synthesis: Stereocontrolled synthesis of a C1C13 segment of concanamycin A

Abstract The C 1 C 13 segment 6 of concanamycin A ( 1 ) was prepared by a highly stereocontrolled route (87% overall ds) in 16 steps from the ester 9 . Key steps are the one-pot aldol/reduction, 8→12 , and the HWE reaction, 18 + 19→6.

The total synthesis of (−)-tetrahydrolipstatin

Abstract Careful control during the bromolactonisation of β,γ-unsaturated acid 3 was required to afford regioselectively the trans -β-lactone 4 as the major diastereomer. Radical debromination of 4 followed by a three-step sequence of reactions afforded the lipase inhibitor (−)-tetrahydrolipstatin.

Osmium-Catalyzed Vicinal Oxyamination of Alkenes by N-(4-Toluenesulfonyloxy)carbamates

N-(4-toluenesulfonyloxy)carbamates based on a range of common amine protecting groups serve as preformed nitrogen sources in the intermolecular osmium-catalyzed oxyamination reaction of a variety of mono-, di-, and trisubstituted alkenes. The reactions occur with low catalyst loadings and good yields and afford high regioselectivity for unsymmetrically substituted alkenes.

Application of olefin metathesis to the synthesis of ABE ring analogues of methyllycaconitine

The synthesis of four novel ABE ring analogues of methyllycaconitine (MLA) is reported, employing olefin metathesis as the key step for appending the seven-membered B ring onto an AE bicyclic ring system. This strategy allows the stereodivergent synthesis of ABE ring analogues in which the stereochemistry of the AB ring junction is well defined. The compounds are designed as ligands to study binding and function of the α7-nAChR.