Michael V. Perkins

TOTAL SYNTHESIS OF (-)-DENTICULATINS A AND B USING EFFICIENT METHODS OF ACYCLIC STEREOCONTROL

Abstract The total synthesis of (−)-denticulatin B (2) was achieved in 9 steps (20% yield), with 70% overall diastereoselectivity, starting from the ethyl ketone (R)-9. Most of the stereochemistry was introduced by substratebased control. Key steps include the boron-mediated aldol/reduction, 9 → 22, the titanium-mediated aldol coupling, 26 + 8 → 38, and the directed cyclisation, 35 → 2. Epimerisation at C10 in 35 led to (−)-denticulatin A (1).

Studies in polypropionate synthesis: stereoselective synthesis of (−)-denticulatins A and B

Abstract ( )-Denticulatin B(2) was prepared in 9 steps (20% yield) with 70% overall ds starting from the ethyl ketone (R)-8. Key steps are the novel boron-mediated aldol/reduction, 8 → 12, the titanium aldol couping, 6 + 5 → 18, and the HF-pyridine cyclisation, 20 → 2. Epimerisation at C10 in 20 led to ( )-denticulatin A (1).

Two O-methyltransferases involved in the biosynthesis of methoxypyrazines: grape-derived aroma compounds important to wine flavour

Methoxypyrazines (MPs) are volatile, grape-derived aroma compounds that contribute to the distinct herbaceous characters of some wines. Although the full pathway leading to MP production has not been elucidated, there is strong evidence that the final step involves the methylation of non-volatile hydroxypyrazine (HP) precursors. Two cDNA encoding O-methyltransferases (OMTs) that have homology to an enzyme previously purified and shown to catalyse the methylation of HPs were isolated from Cabernet Sauvignon. Recombinant protein from the cDNAs (VvOMT1 and VvOMT2) was produced in E. coli and activity assays demonstrated that both encode OMTs able to methylate HPs to produce MPs, however both showed greatest activity against the flavonol quercetin. VvOMT1 has higher catalytic activity against isobutyl hydroxypyrazine compared to isopropyl hydroxypyrazine, whereas the converse is true for VvOMT2. The timing of the expression of VvOMT1 in the skin and the flesh of developing Cabernet Sauvignon grape berries was associated with the period of MP accumulation in these tissues, while VvOMT2 expression was greatest in roots, which were found to contain high levels of MPs. The MP composition of these tissues also reflects the relative levels of expression of these genes and their substrate preference. The identification of genes responsible for MP production in grapevine will help in understanding the effect of different viticultural and environmental factors on MP accumulation.

A methyltransferase essential for the methoxypyrazine‐derived flavour of wine

Methoxypyrazines are a family of potent volatile compounds of diverse biological significance. They are used by insects and plants in chemical defence, are present in many vegetables and fruit and, in particular, impart herbaceous/green/vegetal sensory attributes to wines of certain varieties, including Cabernet Sauvignon. While pathways for methoxypyrazine biosynthesis have been postulated, none of the steps have been confirmed genetically. We have used the F2 progeny of a cross between a rapid flowering grapevine dwarf mutant, which does not produce 3-isobutyl-2-methoxypyrazine (IBMP), and Cabernet Sauvignon to identify the major locus responsible for accumulation of IBMP in unripe grape berries. Two candidate methyltransferase genes within the locus were identified and one was significantly associated with berry IBMP levels using association mapping. The enzyme encoded by this gene (VvOMT3) has high affinity for hydroxypyrazine precursors of methoxypyrazines. The gene is not expressed in the fruit of Pinot varieties, which lack IBMP, but is expressed in Cabernet Sauvignon at the time of accumulation of IBMP in the fruit. The results suggest that VvOMT3 is responsible for the final step in methoxypyrazine synthesis in grape berries and is the major determinant of IBMP production.

New marine natural products from sponges (Porifera) of the order Dictyoceratida (2001 to 2012); a promising source for drug discovery, exploration and future prospects.

The discovery of new drugs can no longer rely primarily on terrestrial resources, as they have been heavily exploited for over a century. During the last few decades marine sources, particularly sponges, have proven to be a most promising source of new natural products for drug discovery. This review considers the order Dictyoceratida in the Phylum Porifera from which the largest number of new marine natural products have been reported over the period 2001-2012. This paper examines all the sponges from the order Dictyoceratida that were reported as new compounds during the time period in a comprehensive manner. The distinctive physical characteristics and the geographical distribution of the different families are presented. The wide structural diversity of the compounds produced and the variety of biological activities they exhibited is highlighted. As a representative of sponges, insights into this order and avenues for future effective natural product discovery are presented. The research institutions associated with the various studies are also highlighted with the aim of facilitating collaborative relationships, as well as to acknowledge the major international contributors to the discovery of novel sponge metabolites. The order Dictyoceratida is a valuable source of novel chemical structures which will continue to contribute to a new era of drug discovery.

Microbial degradation of illicit drugs, their precursors, and manufacturing by-products: implications for clandestine drug laboratory investigation and environmental assessment

Chemicals associated with clandestine drug laboratories are often disposed of covertly into soil, sewerage systems, or public waste management facilities. There are two significant issues relating to such dumps of materials; they might contain valuable evidence as to drug manufacture, and they might be a source of pollution. This study presents initial findings in relation to the impact microorganisms from environmental sources have upon drugs, their precursors, and manufacturing by-products. The aim of this study was to identify which chemicals associated with clandestine drug laboratories persist in the environment in order to allow forensic drug chemists to link discarded residues with the method of manufacture, and to allow the environmental impact of clandestine drug laboratories to be assessed accurately. When exposed to soil microorganisms, phenyl-2-propanone (P2P) was rapidly metabolized into mixtures of 1-phenyl-2-propanol, 1-phenyl-1,2-propanedione, 1-hydroxy-1-phenyl-2-propanone, 2-hydroxy-1-phenyl-1-propanone, and the two diastereoisomers of 1-phenyl-1,2-propanediol. On the other hand, when exposed under the same conditions, methylamphetamine sulphate (MAS) remained virtually unchanged. Implications relating to evidence gathering for forensic purposes and to environmental assessment of clandestine drug laboratories are discussed.

Biomimetic Total Synthesis of Gracilioethers B and C

Total syntheses of the marine polyketide metabolites gracilioethers B and C have been realized in 9 steps (40% overall yield) and 10 steps (34% overall yield), respectively. The [2(5H)-furanylidene]ethanoate (furanylidene) motif was constructed in a transacetalization/dehydration cascade of an advanced β-ketoester intermediate, which was designed to mimic a postulated biosynthetic precursor to the natural products. The relative and absolute configurations of gracilioethers B and C are confirmed as (6R,8R) and (6R,8R,11S), respectively.

Determining the Methoxypyrazine Biosynthesis Variables Affected by Light Exposure and Crop Level in Cabernet Sauvignon

Methoxypyrazines are sensorially potent volatile compounds responsible for herbaceous/vegetal attributes in wines made from certain grape varieties. The biosynthesis of these compounds in grape berries is known to occur via a pathway that involves the methylation of hydroxypyrazine intermediates. Certain viticultural management regimes can be used to alter methoxypyrazine concentrations in fruit of those varieties that have the genetic capability of producing them. This study explored the effect of light exposure and crop level on 3-isobutyl-2-methoxypyrazine (IBMP) concentrations in Cabernet Sauvignon fruit to better understand the effect of these variables on the concentration of the precursor 3-isobutyl-2-hydroxypyrazine (IBHP) and the expression of a methyltransferase gene (VvOMT3) responsible for the final step in methoxypyrazine biosynthesis. Light was found to reduce the expression of VvOMT3 and the concentration of IBHP, suggesting that a combination of these factors reduces IBMP concentration when fruit has greater light exposure. In contrast, reducing the crop level of vines to less than half of that of controls did not have a significant effect on IBHP concentration or VvOMT3 expression, despite the treatment causing a significant increase in IBMP concentration. IBMP appears to be synthesized in the flesh of the berry which suggests differences in berry size may explain the crop level effect on IBMP concentrations.

Total Synthesis of a Hemiacetal Polypropionate from Siphonaria australis

A highly stereocontrolled synthesis of (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one has been achieved in 13 steps and an overall yield of 7.6% from (R)-(+)-4-benzyloxazolidin-2-one. This substrate-controlled asymmetric synthesis utilized Paterson’s lactate-derived chiral ketone (2S)-2-(benzoyloxy) pentan-3-one to generate the 5R and 6S stereocentres and alkylation of an Evans’ auxiliary to generate the remote side-chain 3R stereocentre. Furthermore, a novel, highly efficient, and selective strategy was used to generate an enol trimethylsilyl ether whose subsequent Mukaiyama aldol condensation gave the acyclic precursor to the final product. A thermodynamically controlled cyclization then gave (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one with control of the 2S and 3R stereocentres. The NMR spectroscopic data and optical rotation obtained for synthetic (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one were consistent with those reported for the hemiacetal isolated from Siphonaria australis, and thus, prove the absolute and relative configuration of the natural product.

Stereoselective synthesis of an isomer of Membrenone-C via an aldol based two directional chain extension

Abstract An isomer of Membrenone-C was prepared in 8 steps (17% yield) with 93% overall ds starting from the ethyl ketone ( S )- 10 . Key steps are the boron-mediated aldol followed by anti selective reduction, giving the C 6 C 10 stereochemistry, the two direction chain extending double titanium aldol coupling, 16 + 20 → 19 , and the TFA promoted double cyclisation/dehydration giving an isomer of Membrenone-C, 21 → 4 .