No abstract

In the absence of its natural reaction partner (the N-H proton of the L-cysteinyl-D-valine amide bond), the proposed hydroperoxide intermediate appears to attack the putative thioaldehyde species directly. These results shed light on the events preceding beta-lactam closure in the IPNS reaction cycle, and enhance our understanding of the mechanism for reaction of the enzyme with its natural substrate.

A biosynthetically inspired synthesis of (+)-liphagal has been achieved from (+)-sclareolide in 13 steps (9% overall yield). The key step is a biomimetic ring expansion of a highly stabilized benzylic carbocation, which generates the seven-membered ring and the benzofuran of the natural product in a single cascade reaction.

In 1898 Curtius and Lorenzen reported that benzenesulfonyl chloride reacted with hydrazine to give (phenylsulfonyl)hydrazine.1 As expected, this condensed cleanly with benzaldehyde or acetone, forming the crystalline (phenylsulfonyl) hydrazone derivatives. These early workers noted both the acidity of these substances, associated with the N-H proton adjacent to the electron-withdrawing phenylsulfonyl group, and their instability. Decomposition by heating gave the readily expelled benzenesulfinic acid and nitrogen. Bamford and Stevens, 54 years later, first applied these observations in a useful synthetic procedure.2 Subsequently, in 1967, ketone (4-tolylsulfonyl)hydrazones were used as precusors for the synthesis of vinyllithium reagents.3 The Shapiro reaction was born.Ketones (1) condense readily with (4-tolylsulfonyl)hydrazine to give the invariably crystalline (4-tolylsulfonyl)hydrazones (tosylhydrazones) (2). These de-.r2 TsNHNH; r'x/%^2 nSvU rivatives contain an array of acidic functionality. Thus, reaction with sodium hydride or methoxide or n-butyllithium gives the N-monoanion 3. Anion 3 contains two good leaving groups: the 4-toluenesulfinate anion and nitrogen. Thus, on warming up to 150-200 °C, 3 decomposes, giving the carbene 4, which undergoes, for example, bond insertion reactions. This tosylhydrazone (2) to carbene (4) transformation is termed the aprotic Bamford-Stevens reaction.2 Alternatively, the monoanion 3 may be further metalated with n-BuLi to give the C,N-dianion 5. This intermediate (5) still contains the same two good leaving groups, and it readily decomposes at 0-25 °C, giving the vinyl carbanion 6. Intermediate 6 is then available for interception with an electrophile (E+) to give the product alkene 7. Since, in principle, the electrophile can be easily varied, the range of alkenes 7 available is Robert M. Adlington received his B.Sc. (1977) and Ph.D. (1980) from Imperial College of Science and Technology, University of London. He Is currently a member of Jack E. Baldwin's research team at Oxford University. Anthony G.M. Barrett obtained his B.Sc. (1973) and Ph. D. (1975) at Imperial College, under Sir Derek Barton's guidance. In 1975 he became a faculty member at Imperial. His current research Interests are the development of new synthetic methods and the total synthesis of several natural products.

The biosynthesis of the meroterpenoid guajadial was previously hypothesized to occur via a hetero-Diels-Alder reaction between caryophyllene and an o-quinone methide. This hypothesis has been verified via the biomimetic synthesis of guajadial and psidial A in an aqueous three-component coupling reaction, between caryophyllene, benzaldehyde, and diformylphloroglucinol.

Biomimetic syntheses of three polycylic polyprenylated acylphloroglucinol natural products isolated from Hypericum papuanum, ialibinone A, ialibinone B, and hyperguinone B, have been accomplished by selective oxidative cyclizations of the proposed biosynthetic precursor 5, which was synthesized from phloroglucinol in three steps.

Isopenicillin N synthase (IPNS) catalyses conversion of the linear tripeptide delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to isopenicillin N (IPN), the central step in biosynthesis of the beta-lactam antibiotics. The unsaturated substrate analogue delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-vinylglycine (ACvG) has previously been incubated with IPNS and single product was isolated, a 2-alpha-hydroxymethyl isopenicillin N (HMPen), formed via a monooxygenase mode of reactivity. ACvG has now been crystallised with IPNS and the structure of the anaerobic IPNS:Fe(II):ACvG complex determined to 1.15 A resolution. Furthermore, by exposing the anaerobically grown crystals to high-pressure oxygen gas, a structure corresponding to the bicyclic product HMPen has been obtained at 1.60 A resolution. In light of these and other IPNS structures, and recent developments with related dioxygenases, the [2 + 2] cycloaddition mechanism for HMPen formation from ACvG has been revised, and a stepwise radical mechanism is proposed. This revised mechanism remains consistent with the observed stereospecificity of the transformation, but fits better with apparent constraints on the coordination geometry around the active site iron atom.

A homology derived molecular model of prostate specific antigen (PSA) was created and refined. The active site region was investigated for specific interacting functionality and a binding model postulated for the novel 2-azetidinone acyl enzyme inhibitor 1 (IC(50) = 8.98 +/- 0.90 microM) which was used as a lead compound in this study. A single low energy conformation structure II (Figure 2) was adopted as most likely to represent binding after minimization and dynamics calculations. Systematic analysis of the binding importance of all three side chains appended to the 2-azetidinone was conducted by the synthesis of several analogues. A proposed salt bridge to Lys-145 with 4 (IC(50) = 5.84 +/- 0.92 microM) gave improved inhibition, but generally the binding of the N-1 side chain in a specific secondary aromatic binding site did not tolerate much structural alteration. A hydrophobic interaction of the C-4 side chain afforded inhibitor 6 (IC(50) = 1.43 +/- 0.19 microM), and polar functionality could also be added in a proposed interaction with Gln-166 in 5 (IC(50) = 1.34 +/- 0.05 microM). Reversal of the C-4 ester connectivity furnished inhibitors 7 (IC(50) = 1.59 +/- 0.15 microM), 11 (IC(50) = 3.08 +/- 0.41 microM), and 13 (IC(50) = 2.19 +/- 0.36 microM) which were perceived to bind to PSA by a rotation of 180 degrees relative to the C-4 ester of normal connectivity. Incorporation of hydroxyl functionality into the C-3 side chain provided 16 (IC(50) = 348 +/- 50 nM) with the greatest increase in PSA inhibition by a single modification. Multiple copy simultaneous search (MCSS) analysis of the PSA active site further supported our model and suggested that 18 would bind strongly. Asymmetric synthesis yielded 18 (IC(50) = 226 +/- 10 nM) as the most potent inhibitor of PSA reported to date. It is concluded that our design approach has been successful in developing PSA inhibitors and could also be applied to the inhibition of other enzymes, especially in the absence of crystallographic information.