Michael H. Block

Design, synthesis and evaluation of a novel series of spiroketals basedon the structure of the antibacterial gyrase inhibitor novobiocin

Molecular modelling has been used in conjunction with crystallographic and biological data in an attempt to design compounds that mimic the structure and activity of the coumarin antibiotic novobiocin 2. Calculations on four conformations of a 6,6-spiroketal system 8–11 suggest that whilst 9 should be the lowest energy, 8 should also be readily accessible and this conformation overlays well with the coumarin and sugar rings of novobiocin. Incorporation of key hydroxy and carbamate groups onto the cyclohexane and a hydrogen bond acceptor onto the aromatic ring suggest 12a as the initial target. The crystal structure of a model compound 24 shows the conformation illustrated in 9 and thus supports the molecular modelling. However, the crystal structure of novobiocin bound to a 24 kD fragment of gyrase B, the target of the coumarin antibiotics, reveals that it is the lactone carbonyl rather than the 4-oxy group which is responsible for a key interaction with an Arg residue and consequently the carboxy group in 12a is misplaced as a corresponding H-bond acceptor. Subsequently, compounds incorporating an extra aromatic (as in 13a) or heteroaromatic ring (as in 45) bearing an H-bond acceptor, along with the extra dimethyl groups on the cyclohexane ring, have been designed. Models of these structures overlay convincingly with novobiocin bound to 24 kD gyrase B. Versatile synthetic routes have been developed allowing these compounds and the underlying hypotheses to be tested. Unfortunately, none of the compounds demonstrates significant enzyme or antibacterial activity, which we attribute to a combination of features, including the lack of a replacement for the noviose methoxy group and the failure to achieve a good stacking (charge transfer) interaction with Arg-76.

Synthetic studies relevant to biosynthetic research on vitamin B12. Part 8. Synthesis of (±)-Faktor-I octamethyl ester

Several variations have been explored of the general synthesis of chlorins which uses a photochemical ring-closure of the macrocycle as the key step. This work leads to a synthesis of (±)-Faktor-I octamethyl ester, a chlorin which arises, in the free-acid form, by aromatisation of the mono-C-methylated intermediate on the biosynthetic pathway to vitamin B12. This synthesis confirms the structure of Faktor-I and allows labelled samples to be prepared.

Synthesis relevant to vitamin B12 biosynthesis: synthesis of (±)Faktor-I octamethyl ester

Faktor-I, the chlorin which arises by aromatisation of the mono-C-methylated intermediate on the biosynthetic pathway to vitamin B12, has been synthesized in racemic form as its octamethyl ester by ring-closing the macrocycle photochemically.

CARBONYLATIVE COUPLING OF ORGANOZINC REAGENTS IN THE PRESENCE AND ABSENCE OF ARYL IODIDES: SYNTHESIS OF UNSYMMETRICAL AND SYMMETRICAL KETONES

The utility of the palladium(0) catalysed reaction of the iodoalanine-derived organozinc reagent 6a with functionalised aryl iodides, under a carbon monoxide atmosphere, to give protected 4-aryl-4-oxo α-amino acids 8, is illustrated by a short synthesis of L-kynurenine 4. Treatment of functionalised organozinc reagents with catalytic tetrakis(triphenylphosphine)palladium(0) under an atmosphere of carbon monoxide in the absence of any electrophile leads to the formation of symmetrical functionalised ketones 9 in good yields. This reaction is illustrated by a one-step synthesis of protected (2S,6S)-4-oxo-2,6-diaminopimelic acid 9a from commercially available compounds. It has been established that adventitious molecular oxygen plays a key role in the formation of the symmetrical ketones 9, and that rigorous exclusion of oxygen can result in substantially higher yields of ketones 8 in the cross-coupling with some aromatic iodides.

A novel stereoselective synthesis of the macrocycle of haem d1 that establishes its absolute configuration as 2R,7R

A novel route to isobacteriochlorins is developed that allows the stereoselective synthesis of the macrocycle of haem d1 and so establishes its absolute configuration 2R,7R.

Alkylation and acylation of 5-phenylsulphonyl- and 5-cyanobutyrolactones

Abstract Conditions are reported for the alkylation and acylation alpha to the sulphonate of a butyrolactone carrying a 5-phenylsulphonyl group. Yields are good if the alkyl or acyl halide is sufficiently reactive. The phenylsulphonyl group is initially retained and studies of its elimination are described. The corresponding system with nitrile in place of phenylsulphonyl appears to be a less satisfactory synthon for the desired chemistry.

HAEM D1: STEREOSELECTIVE SYNTHESIS OF THE MACROCYCLE TO ESTABLISH ITS ABSOLUTE CONFIGURATION AS 2R,7R

Although the gross structure of haem d1 1 has been established, the absolute stereochemistry at C-2 and C-7 is unknown. An unambiguous stereoselective synthesis of the ester of the metal-free macrocycle corresponding to haem d1 has been completed which establishes the absolute configuration of the natural cofactor as 2R,7R. Haem d1 is thus shown to match stereochemically other biologically important macrocycles, e.g. those involved in the biosynthesis of vitamin B12, which are related to isobacteriochlorins and also display 2R,7R configurations. The synthetic sequence used is based on a new procedure for assembly of the western and eastern building blocks and it serves as an efficient general route for construction of isobacteriochlorins.

Haem d1: development of a new coupling procedureleading to the synthesis of isobacteriochlorins1

A new approach has been developed for construction of the western and eastern lactams, e.g. 2 and 3, needed for synthesis of isobacteriochlorins. It involves acylation of pyrroles with lactonic acids to form ketones. These are then efficiently converted into the desired lactams by a short sequence of reactions. All the steps are high yielding and simple to carry out.

Synthetic studies relevant to biosynthetic research on vitamin B12. Part 10. Construction of the east and west building blocks for synthesis of isobacteriochlorins

Studies with model compounds have led to the development of effective methods for (a) linking the pyrrolic rings to the reduced rings present in the isobacteriochlorin system (e.g.4) and (b) for introducing the carbon at C-5 required to complete the macrocycle. In the course of this work, many new pyrrolic systems have been prepared and characterised.

Syntheses relevant to vitamin B12 biosynthesis: synthesis of sirohydrochlorin and of its octamethyl ester

Sirohydrochlorin is an isobacterichlorin isolated (a) as the metal-free prosthetic group of sulphite reductase and (b) as the aromatised form of the di-C-methylated intermediate on the biosynthetic pathway to vitamin B12; the synthesis is described of the natural enantiomer of sirohydrochlorin and also of its octamethyl ester using a mild photochemical route.