DICHLOROMETHANE: Colourless volatile liquid with chloroform-like odour; b.p. 40deg.C; immiscible with water. HARMFUL VAPOUR IRRITATING TO EYES. Avoid breathing vapour. Avoid contact with skin and eyes. O.E.L. 350 mg m-3.
Toxic effects. The vapour irritates the eyes and respiratory system and may cause headaches and nausea; high concentrations may result in cyanosis and unconsciousness. The liquid irritates the eyes. Poisonous if taken by mouth.
Hazardous. Mixtures with Li, Na, N2O4, HNO3 liable to explode.
Spillage. Wear face shield goggles and gloves. Absorb bulk quantities on sand, shovel into lbuckets and transport to safe open area for atmospheric evaporation. Wash site of spillage with water and detergent.
DIETHYL ETHER: Colourless highly volatile liquid with characteristic odour; b.p. 34deg.C; immiscible with water. Liable to form explosive peroxides on exposure to light/air. Peroxides removed by treatment with aqueous sodium metabisulfite. HARMFUL VAPOUR. FORMS EXPLOSIVE PEROXIDES. EXTREMELY FLAMMABLE. Avoid breathing vapour. O.E.L. 1200 mg m-3.
Toxic effects Inhalation of the vapour may cause drowsiness, dizziness, mental confusion, faintness and, in high concentrations, unconsciousness. Ingestion may also produce these effects. Continued inhalation of low concentrations may cause loss of appetite, dizziness, fatigue and nausea.
Hazardous. Peroxide formation can result in subsequent explosion. Powerful oxidants can cause explosion. Reacts vigorously with sulfuryl chloride.
Fire Hazard Flash point -45deg.C, ignition temp. 180deg.C; extinguish fire with CO2.
Spillage Clear area, shut off all sources of ignition. Organise effective ventilation and allow to evaporate.
DIOXAN: Colourless almost odourless liquid; b.p. 101deg.C; miscible with water. Liable to form explosive peroxides on exposure to light/air. Peroxides removed by treatment with aqueous sodium metabisulfite. HARMFUL VAPOUR. FORMS EXPLOSIVE PEROXIDES. EXTREMELY FLAMMABLE Avoid breathing vapour. O.E.L. 360 mg m-3.
Toxic effects. The vapour irritates nose and eyes and this may be followed by headache and drowsiness. High concentrations may also cause nausea and vomiting, while injury to the liver and kidney is possible. Similar effects when taken by mouth.
Hazardous. Explosive peroxides formed on exposure to air/light. Reacts explosively with Ni(R) at >210deg.C. Addition complex with SO3 explodes on storage.
Fire hazard. Flash point 12deg.C; ignition temp. 180deg.C; extinguish fire with CO2.
Spillage . Clear area, shut off all sources of ignition. Mop up with plenty of water and run to waste. Organise effective ventilation and evaporate remaining liquid.
ETHYL ACETATE: Colourless liquid with fragrant odour; b.p. 77deg.C; slightly soluble in water.
EXTREMELY FLAMMABLE. IRRITATING TO EYES AND RESPIRATORY SYSTEM. Avoid breathing vapour. Avoid eye contact. O.E.L. 1400 mg m-3.
Toxic effects. The vapour may irritate the eyes and respiratory system. The liquid irritates the eyes and mucous surfaces. Prolonged inhalation may cause kidney and liver damage.
Fire hazard. Flash point -4.4deg.C; ignition temp. 427deg.C; extinguish fire with CO2.
Spillage . Clear area, shut off all sources of ignition. Wear face shield goggles and gloves. Absorb bulk quantities on sand, shovel into buckets and transport to safe open area for atmospheric evaporation. Wash site of spillage with water and detergent.
PHOSPHORIC ACID: Colourless viscous liquid, miscible with water . CORROSIVE . CAUSES BURNS. Avoid eye contact. O.E.L. 1 mg m-3.
Toxic effects The liquid burns the eyes and skin severely. If taken by mouth, there would be severe internal damage.
Spillage Wear face shield goggles and gloves. Spread sodium carbonate liberally over the spillage and mop up cautiously with plenty of water. Run to waste with running water.
PYRIDINE: Colourless liquid with a sharp penetrating odour; b.p. 115deg.C; miscible with water. HIGHLY FLAMMABLE. HARMFUL VAPOUR. IRRITATING TO EYES, SKIN AND RESPIRATORY SYSTEM. Avoid breathing vapour. Avoid contact with skin and eyes. O.E.L. 15 mg m-3.
Toxic effects. The vapour irritates the respiratory system and may cause headache, nausea, giddiness and vomiting. The vapour and liquid irritates the eyes and may cause conjunctivitis. the liquid may irritate the skin and cause dermatitis. Affects the central nervous system if taken by mouth and large doses act as a heart poison.
Hazardous. Complex with CrO3 unstable and decomposes violently when dry. Reacts violently with N2O4.
Fire hazard. Flash point 20deg.C, ignition temp. 482deg.C; extinguish fire with CO2.
Spillage Clear area, shut off all sources of ignition. Mop up with plenty of water and run to waste. Organise effective ventilation and evaporate residual liquid.
The Penicillin to Cephalosporin Conversion
The benzhydryl ester of penicillin V [[beta]]-sulphoxide 1, on heating, undergoes reversible ring opening to give the derived sulphenic acid 2. This may be intercepted by a nucleophile, or an electrophile, to produce diverse novel [[beta]]-lactam systems2. In the presence of a trace of acid, the sulphonium salt 3 is formed. This, on deprotonation, gives the desacetoxycephalosporin analogue 4. Clearly since [[beta]]-lactams are sensitive molecules, the choice of the acidic catalyst is crucial for success.
The rearrangement of the penicillin 1 to the cephalosporin 4 may be conveniently followed by t.l.c. and the product isolated by flash chromatography.
1. Dioxan (20ml), purified by reflux over and distillation from sodium metal under dry nitrogen (DO NOT DISTIL TO DRYNESS), can be obtained from the communal still (see Technician). Collect the dioxan, immediately prior to use, in a dried flask.
2. Redistilled dichloromethane and ethyl acetate (~150 ml of each) will be provided.
3. Establish a t.l.c. system for the [[beta]]-lactam ester 1 to give an RF value of about 0.3. Use a mixture of ethyl acetate and dichloromethane. The spots can be visualised under the u.v. lamp and by placing the plates in a jar containing a few crystals of iodine.
4. Record and interpret the i.r. and n.m.r. spectra of the penicillin.
Rearrangement of Penicillin V [[beta]]-Sulphoxide 1.
In a 25ml round bottomed 2 or 3 necked flask, dissolve dry pyridine (10ul) and 85% orthophosphoric acid (14ul) in dry redistilled dioxan (see Technician) (10ml). Measure the acid and base using a Gilson micropipette with a new tip for each reagent. Add the penicillin ester 1 (0.50g) as provided, and heat the mixture to reflux overnight under nitrogen (CARE: check all joints). Do not purge the system continuously with N2 since this will evaporate the solvent. Before commencing heating, flush the flask with N2 and subsequently maintain a slight positive pressure of N2 applied to the top of the condenser, via a T-piece with a liquid paraffin bubbler (see Demonstrator / Technician for details). Allow the solution to cool. Recheck the t.l.c. If necessary, adjust the solvent composition (to x% EtOAc/DCM) to give an RF of ~0.5 for the product.
Flash Chromatography and Isolation of the Product
Cover the sintered glass column with the plastic safety webbing provided and clamp it vertically over a conical flask. In a conical flask, slurry Kieselgel H (12g) with dichloromethane (DCM). Pour the slurry into the column and wash in the residue with more DCM. Using the hand bellows, force the DCM through the column and thus compact the silica. It is essential that the level of solvent is not forced beyond the top of the silica otherwise channels will form in the silica and its effectiveness destroyed.The surface of the silica pad must be as flat as possible. With extreme care, add more DCM to the column; do not disturb the silica surface. Pouring the solvent carefully down the side of the column over the blade of a wide spatula is helpful. Again, force the solvent through the column so that the surface of the silica is just damp with DCM. If cracking or channelling occurs, repack the column.
Evaporate the dioxan solution of the product to complete dryness. dissolve the residue in the minimum volume of DCM and carefully apply dropwise to the surface of the silica. Wash on the last traces with a further small volume of DCM. Using the bellows, carefully force the liquid onto the column so that the level of liquid is again just flush with the silica surface. Rinse the edges of the glass with a little more DCM and again force it onto the column.
Fill the column with x/2 % EtOAc / DCM (50ml). Force the solvent rapidly through the column and collect 5 x 10ml fractions. Take care that the bung does not fly out during the operation! Further elute the column with x % EtOAC / DCM (50ml). Check all 10 fractions by t.l.c. Combine those containing the pure product. Rotary evaporate to dryness in a pre-tared 250ml r.b. flask. Check the weight of the residue. Using small volumes of DCM, transfer the residue via filtration through a Pasteur pipette containing a cotton wool plug, to a small conical flask. Evaporate (fume cupboard) to dryness and recrystallise the residue from ethyl acetate. Dry the product under vacuum overnight at room temperature. Record the yield, m.p., n.m.r. and i.r. spectra and compare with those of the starting penicillin. Transfer the product to a small labelled sample tube.
Comment on the spectral data.
1. W.C. Still, M. Kahn and A. Mitra, J. Org. Chem., 1978, 43, 2923.
2. R.D.G. Cooper and D.O. Spry, 'Cephalosporins and Penicillins. Chemistry and Biology', Ed. E.H. Flynn, Academic Press, New York, 1972, p. 183; P.G. Sammes, Chem. Rev., 1976, 76, 113 and references there cited.