ESPECIFICACIONES DE LUBRICANTES SELECCIONADOS

A s the first strategy did not give satisfactory results, it appeared necessary to find an alternative to serine (3-lactone and to change the photolabile ^ -p ro tectio n. A t that tim e in the project, a new m ethod to synthesise lanthionine residues via iodoalanine residues was published by D ugave and Ménez.^^ The route seem ed viable and proceeded in high yields. A new retrosynthetic pathw ay was therefore planned (F ig u re 49).

P , H N ^ CO2H CO2P2 N H F m oc ? i = 6-n itr o v e r a tr y lo x y c a r b o n y l ? 2 = 4 -m e th o x y p h e n a c y l TrtHN H N CO2P2 CO2P2 C02tBu N H F m oc TrtHN

+

Figu re 49: R etrosyn th etic p ath w ay to build the target lan th ion in e residue via iod oalan ine.

6-N itroveratryloxycarbonyl was chosen to replace 3,5-dim ethoxybenzyloxy­ carbonyl as the A -protection as it had also been used to protect am ino acids and reasonable cleavage yields w ere r e p o r te d .4 - M e th o x y p h e n a c y l w as retained as the photolabile carboxy protection.

2. 1. Synthesis o f the cysteine m oiety

Starting from &rgrf-butylthiocysteine, tw o protection steps w ere carried out. As noted previously (see 1. 1. ), it was necessary to initially protect the thiol function or it w ill be protected w ith a tert-h\xiy\ group in the first step. Three sim ple steps gave the desired cysteine residue (Figure 50).

S S tB u ^ SStB u .|\\\H H 2 N ' ^COgH ^ ^ O tB u (83%) " ^ C O z t B u (1 1 ) F m ocC l N a2C0 3, d ioxan e , S H SStBu

JQh

_

JQh

Fm ocH N C0 2tBu T H F /H tO F m ocH N C0 2tBu ( 4 ,% f

F igu re 50: Synthesis o f cysteine (13) from 5'-t^r/-butylthiocysteine.

The two first steps proceeded in high yields. The two protected interm ediates w ere successfully purified by flash colum n chrom atography.

The last step is the disulfide cleavage required to free the thiol function o f the cysteine residue. In the original pathway tow ards lanthionine r e s id u e s ,c y s tin e w as used to give a cysteine residue that was then used straightaw ay alongside iodoalanine to m ake lanthionine. The disulfide cleavage o f the dim er was perform ed w ith tributylphosphine. It had been decided to use 5'-ferr-butylthiocysteine instead o f cystine as prelim inary tests show ed that cystine was not dissolved in a m ixture o f tert-h\xXy\ acetate and sulfuric acid. The cleavage o f the thiol protecting group on iS-f^rf-butylthiocysteine should lead to a sim ilar cysteine residue than the one described in the original reference.

Tributylphosphine w as used for this cleavage, in wet THF. The m ain draw back o f the reaction is the form ation o f tert-bvXyX thiol as a by-product, w hich presented problem s in purification. Flash column chrom atography was carried out but by-products resulted from the presence o f tert-h\xXy\ thiol. The reaction proceeded to give a rather low yield (41 %).

It was consequently decided to avoid this problem by using cystine from the start o f the pathw ay (Figure 51) (so there is no need o f thiol protection and cleavage o f the dim er w ill give the cysteine m onom er w ithout by-product).

H C IO4 ► 'O tBu (96% ) s- ^H2N" C02tBUy (1 4 ) 2 Fm ocC l jY-methylmorphoIine THF (82% ) SH PBU3 / ^

---

J L

F m ocH N C O jtB u THF/H^O \ F m ocH N

(1 3 ) (91% ) \ ^

F ig u r e 51: S y n th e s is o f c y ste in e (1 3 ) from c y stin e .

Tests were carried out in acidic conditions to find an acid in w hich cystine dissolves well, how ever perchloric acid was the only suitable acid, as confirm ed by reports in the l i t e r a t u r e . T h e reaction proceeded in high yield, im proving on the literature. The key elem ent o f the reaction is the increase o f the pH in the w ork-up. It is im portant to reach at least pH 9-10 to recover m ost o f the title com pound. The next step m ust be carried out quickly afterwards as cystine bis-ferf-butyl ester degrades, turning yellow.