2.3.3.3 Synthesis of the rare AATp-Gal building block.
2.3.3.3.1 Reports from the literature.
2-Acetamido-4-amino-2,4,6-trideoxy-D-galactopyranose is present at the cell surface of a number of bacterial polysaccharides70,71. There are several reports of synthesis of derivatives of this monosaccharide72,73 but here only those with some application to this work74,75 will be discussed. The protecting group or precursor for the acetamido group at position 2 must be azido due to the configuration at the anomeric site of this monosaccharide in the natural compound (linkage between O and P). The protecting group of the amino group at position 4 must be different from azido, because at the end of the synthesis these two amino groups appeared in different forms and we had already chosen the azido for the position 2.
In 1992, van Boom et. al.74 published a synthesis of this monosaccharide (Scheme 25) and used it as donor and acceptor. But this synthesis had some points that are necessary to discuss;
first the reduction of the oxime at position 4 to get the benzyloxycarbonyl protected amino
2. Theoretical Part.
Therefore this strategy in its original version was not suitable for us.
O
Scheme 25: Van Boom s strategy for synthesizing AATp residue74.
The second strategy appeared in 2004 and was reported by Grindley et. al.75 (Scheme 26). In this case the synthesis is very short with high yields, but the final compound is protected with a methyl group at the anomeric position, and its removal needs harsh procedures. The major problem of this synthesis is that the protection of the amino group at the second position is already in acetamido form, making it impossible to glycosylate this monosaccharide with good yields at this stage. Therefore this strategy in its original version was not suitable for us either.
2. Theoretical Part.
Scheme 26: Grnidley s synthetic strategy75.
2.3.3.3.2 Our Approach.
In our first strategy (Scheme 27) we started from D-glucosamine hydrochloride which through a diazo transfer reaction67 was converted into the peracetylated azido D-glucose (81) that was glycosylated with thiophenol ( 82), following the same procedure as already described to protect the anomeric hydroxy group. 4,6-O-Anisaldehyde benzylidene protection, further allylation of the free hydroxy group at position 3 ( 83) and removal of the 4,6-O-anisaldehyde benzylidene protection yielded the diol intermediate (84).
This diol was used to form a 4-N,6-O oxazoline76 in galacto configuration in one pot reaction.
The reaction began with a trichloroacetimidate formation in the primary position, and subsequent triflate in the secondary one. After adding 5 equivalents of Hünig´s base the oxazoline (85) is formed in 12 hours with high yields. Hydrolysis under acidic conditions gave the free amino group in galacto configuration which was protected with benzyloxycarbonyl (86). The free hydroxyl group at position 6 was in turn converted into the tosylate ( 87) and further treatment with Bu4NI in acetonitrile to convert it into the iodide resulted in decomposition of the molecule. This decomposition could be initiated with a formation of a 4 membered ring due to a nucleophilic attack of the nitrogen to the carbon at position 6, which in turn also decomposes.
Having found this problem we decided to change the approach; it was clear that, at least, with the protecting groups we had, it was necessary to first reduce the position 6 and then generate the amino group in position 4. It is important to remember that the azido group in position 2 is
2. Theoretical Part.
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not stable under many reducing conditions, therefore the reducing method must be relatively mild, and consequently it was not possible to directly reduce the tosylate.
O
Scheme 27: First intent to synthesize AATp residue.
Our second approach was a variation of the first one, just changing the order of the reactions.
In this case the position 6 is reduced first, and later we tried to form the oxazoline ring.
In this strategy (Scheme 28), the thiophenol glycoside of azido D-glucose was selectively tosylated in the hydroxyl group on position 6 with ( 88) tosyl chloride in pyridine at low temperature. It was then tried with either 2,3 diol or 2,3 diactetyl, but the diacetylated compound was preferred due to the high polarity of the diol.
The tosylate is converted into the iodide (89) and this is reduced using NaCNBH3 in HMPT or DMPU at 95° for 12h ( 90). After removal of acetates, we followed a reported procedure where 5 membered ring oxazolines are prepared77. The hydroxy group at position 3 is selectively silylated with TBSCl and imidazole in DMF ( 91), and the hydroxy group at position 4 was mesylated with MsCl and pyridine in DCM ( 92). At -30° the silyl protecting group could be removed with TBAF without affecting the mesyl protecting group ( 93). The free hydroxy group was converted into the trichloroacetimidate following the same strategy as in the previous case, and subsequently Hünig´s base was added to form the oxazoline, but this
2. Theoretical Part.
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time there were no oxazoline formation. The explanation could be that in this case this five membered ring is too strained to form the imino double bond, and therefore the cycle is not formed.
Scheme 28: Second approach to the synthesis of AATp.
After these two attempts, it was clear for us that even when the approach of the oxazoline was short and effective, it did not fit into our scheme of synthesis; and that was the reason why we decided to design a different strategy.
In this third approach the idea was to keep the same reaction order; however it means reduce the position 6 first to obtain the deoxy sugar, and then find a suitable method to generate the amino group in galacto configuration.
This strategy (Scheme 29) started just as in the previous cases; from D-glucosamine hydrochloride and its conversion into azido D-glucose; in this case the anomeric protecting group was p-methoxyphenyl, which was obtained through a glycosylation step with the peracetylated azido D-glucose using TfOH as Lewis acid ( 94). The acetate groups were removed and a 4,6-O-anisaldehyde benzylidene protection was chosen ( 95) to let free only the hydroxyl group at position 3, which was acetylated ( 96) under normal Ac2O/Py conditions. The benzylidene group was submitted to a reductive opening with BH3.
THF
2. Theoretical Part.
After removal of the p-methoxybenzyl protection of hydroxyl group at position 4 ( 101), it was tried to generate the galacto configuration of an amino protected moiety through inversion of the configuration using triflate as leaving group and potassium phtalimide as nucleophile, but the yields were poor. A second intent was trying to invert through a Mitsunobu reaction using phtalimide as nucleophile. This reaction gave very poor yields when THF was used as solvent; a small improvement was observed when toluene was used as solvent at room temperature, and the yields were up to 74% when the reaction was performed in toluene at 50° ( 102).