Recent results in synthetic glycochemistry with iron salts at Orsay-Gif - Carbohydrate Chemistry: Chemical and Biological Approaches - page 130

Adding an excess of acetic anhydride (10 equiv) and 5 mol% of
FeCl 3 6H 2 O furnished the expected 2,2'-di-O-acetylated a,a- D -trehalose
62 in an isolated 41% yield. The one-pot bis-reductive benzylidene ring
opening required the successive addition of 10 equiv of Et 3 SiH and 15
mol% of the catalyst to afford the expected compound 63 in a moderate
overall yield of 28%. Finally, the iron( III ) chloride-catalyzed tandem
protocol was also successfully applied to a non-symmetric disaccharide.
Per-O-silylated-a-methyl maltoside 64 was cleanly converted to the mono-
benzylidene bis-benzyl derivative 65 (Scheme 15). However, a higher
loading of the catalyst (15 mol%) was required to reach 51% yield.
4.3 Iron( III ) chloride in key steps to bioactive mycobacterial
sulfoglycolipid analogues
Tuberculosis still remains a major health problem in the world, with
almost 1.5 million deaths annually. Sulfoglycolipids were identified as
new mycobacterium antigens able to control mycobacterial infection 42
and appeared to be promising additive candidates for the development of
a new tuberculosis vaccine. These complex metabolites are acylated and
sulfated a,a- D -trehalose derivatives which are found in the cell wall of
Mycobacterium tuberculosis (Scheme 16). 43 Ecient access to di-O- and
tetra-O-acylated sulfoglycolipid analogues have been developed by using
the iron( III ) chloride-catalyzed tandem protocol for the functionalization
of C 2 symmetric a,a- D -trehalose. 44,45
The synthesis started with the FeCl 3 6H 2 O-catalyzed tandem pro-
cedure providing symmetrical trehalose compound 61, as reported above
(Scheme 15). The one-pot desymmetrization of trehalose was possible by
terminating the tandem sequence with the addition of palmitoyl chlor-
ide. In this case, FeCl 3 6H 2 O catalyzed the esterification at the C-2 pos-
ition of trehalose, to give mono-O-palmitoyled trehalose 66 in a moderate
27% yield (Scheme 17). Desymmetrization was better performed by
mono-t-butyldimethylsilylation at low temperature and regioselective
mono-de-O-benzylation of the less hindered benzyl group under catalytic
hydrogen transfer conditions led to the key intermediate 67. 46 The two
Scheme 16 Structures of some of the natural sulfoglycolipids from Mycobacterium
tuberculosis.