A- -Mannan. synthesis

Scheme 37. a-(1 2)-mannan synthesis with a polymer-supported acceptor. 

The synthesis of an alternating antigenic jS-(l—>3)-jS-(l—>4)-mannan common to Rhodotorula glutinis, Rhodotorula mucilaginosa, and Leptospira biflexa provided the occasion for the comparison of linear and block approaches. A linear synthesis was first undertaken and provided a relatively straightforward access to the hexaose 313 (Scheme 51).134,146 The most noteworthy feature of this synthesis was the protection of the /1-donor 305 destined to become the - 4)-D-mannopyranosyl-/i-(l unit in the polymer as the 4-methoxybenzylidene acetal rather than as the simple benzylidene acetal employed in the /1-donor 301 precursor to the ultimate 3) - D- m an n opy rano -... 

This in turn resulted in the introduction of the sterically minimal 2-propargyl ether group to compensate for the presence of a bulky group at 0-3, with positive results in a number of systems.149 Unfortunately even this innovation was not sufficient for the l>-( 3)-mannan synthesis, as is clear from the contrast in the selectivities of the di- and trisaccharide preparations presented in Scheme 53.150... 

In S. oerevisiae a similar mannan modification reaction occurs that involves the addition of mannosylphosphate units to the trisaccharide sidechains (3 ). The function of these units is unknown but they could obviously control the charge on the cell surface, which may be important in some environmental situations. We have obtained two types of mutants that regulate this process. They segregate independently, but both result in a mannan without phosphate. One mutation mnn4) (22) is dominant and probably affects a locus that regulates synthesis of the mannosylphosphate transferase (Table III) the other is recessive... 

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