long time ago I found my own solution but forgot to put it here. So, here it is:
folder = ['/scratch ...']
fields = fieldnames (datastructold) ;
for f = 1:size(fields,1) ;
varname = genvarname(fields{f}) ;
extract = {datastructold.(varname)};
datastructnew.(varname) = extract ;
end
v = genvarname([ 'name' ]) % name for variable
S.(v) = datastructnew
save([ folder v '.mat' ] , '-struct' , 'S' ) ; % name for .mat