[nj_bartel]

It seems like C6 attack would be preferential to obtain the 6 membered ring.

[Dan]

They do tend to adopt a 6-membered ring as the free monosaccharides in aqueous solution. Most ketohexoses (I'm talking about hex-2-uloses or 2-ketoses here) exist predominantly in pyranose form in solution - psicose is the only one of the four in which the furanose form is slightly favoured at about 55% furanose, 45% pyranose. Sorbose and tagatose are both around 95% pyranose and fructose around 70% pyranose.

For a nice nmr paper see:

Stephen J. Angyal and Geoffrey S. Bethell, Aust. J. Chem., 1976, 29, 1249-65

[nj_bartel]

Hmm, thanks!  For some reason my biochem text tends to draw them in furanose form.