Tagatose conformation

In the synthesis of D-tagatose from the more common D-fructose, 1-O-benzoyl-2,3-O-isopropylidene-P-D-fructopyranose afforded two products identified as l-O-benzoyl-5-0-benzyl-2,3-0-isopropylidene-P-D-fructopyranose (97%) and its 4-O-benzyl isomer (2.8%). The skew-boat 6S4(D) conformation with an oxygen atom at C-5 adopting a quasi-equatorial position is responsible for the unexpected regioselectivity observed [136]. Conformational equilibria may also be a reason for the non-exclusive, though preferential substitution at 0-3 of benzyl 4-0-benzyl-6-deoxy-a-L-talopyranoside (9) [142]. Even in this case, however, no tri-O-benzyl derivative was formed and no starting material 9 remained, the total isolated yield of 10 and 11 being 87%. 

With D-tagatose (D-/yxo-hexulose) and D-manno-heptulose in the CA conformation, the hydroxyl group at C-3 is axial this gives rise to an anomeric effect that favors the a-pyranose form. Hence, it is not surprising that these compounds exist almost exclusively in the a-pyranose form. The equilibrium states for D-fructose (d-arabino-hexulose), turanose, lactulose, and perseulose (h-galacto-heptulose), are, however, quite different. 

Tagatose, d-conformation of, 32 infrared spectrum of, 39 Talitol, 1 -amino-1,5-anhydro-1,6-dideoxy-L-, 138... 

The CD and conformational properties of acyclic derivatives of fructose, sorbose, tagatose, and four deoxy derivatives of fructose have been reviewed [14],... 

Draw the most stable conformation of the most stable cyclic hemiacetal form of tagatose. 

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