Determining Chirality Chair Conformation

[northpenn]

Is there a simple way to determine the chirality of a sugar in chair conformation? TBR states that L and D are determined by the chirality of the penultimate carbon but I always incorrectly determine the chirality when looking at carbons in the chair conformation. I know almost all naturally occuring sugars are in D conformation but a couple of the passages mention L-fucopyranose which is a naturally occurring sugar in the L conformation that tripped me up big time.

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[orgohacks]

Is there a simple way to determine the chirality of a sugar in chair conformation? TBR states that L and D are determined by the chirality of the penultimate carbon but I always incorrectly determine the chirality when looking at carbons in the chair conformation. I know almost all naturally occuring sugars are in D conformation but a couple of the passages mention L-fucopyranose which is a naturally occurring sugar in the L conformation that tripped me up big time.

If you look at the structure of L-fucopyranose as shown in the linked diagram, the final stereocenter shows the methyl group axial. If the final stereocenter has the substitutent axial and the H equatorial, it is likely D. If you open up this molecule and draw the Fischer projection, that is the ultimate test.


http://commons.wikimedia.org/wiki/File:Alpha-L-Fucopyranose.svg

note: this assumes that the sugars are drawn in the configuration shown, with the O at the top right and the carbons C1-C2-C3 etc tracing a counterclockwise path.

hope this helps - James