[orgo814]

"In addition to lactose, how many other heterodisaccharides can D-galactose and D-glucose form?"

The answer is 39 in addition to lactose. But, I'm struggling to find a logical way to solve this. I look at number of chiral stereocenters to see stereoisomers but that did not help. I looked at beta and alpha combinations but that also falls way short of 39. Please help

[Dan]

I get 20 (including lactose) considering pyranose forms only.

If I consider two disaccharides different if they vary only by the configuration at the non-glycosidic anomeric centre, which would not be correct to do, I get 36. If I forgot to remove degenerate pairs from that list, which would not be correct to do, I would have 40 (which matches the answer, that may be coincidence though).

If I consider combinations of pyranose and furanose forms I get 80.

So I'm stumped as well... I can only get 40 by making two mistakes.

If it doesn't get solved on here with someone else's help, please post the solution when you have it.

[orgo814]

My professor confirmed it was a book mistake. Thanks anyways though... I was there for a half hour trying to solve it!

[billnotgatez]

@butlerw2 Out of curiosity
What should have the answer been according to your professor?

[Babcock_Hall]

I was also interested in that question.  After some thought, I agree with Dan that the number is probably 20, as long as we restrict ourselves to pyranoses.

[orgo814]

He said about half that number so probably around 20