Hello Prabhakaran, 

Is your concern only with the curcumin or the other phenolic compounds usually found as plant secondary metabolites? Since I am working on post modifications of such compounds, so far I can say curcumin is not found in its glycoform in nature. And indeed, glycans are added chemically or enzymatically to produce their glycosides which later enhance its water solubility and sometimes alter in biological activities as well. 

And the other answer I would say is, it is rather easy and cost effective to synthesize the phenolic glycosides  by enzymatic and chemical modification approach instead of extracting direct from plant source.

Hello Prakash, thank you so much for your answer. Yes I am concerned particularly in curcumin and other phenolic compounds with similar pharmacological activities. I am looking for ways to not only enhance their bioavailability but also their potency towards specific pathways. But as far as i know curcumin is the only phenolic compound that doesn't exist in glycoside form in nature. So just wondered if there is any special reason for that. and you mentioned that these glycosides sometimes alter their biological activities as well. could you refer me to some paper that actually demonstrates that? thank you.

No, curcumin is not that exceptional, flavanols (catechin, procyanidins etc) are also generally not glycosylated in nature.

Dear catherine, thank you for clearing that up. 

There are a lot of natural products which are not glycosylated in nature, and also the comounds there which are both glycosylated and the aglycone is also isolated from nature