Crocetin glucosylation

Moraga, A.R. et al., Glucosylation of the saffron apocarotenoid crocetin by a gluco-syltransferase isolated from Crocus sativus stigmas, Planta 219, 955, 2004. 

The esterification can also be carried out as a one-pot reaction without isolation of the azolide for example, an 80% yield of crocetin bis(/ -D-glucosyl ester) was obtained via the triazolide. 1953 Reactions of 8 -apo-/ -carotene-8 -oic acid with D-galactose or lactose were claimed to proceed also with a high degree of regioselectivity.11963... 

The ratio of the a- to the j -anomeric D-glucosyl ester can be influenced by changing the reaction conditions. In DMF the a-anomer of the crocetin bis(D-glucosyl ester) was formed in about 70% yield.[196] Esterification of D-glucose with the imidazolides of benzoic acid or stearic acid in pyridine furnished a mixture of the a- and j -anomers of the C(l) glucosyl ester.[196]... 

For the partial synthesis, a regio- and stereoselective reaction is required, in which a bond with defined stereochemistry is formed exclusively between the carboxylic acid and the carbohydrate at the anomeric carbon atom. Due to the polyene system and the ease of the hydrolysis of the ester bond at the anomeric carbon atom, the application of protecting groups on the carbohydrate is restricted. Therefore a method using unprotected carbohydrates has been developed. Crocetin-di(P-D-glucosyl) ester (543) was synthesized in 70% yield by the reaction of crocetin-diimidazolide (4) or crocetin-di(l,2,4-triazolide) (5) and unprotected p-D-glucose in pyridine in the presence of a base [5]. The esterification takes place exclusively at the anomeric carbon atom and produces only the p-anomer [6]. The method was also applied to 8 -apo-P-caroten-8 -oic acid (486) and to other carbohydrates [7]. 

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