The rule really is y, not u, for Greek upsilon. That really *really* surprised me.
I went to the OED, and it didn’t tell me much:
Etymology: First formed as French glucose (Dumas 1838, in Compt. Rend. VII. 109); compare Greek γλυκύς sweet and -ose suffix.
The English Wikipedia didn’t tell me much more.
But you know, there are other Wikipedias, and they often say things the English Wikipedia doesn’t. And since the word was coined in French, I took a chance that it might have said what was on Dumas’ mind. My translations.
En 1838, un comité de l’Académie des sciences composé des chimistes et physiciens français Thénard, Gay-Lussac, Biot et Dumas, décide d’appeler le sucre se trouvant dans le raisin, dans l’amidon, et dans le miel du nom de glucose, en fournissant comme étymologie le grec τὸ γλεῦκος / gleukos, vin doux. Émile Littré ayant donné une autre étymologie, l’adjectif γλυκύς / glukus (« de saveur douce »), la racine habituelle est devenue glyc-(l’upsilon grec donnant un y), comme dans glycémie et glycogène.
“In 1838, a committee of the Academy of Sciences, composed of the French chemists and physicists Thénard, Gay-Lussac, Biot and Dumas, decided to call the sugar found in grapes, starch and honey with the name glucose, providing its etymology as the Greek gleukos ‘sweet new wine’. Émile Littré had provided an alternative etymology, the adjective glykys ‘sweet’, so the usual root in derivations is glyc-, as in glycaemia and glycogen.”
And the French Wikipedia adds a footnote with the actual 1838 article derivation:
Louis Jacques Thénard, Louis Joseph Gay-Lussac, Jean-Baptiste Biot et Jean-Baptiste Dumas, « Rapport sur un mémoire de M. Péligiot, intitulé: Recherches sur la nature et les propriétés chimiques des sucres », Comptes rendus hebdomadaires des séances de l’Académie des sciences, 2 juillet 1838, p. 106-113 (lire en ligne [archive]) :
« Il résulte des comparaisons faites par M. Péligot, que le sucre de raisin, celui d’amidon, celui de diabètes et celui de miel ont parfaitement la même composition et les mêmes propriétés, et constituent un seul corps que nous proposons d’appeler Glucose(γλευϰος, moût, vin doux). »
“From comparisons made by Mr Péligot, it turns out that the sugar in graps, starch, diabetics and honey have the identical composition and properties, and involve a single constituent which we propose to call glucose (γλεῦκος, ‘must, sweet wine’).”
The transliteration of Greek <ευ> as <u> is also irregular; it is conventionally <eu>, as in leucocyte or rheumatism. But there is a tendency to transliterate <ευ> as <u> in French: cf. leucocyte, but rhumatisme.
(Why yes, I have found an error in the OED. I’ve emailed them.)
Btw, noone told the Greeks the word is derived from gleukos; in Greek the word is γλυκόζη glykozē.
EDIT: Thanks, Chad Turner. Some Greek upsilons are spelled in English as <u>; notable instances are kudos and hubris. Per both Merriam-Webster’s Manual for Writers and Editors and The History of English Spelling (9781405190237): Christopher Upward, George Davidson (PDF draft chapter here: http://www.aston.ac.uk/EasysiteW…), the <u> is a 19th–20th century convention, subsequent to the obligatory latinisation of Greek loans. Notice that it’s Hellenising kudos, not Latinising cydus.