In Greek, as it is used in Scientific terminology, the prefix is "pent-." As with other such prefixes, it is left like that if the next part of the word begins with a vowel. If the next letter is a consonant, an "-a" is added to make the connecting form of the prefix.
Pent- angle (not "penta- angle")
Penta- gon (not "pent- gon")
In your specific example, if you drop the numbers and hyphens, you get "pentadiene," which is correct. And which interestingly enough actually has TWO numeric prefixes: "penta-" and "di-."