It's actually closer to NA-tyu-ral. The 'u' gets a 'y' sound before it, as in "universe". "Na-choral" is a little sloppy-sounding.

the pronunciation of the word natural ˈnætʃrəl you can hear it here : http://www.oxfordadvancedlearnersdictionary.com/dictionary/natural_1

au naturale :P

it's na-choral and here you can hear how to pronounce it http://www.macmillandictionary.com/dictionary/british/natural or http://www.forvo.com/word/natural#es

Nachral consider the first [a] is long but the second is short.