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Pyruvic acid on reduction with yeast gives (-) lactic acid while with $\ce{NaBH4}$ it reduces to (+) lactic acid. How?

I know this has something to do with stereoselectivity. But I don't know which one would be a correct answer.

It would be more helpful if you could explain a bit about stereo-selective reactions. I know what it is, what I want to know is, how to find which of the products will be stereochemically preferred one, just like in this question.

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    $\begingroup$ There is no way that borohydride reduction of pyruvic acid can give anything other than racemic lactic acid. $\endgroup$ – iad22agp Oct 25 '15 at 22:50
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Enyzmatic reduction (as would occur in yeast) could easily be a stereoselective process - probably via selective delivery of a hydride from NADH.

As the other respondent notes, there is simply no way that combining achiral reagents (pyruvic acid and NaBH4) can yield anything other than a racemic product.

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  • $\begingroup$ That is the exact reason, I'm asking this question for.I have read it in O P Tondon's organic chemistry for competitions book. $\endgroup$ – shaistha Oct 26 '15 at 8:56
  • $\begingroup$ As for the answer, how may I know or predict ,the product formed after stereoselectivy. Is there a way to predict what should be the major product or something? $\endgroup$ – shaistha Oct 26 '15 at 8:59

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