[taurean]

I am always confused with the concept of exo and endo isomers.  I am unable to identify exo and endo isomers comfortably.  Please someone clarify the subject for me.  Thanks.

[Markovnikov]

Are you talking about DA endo and exo? In that case, I always check the hydrogens, how they are pointing. ^^

[taurean]

Are you talking about DA endo and exo? In that case, I always check the hydrogens, how they are pointing. ^^

Thanks for your reply.  Could you please explain with an example?

[stewie griffin]

I always just figure out what the endo transition state would be. That way I know what the endo product would be. If the product given doesn't match, then it must have been an exo transition state.
BTW, endo transition state is where the dienophile has an electron withdrawing group (like a carbonyl) pointing "towards" the diene rather than "away". I realize that's vague without a drawing, but wikipedia has some drawings that may be of help (http://en.wikipedia.org/wiki/Diels–Alder_reaction). The transition state they draw under the "endo addition rule" is how I draw my transition states.

[g-bones]

Endo/exo refers to the substituents in a bridged system.  If the substituents are on the same side as the larger bridge than this is the endo system, but if the are on the same side as the smaller bridge head, than these are exo.  For instance imagine a DA with cyclopentadiene and maleic anhydride.  typically the endo product is seen and this is when the carbonyls are on the same side as the bridgehead bearing the alkene (two carbons).  the exo product would be if they were on the same side as the methylene bridge (one carbon).  hope this helps.