Clarification of ENDO and EXO The Diels-Alder reaction can proceed via an ENDO or EXO transition structure. When there is a double bond (π bond) ATTACHED to the dieneophile the ENDO transition structure is favored. For example, consider the following dieneophiles: O
O
O
OCH3
OCH3
dieneophiles with π group
dieneophiles without π group
IF THE DIENEOPHILE HAS A π GROUP then consider the two possible orientations of the dieneophile in the transition structure. In one arrangement the π group is underneath the diene and this is termed the ENDO transition structure and consequently gives rise to the ENDO product. In arrangement B, the π group is pointed away (not underneath) the diene and this is termed an EXO transition structure and consequently gives rise to the EXO product. While the EXO transition structure has less steric interactions, the ENDO TRANSITION STRUCTURE IS FAVORED DUE TO A FAVORABLE INTERACTION BETWEEN THE π GROUP AND THE p-ORBITALS ON THE DIENE. O
+
OCH3
CH3
O
O
H H
H3CO
H3CO
H H3C
ENDO transition structure H
O
H
H3C
ENDO PRODUCT
H
OCH3
OCH 3
H3C
O
O
CH3 H O
O H
OCH3
+
H
OCH3
OCH3
H3C
EXO transition structure
H
O
O
H3C H
OCH3
EXO PRODUCT
OCH3 H
H3C
WHEN A π GROUP IS ATTACHED TO THE DIENEOPHILE THE MAJOR PRODUCT IS THE ENDO PRODUCT. A SHORTCUT TO PREDICTING THE CORRECT STEREOCHEMISTRY OF THE ENDO PRODUCT IS AS FOLLOWS: Look at the s-cis conformation of the diene. If the groups attached to the diene point away from one another (see below) they will end up pointing away or “into the page” in the product. If they point towards one another they will point towards you or “out of the page” in the product. Following this approach, the π group on the dieneophile ALWAYS ends up “pointing away” or “into the page”. point away from one another